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The Origin and Evolution of Earth (L4 "Ur Minerals, First Crystals in the Cosmos")

The first mineral in the cosmos was diamond.

Formed in the carbon-rich envelopes of exploding stars. Temperatures were still high but cooling, allowing tiny crystals of carbon to form. Soon other crystals followed. These were the first minerals. Together, these dozen or so minerals began to seed the universe with their own dust, becoming the initial raw-materials for the formation of any Earth-like planet. There is beauty in the idea that the first mineral in our universe was diamond. But it astonishes that until just a few years ago, in November of 2008 to be exact, no one seems to have asked when and where the first mineral formed. What was the first mineral in the universe? That basic question had not been asked.

Think about this. Scientists have long asked when the universe itself began, or when Earth was formed, or when life emerged from the primordial soup. We're constantly improving our estimates of when the first plants, or land animals, or primates evolved. Indeed, it seems to be the most basic question in almost any field of study, to ask how things got started. But in spite of an extensive search, I can't find a single mention of the oldest mineral anywhere in the vast literature of geology. In this lecture we're going to look closely at that question. The question of the beginning of cosmic mineralogy, which in a very real sense is the beginning of all the rocky planets like Earth.

Charlemagne: Father of Europe (L1 "The Making of Emperor Charlemagne")

It was Christmas Day 800. Charles, as he was then called, was spending Christmas in Rome. It was still the largest and greatest of all the cities in Christian Europe, with a resident population of perhaps 20,000 people. Admittedly, some eight hundred years earlier, Rome had housed one million people. The Rome that Charlemagne visited consisted mostly of empty, abandoned buildings. But even in its diminished state, Rome was still larger than any town north of the Alps. In Charlemagne’s own native kingdom, the Kingdom of the Franks, even the largest towns such as Paris had only perhaps 5,000 residents.

Rome’s reputation rested on much more than its former size. Its reputation rested, above all else, on its many old churches. Charlemagne attended Christmas Mass in one of those churches: the basilica of Saint Peter, an early version of the building located in what is today Vatican City. On this day, Charlemagne had shown up for Christmas Mass dressed in Roman clothing, which he had worn perhaps once before in his life: he sported a long Roman tunic, a Roman cloak, and pointed Roman shoes. Given how he was decked out, wearing spiffy Roman clothes far removed from the usual Frankish wardrobe that he preferred, it was almost as if Charlemagne was expecting something unusual to happen.

Presiding over Christmas Mass was Pope Leo III. As pope, on Christmas Eve and Christmas Day, he had to officiate at four different masses held at four different locations: the mass at St. Peter’s was the fourth of four. The grueling cycle of masses that he had to lead may well have left him looking the worse for wear.

Certainly his five years as pope had taken a toll on him. In 799, just one year earlier, assailants had mutilated Pope Leo’s tongue and eyes as part of an effort to depose him. Leo had fled to Charlemagne for support, Charlemagne had helped Leo retain the papacy, and now, just a year later, both men were together again, in Rome.

Christmas Mass at St. Peter’s in 800 did indeed take an unusual turn. During the Mass, Pope Leo III placed a crown on Charlemagne’s head. Inhabitants of Rome attending mass acclaimed Charlemagne as emperor: “To the august Charles, crowned by God, the great and peaceful emperor of the Romans, life and victory!” That they shouted these acclamations in unison suggests rehearsal and preparation. The pope then performed a gesture of submission to Charlemagne—most likely, the pope prostrated himself before the new emperor, a gesture called proskynesis.

The newly crowned emperor, in turn, expressed his continuing veneration for St. Peter’s basilica and for other Roman churches by giving them gifts: valuable liturgical items such as crucifixes and chalices, made from silver and gold, and adorned with jewels.

When Leo III crowned Charlemagne as emperor in 800, there had been no emperor in western Europe since 476; back in that year, a barbarian general named Odoacer had deposed the last western Roman emperor. Between 476 and 800, there had been kings and kingdoms in western Europe, and plenty of them.

Italy had been home to one of those kingdoms, the Kingdom of the Eastern Goths, or Ostrogoths. The surviving eastern half of the Roman Empire, which historians call the Byzantine Empire, toppled the Ostrogothic Kingdom in the mid-6th century and reimposed imperial rule.

But even before the end of the 6th century, a new and independent kingdom emerged in northern and central Italy, the Kingdom of the Lombards The Iberian peninsula had been home to several kingdoms, chief among them the Kingdom of the Western Goths, the Visigoths. It had succumbed to Muslim conquerors two generations before Charlemagne’s birth. In what had once been Roman Gaul, there had been various Frankish-ruled kingdoms, centered on regions such as Burgundy, Aquitaine, Neustria, and Austrasia. Charles’s grandfather had held high office in Austrasia, and wielded considerable influence in Neustria as well. Charles’s father, like Charles himself, had ruled them as their king.

In what had once been Roman Britain, there were even more kingdoms, usually seven in number, modestly sized and ruled by Anglo-Saxons. Europe had not always been a continent of disunited kingdoms. Under the Roman Empire, a single Roman emperor had ruled all of these lands. Empire and the office of emperor stood, above all else, for unity and unified rule. Charlemagne’s imperial coronation in 800 raised the possibility that Europe would no longer consist of a multiplicity of kingdoms.

Perhaps Europe’s future would be that of a single empire, like contemporary China under the Tang dynasty, about which Charlemagne knew next to nothing. Or like the Byzantine Empire, about which Charlemagne knew a great deal more, and with which he had substantial dealings throughout his life. The Byzantine Empire was Greek-ruled, and its capital was at Constantinople. That city housed perhaps as many as two hundred thousand residents, which would have made it ten to twenty times larger than any town or city over which Charlemagne ever ruled.

Charlemagne’s imperial revival was audacious. And it caused political and conceptual problems.

By reviving the imperial title, Charlemagne had raised awkward questions about the principle of imperial unity. The entity that we call the Byzantine Empire still called itself the Roman Empire, and its emperor still called himself the Roman Emperor. And they were right to do so. The Byzantine Empire was a direct continuation of the Roman Empire; when the last western emperor had been deposed in 476, the eastern emperor had remained in office.

As a result of Charlemagne’s imperial coronation, now there were two emperors claiming to rule over the Romans. The Byzantines had liked it better when there was just one such emperor. Even worse—far worse— Charlemagne was a Frank. The phenomenon of co-emperors had a long history within the Roman world; in such cases, the pretense of imperial unity could be preserved, provided that all the emperors were Romans. Charlemagne’s Frankishness left little or no room for such a pretense. Moreover, Emperor Charlemagne did not rule over all the places that had once been part of the western half of the Roman Empire. He never ruled over the entire Iberian peninsula, or any part of the British Isles.

Still, the empire over which he did rule was larger, by far, than any European kingdom that had existed since the deposing of the last western Roman emperor in 476. During his reign as king and then emperor, Charlemagne doubled the size of the territories over which he ruled. Charlemagne’s Empire was clearly in the ascendant, which was more than could be said for its chief rivals. The Byzantine Empire had lost most of its territory to the Islamic Conquests of the seventh and early eighth centuries; it was still early the process of an uncertain recovery.

The house of Islam, after an explosive century of expansion, had itself started to fragment in the 8th century, a process that would eventually lead to the emergence of multiple, rival caliphates. By contrast, Charlemagne’s Frankish-ruled empire seemed well-positioned to continue expanding, bringing about an even broader political and cultural unification of Europe.

Fast-forward 1,150 years later. In 1950, the German city of Aachen, located on the border with Belgium and the Netherlands, created an annual prize to honor those who worked to foster West European understanding. The name of the award is the Charlemagne Prize.

In 1990, as the Warsaw Pact and then the Soviet Union were disintegrating, Aachen broadened the criteria for winning. Since then, the prize has honored those who worked for the “overall unification of the peoples of Europe”— not just Western Europe anymore, but all Europe. Among those who have been awarded the Charlemagne Prize are Winston Churchill and Emmanuel Macron, Henry Kissinger and Bill Clinton, Pope Francis and the entire population of Luxembourg—a rather eclectic list.

The city of Aachen and Charlemagne had close historical ties. Aachen was Charlemagne’s favorite residence late in life; he built his most famous palace there.

Aachen’s post-World War II invocation of Charlemagne was not unique to that city. Today, several departments of the European Commission (which constitutes the executive branch of the European Union) are housed in the European Commission Charlemagne Building, built in Brussels in 1967. Two years after moving into their Charlemagne building, the European Commission itself won the Charlemagne Prize.

And those bent upon European unification of a quite different sort than that envisioned by the European Union have sometimes embraced Charlemagne, too, as a precursor, an inspiration, and a symbol.

In September 1944, Germany’s ruling Nazi party organized the Waffen Grenadier Brigade of the SS Charlemagne. The SS Charlemagne consisted primarily of French volunteers; after the war, these volunteers stated that they had served in the SS Charlemagne because they loved and wanted to defend Europe. Admittedly, these retrospective justifications might have been intended to obscure other motives. Nonetheless, the fact that veterans of the SS Charlemagne offered the “love of Europe” as a plausible explanation for their service is evidence of how, more than one thousand years after Charlemagne’s death, people across the political spectrum linked Charlemagne and Europe.

That link extends all the way back to Charlemagne’s own lifetime. At some point between 800 and 803, an anonymous poet attached to Charlemagne’s court wrote an epic poem, only part of which survives today, that modern scholars have dubbed “The Paderborn Epic.” The poet bestowed on Charlemagne several appellations intended to flatter him. One was “Lighthouse of Europe.” That one never caught on. The poet’s “Apex of Europe” did not catch on either.

“Father of Europe,” on the other hand—pater Europae—did catch on. About thirty years after Charlemagne’s death, a chronicler named Nithard likened Charlemagne to a good father who had made beneficial bequests to Europe: “Charles of happy memory, and deservedly known as the Great, called emperor by all nations and dying at a ripe old age, left Europe filled with every good thing.”

Note how Nithard’s testimony reveals that, already by the 840s, Charles was known as Charles the Great: Karolus magnus in Latin. Centuries later, as early German and early French came into being, that became Karl der Grosse in German, and Charl-le-magne in French, whence the English Charlemagne.

But even “father of Europe” is understated compared to some of the accolades that contemporaries heaped on Charlemagne. The anonymous author of “The Paderborn Epic” describes the recently crowned emperor Charlemagne thus:

HE IS POWERFUL, WISE, KNOWING, PRUDENT, BRILLIANT, APPROACHABLE, LEARNED, GOOD, MIGHTY, VIRTUOUS, GENTLE, DISTINGUISHED, JUST, PIOUS, A FAMOUS WARRIOR, KING, RULER, VENERABLE SUMMIT, AUGUST, BOUNTIFUL, DISTINGUISHED ARBITER, JUDGE, SYMPATHETIC TO THE NEEDY, PEACEMAKING, GENEROUS, CLEVER, CHEERFUL, AND HANDSOME.

Not everyone has viewed Charlemagne in such a positive light, not during his lifetime, and not since then. Ten years after Charlemagne’s death in 814, a monk named Wetti, who was himself dying, experienced a religious vision. He related what he had seen to his fellow monks, one of whom composed a prose work called the “Vision of Wetti,” in 824. Another monk expanded on this work in verse form three years later.

According to these accounts, Wetti had seen individuals undergoing purgatorial punishment for the sins that they had committed while alive. Among those whom Wetti saw was someone whom he clearly recognized, a former ruler over Italy and the Roman people. An angel told Wetti that God had denied this ruler immediate entrance into heaven because the ruler had “defiled himself with vile lechery.” As punishment, a wild animal was lacerating and tearing off the ruler’s genitals. Wetti does not name the ruler, but the verse version of 827 takes the form of an acrostic poem. The first letters of each line, when put together, spell out a name and a title: Carolus imperator, Charles the emperor, which is to say, Charlemagne.

In sum, many have lauded Charlemagne; others have regarded him as having much to answer for. And this debate has been going on for over a millennium. What was it about Charlemagne that has caused such fascination with him? What did he do to generate so much controversy? Why have opinions about Charlemagne diverged so much?

A Field Guide to the Planets (L3 "Venus, the Veiled Greenhouse Planet")

Humans have been to the Moon. Where should we go next? Although the answer is usually assumed to be Mars, let's consider travel to the planet Venus. Venus is the brightest object in our sky, after the Sun and Moon, inspiring names such as Morning Star and Evening Star. The modern name Venus comes from the Roman goddess of love and beauty. Venus is a planet quite similar to Earth. Some even call Venus Earth's twin planet. Venus's diameter and average density are 95% those of Earth. Venus's orbit is also closest to Earth's orbit, at 72% the distance. That means travel time from Earth to Venus could be less than travel time from Earth to Mars.

And there are other Earthly features, too. Suppose we travel to Venus with equipment that can open to form a giant airship, like the Goodyear Blimp. NASA has simulated how the parachute for such a craft might open high in the Venus atmosphere, how a giant balloon might be extended, how it could be inflated and become a floating airship high in the atmosphere.

At around 50-65 kilometers above the surface, we find that Venus offers some conditions surprisingly similar to the surface of Earth. The temperature, atmospheric pressure, and even shielding from the Sun's radiation are comparable to what we take for granted at Earth's surface. Compared to Mars or open space, the atmosphere of Venus would have less radiation from the Sun, which would make long-term Venus missions much less hazardous for astronauts and equipment. In fact, if you were going to pick a single destination anywhere in the solar system that's most similar to Earth, floating high in the atmosphere of Venus might be just the ticket.

Of course, there are challenges. For one thing, once we enter the atmosphere of Venus, we can't see anything. There is a shroud of highly reflective cloud and haze blanketing the planet. The veil hiding Venus allowed older science fiction writers some free reign in imagining Venus's surface. Could it be a jungle? A desert? How about an ocean world? All these ideas were motivated by the fact that Venus is closer to the Sun than Earth is. Temperatures were expected to be hotter. That's true. But we now know that the atmosphere of Venus makes it much hotter than any jungle or desert, in fact, twice as hot as the highest setting of the oven in your kitchen.

Venus's atmosphere is also much denser, and under higher pressure, than on Earth. Remember that the pressure 50 kilometers above Venus's surface is similar to the pressure at Earth's surface. That means if we go further down in Venus's atmosphere, the pressure and density increase. At Venus's surface, the pressure is 92 bars. That's 92 times Earth's atmospheric pressure at sea level. It's about the same pressure you would experience if you were under a kilometer of water in Earth's oceans.

And don't bother bringing champagne to celebrate once you reach the surface. Champagne is bottled on Earth at a pressure of only 2 bars. If the bottle isn't crushed by the incredible pressure on Venus, opening the bottle isn't going to allow carbon dioxide to escape and become bubbly. Instead of depressurizing, the contents of the bottle would pressurize much more.

Now that we're at the surface, let's compare the atmospheres of Venus and Earth from the ground up. On Earth, the bottom layer of the atmosphere, called the troposphere, extends from the surface up to an average of about 10 kilometers. This is the layer of the atmosphere where most of the weather happens. Three-quarters or more of all atmospheric mass is here. The defining characteristic of the troposphere is this is where temperature decreases with height due to convection. Venus has a troposphere, but it extends from the surface up to about 65 kilometers, so it's, on average, 6½ times taller. And again, you have to be at least 50 kilometers high in this troposphere to experience the pressure we have at Earth's surface.

But while Venus has this extremely tall troposphere layer, the other layers are smaller than those on Earth, and the total height of the Venus atmosphere is actually less than Earth. Venus transitions directly from troposphere to a relatively calm mesosphere, which means middle atmosphere. Outermost is an exosphere, similar to Mercury, where particles no longer behave like a gas because the density is too low for collisions to happen. For Venus, the exosphere starts at about 220 to 350 kilometers altitude. In contrast, Earth's exosphere starts at about 600 kilometers altitude. So even though Venus's troposphere is taller, and very dense, the total atmosphere is only 1/2 the height of Earth's atmosphere.

What's in the air is also very different. Venus's atmosphere is overwhelmingly carbon dioxide. Earth's atmosphere is 3/4 nitrogen. Why are their compositions so different? It turns out that Earth, as a planet, still has about the same amount of carbon dioxide as Venus; it's just that Earth removes it from the atmosphere. Earth buries carbon dioxide in carbonate rocks. And plants convert it to oxygen in the air.

Burial of carbon dioxide in rocks happens mostly in our oceans. These carbonate rocks are then recycled back into Earth through plate tectonics. Essentially, Earth's carbon cycle regulates the amount of carbon dioxide in the atmosphere. Any oceans Venus may have had early in its history boiled away when the temperatures increased due to a runaway greenhouse effect. So, Venus didn't have oceans to soak up carbon dioxide from the atmosphere. Venus also doesn't have plate tectonics to remove carbon from the atmosphere by sending carbonate rocks deep into the planet. And of course, Venus never had plants that trap even more carbon dioxide from the air and replace it with oxygen.

The result is there is nowhere on Venus for the carbon dioxide to go, except the atmosphere. Interestingly, if you were to take all the carbon locked away in all the rocks and plants on Earth and move it to the atmosphere in the form of carbon dioxide gas, Earth would have atmospheric pressure and composition quite similar to Venus.

Understanding Gravity (L2 "Free Fall and Inertia")

Last time we began our exploration of gravity with Isaac Newton and the famous story of the apple. This time let's start with a more extreme example of free fall. In 2012 an Austrian adventurer named Felix Baumgartner did something extraordinary: he rode a balloon to an altitude of 39 kilometers, more than 24 miles above the ground. Then he opened the door of his gondola and jumped out. Baumgartner fell for more than four minutes before he opened his parachute. It was the highest skydive in history, breaking a record more than 50 years old. It was a rather dramatic experiment in gravity.

Some of the most important insights about gravity and about mechanics, the science of force and motion, a subject of extreme interest to Felix Baumgartner in his jump, some of those insights actually predate Newton's work by more than half a century. Indeed, these original basic discoveries were not only the inspiration for Newton, but also for Einstein. If Isaac Newton was the father of mechanics, then the grandfather of mechanics was Galileo Galilei.

Let's return for a minute to Felix Baumgartner and his record setting jump from a balloon 39 kilometers above the Earth. At first after he jumps, Baumgartner is in free fall. After one second, he is traveling 10 meters per second, having fallen 5 meters. After two seconds he is traveling 20 meters per second, having fallen 20 meters since the start. After three seconds he is traveling 30 meters per second, having fallen 45 meters. After four seconds he is traveling 40 meters per second, having fallen 80 meters. All of these agree perfectly with our equations for free fall. But pretty soon, after less than a minute, he stops falling any more rapidly. He is no longer accelerating.

In fact, he begins to slow down a little. Why? Is he defying the law of free fall? No.

The reason Baumgartner does not continue to accelerate is that another force has begun to be important: air drag. Once Baumgartner is going fast enough the friction of the air prevents any further speed up. This maximum speed is called his terminal velocity. Terminal velocity depends on mass and shape and air density. In Baumgartner's case his maximum speed is over 375 meters per second, that's faster than the speed of sound. As he falls through denser and denser air, though, that terminal velocity actually gets less. On Earth, at the surface, the terminal velocity of a falling human body is still pretty fast. That's why you need a parachute to slow the terminal velocity even more. On a planet with a much denser atmosphere, the terminal velocity might be a lot slower. You might not need a parachute to land safely.

I once wrote a science fiction story set on the planet Venus. The atmosphere there is more than 50 times denser than on Earth. The hero of my story finds it necessary to skydive without a parachute from an altitude of more than 70 kilometers above the surface of the planet, even higher than Felix Baumgartner. It is just possible, just barely, for my hero to survive because the terminal velocity is so much slower in the dense atmosphere of Venus.

With no atmosphere, as Galileo said, a falling body continues to accelerate, and everything continues to accelerate the same. Of course, Galileo could not actually do an experiment without air. In 1971, the Apollo 15 mission spent a few days exploring the Moon. The Moon, of course, has no appreciable air. On the last day on the Moon, Dave Scott, the mission commander, performed what has to be the coolest physics classroom demonstration in history.

[Video start.]

[Astronaut Dave Scott is speaking] In my left hand I have a feather. In my right hand, a hammer. Well, in my left hand, I have a feather; in my right hand, a hammer. And I guess one of the reasons we got here today was because of a gentleman named Galileo, a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought, where would be a better place to confirm his findings than on the Moon. So, we thought we'd try it here for you. The feather happens to be, appropriately, a falcon feather for our Falcon. And I'll drop the two of them here and, hopefully, they'll hit the ground at the same time. [Scott drops the objects. They land at the same time.]

How about that? [Video clip with Scott ends.]

It is too bad Galileo could not do his experiments on the Moon. There are two things to note about this. First, on the Moon the acceleration of gravity is not the same as on Earth. It's about 1/6th as great. In Lecture 4 we'll find out why. Two, in the absence of air resistance, the feather and the hammer really do fall just the same. Galileo had it exactly right. All objects behave the same in free fall. It's a basic fact about gravity. In fact, the law of free fall contains a secret message that won't be decoded for 300 years until Albert Einstein realizes that it is the key to everything.

What Darwin Didn’t Know: The Modern Science of Evolution (L7 "Rapid Evolution within Species")

Darwin considered evolutionary change to be a process that occurred very slowly. In the Origins of Species, he went out of his way to emphasize the slow pace of evolution. He wrote, “We see nothing of these slow changes in progress,” and he used that word “slow” 144 times throughout the text. Even for the modifications of our domestic breeds, Darwin declared that “the chance will be infinitely small of any record having been preserved of such slow, varying, and insensible change.” But research on a wide range of different species has shown that evolution can actually happen rather quickly. So quickly, in fact, that we can watch it as it plays out in real time.

Ironically, one of the best examples of rapid evolution comes from the Galapagos finches that helped inspire Darwin’s ideas about slow, gradual change. Galapagos finches have since become some of the best studied organisms on the planet, the subject of decades of research by biologists. Among them are Peter and Rosemary Grant, a husband-and-wife team that have spent their careers conducting detailed studies of these birds.

Much of the Grants’ time in the field has been spent on a single small island called Daphne Major. Like many of the smaller islands in the Galapagos, Daphne Major is largely uninhabited, and for good reason. There is no reliable source of freshwater. There aren’t even any trees to provide shade from the equatorial Sun. One advantage of working on the remote, inaccessible island of Daphne Major is that there has never been much of a human presence on the island to affect the natural processes as they play out. Not even tourists stop on Daphne Major. Yet the Grants and their collaborators have returned there every year for more than 4 decades, beginning in 1973.

Another advantage of working on Daphne Major is its small size. At just 84 acres in area, the Grants are able to capture every single finch on the island. By doing so each year, they came to know each of the birds as individuals and have tracked the entire population from year to year. And by taking blood samples, they could determine how all of the birds are related. As they captured each bird, they also recorded many different body measurements, from the length of its legs, to the depth of its beak, to the color of its plumage. And the Grants collected data on the other inhabitants of the island, including the few species of plants that manage to survive in the harsh, volcanic rock.

In 1977, the Galapagos Islands experienced a severe drought. With their detailed data, the Grants were poised to learn how the finches of Daphne Major were affected by the drought. When the Grants returned after the drought, they found that, compared to a population of 751 medium ground finches before the drought, after the drought there were only 90. The Grants took all their usual measurements, and when they analyzed their data, they discovered something no one had ever seen before; evidence that evolution had occurred in a wild species in just one generation! Prior to the drought, the medium ground finches had beaks that ranged in depth from about 8 to 11 mm, with an average of 9.2 mm. After the drought, the surviving finches had an average beak depth of 9.7 mm, an increase of 15%. That might not seem like much, but it’s a big enough difference to make the Grants ask: What happened? Despite experiencing a population bottleneck, the change in beak depth wasn’t caused by random genetic drift; larger beaks were favored by natural selection. The drought had had a big impact on the plants living on Daphne Major, including a plant called spurge that makes small seeds that the finches like to eat. Without their favorite food during the drought, the finches were forced to try to eat the only other source of food on the island, a larger, spiky seed called caltrop. Caltrop seeds are not only spiky, they’re also hard, and the finches struggle to crack them open. But finches with larger, deeper beaks can apply more force to the caltrop seeds and are therefore better at cracking them open. Beaks better adapted to switching to the alternative food source had a better chance of surviving.

This was an exciting result, but the Grants didn’t stop there. They continued returning to Daphne Major every summer, and before long they witnessed natural selection at work again. It was particularly rainy in late 1982 to early 1983, leading to an abundance of the finches’ preferred food, spurge seeds. With lots of tiny seeds to eat, big beaks became more of a liability than an asset, and the average beak depth declined by 2.5%. The Grants had not only shown that natural selection can cause rapid evolutionary change, they had also shown that the traits favored by natural selection can fluctuate as the environment changes.

How would Darwin have reacted to these findings? On the one hand, he would probably have been thrilled to see clear evidence that the mechanism he proposed for evolutionary change works exactly as he had imagined, and the data came from a group of species that he was responsible for bringing to the world’s attention. On the other hand, it showed that Darwin was wrong about evolution being slow. The Galapagos finches proved that natural selection could change a species in a noticeable way in just one generation. Comparison of the genomes of 13 species of Galapagos finches suggests that the evolution of the entire group has happened rapidly. The common ancestor of the 13 species lived just 2 million years ago, and some finch species may have come into existence in just the last 100,000 to 300,000 years. And extrapolating with the help of DNA data, the Grants estimated that a new species might emerge in as little 200 years of sustained change in a single direction. Darwin used the idea that humans can cause other species to evolve as the opening argument in The Origin of Species. He knew that animal and plant breeding would be a familiar concept to his readers, so he began by pointing out how effectively breeders can develop new varieties of crops, livestock, and pets. Pigeons were a prime example. Humans have been breeding or having an impact on pigeons for thousands of years, but how long does it really take to tame a wild animal? At least one line of evidence suggests that domestication could happen quickly.

On a farm in Siberia, near the town of Novosibirsk, live the friendliest foxes you could ever hope to meet. Unlike their wild counterparts, these foxes enjoy human company. They wag their tails, roll over on their backs to have their bellies rubbed, and respond to commands like “sit” or “shake.” In short, they’re a lot like dogs. Which is exactly the point. In 1959, biologists Dmitri Belyaev and Lyudmila Trut began an experiment to see if they could breed foxes to become tame. Part of the motivation was to see if foxes could become domesticated, just as wolves had been thousands of years ago, leading to the first dogs. The other reason for the experiment was that across much of the USSR, foxes were being kept for their fur, but working with them was dangerous because foxes kept in captivity tend to be aggressive. Belyaev and Trut bought foxes from several different farms and began breeding only the those that seemed the least afraid of humans. Those that were aggressive were never allowed to mate. Belyaev and Trut found that the calmer a fox was, the calmer its offspring tended to be. After just a couple of generations they were already seeing calmer behavior on average, and a few foxes were less aggressive than any of the original foxes had been. One fox pup in the 4th generation began wagging its tail when a person approached, a behavior then known only in dogs. By the 6th generation, a few of the pups were licking their caretakers’ hands and rolling over on their backs to have their bellies rubbed. Those dog-like behaviors became more common in each subsequent generation until the vast majority of descendants behaved like dogs. The foxes began to look like dogs, too. Some developed floppy ears, curly tails, and white patches of fur. As was already recognized in Darwin’s time, these traits are found across a variety of domestic animals. Pigs, goats, sheep, and rabbits have floppy ears and white fur patches, and domestic pigs have curly tails. Even though these traits weren’t being selected for in the foxes, they became more common as the foxes became tamer.

Belyaev and Trut showed that foxes can be domesticated in much the same way as dogs, and in just a few decades. We don’t know how long it took the many other species of animals and plants to become domesticated, but if the 40 generations from the foxes experiment are any indication, it might have happened quickly.

Understanding the Periodic Table (L21 "Rare-Earth Elements: Surprisingly Abundant")

Decades ago, the Molybdenum Corporation of America renamed itself Molycorp and later ran a promotional campaign giving away free samples of what are known as ‘rare-earth elements.’ Their slogan was:

“These elements are no longer rare. Try them.”

Most people have no idea how much modern technology has been transformed by including these ultimate team players of the periodic table.

-Color TV colors got better, thanks to europium.

-So-called halogen lights that actually depend on dysprosium.

-Magnets with neodymium have transformed everything from your headphones to the commercial turbines that make possible modern wind power farms.

-Terbium made X-rays became much safer and made solid-state drives possible for data storage.

-Hybrid and electric car batteries depend on 20-30 pounds of lanthanum.

In short, largely unnoticed, the so-called “rare earth” elements have become unsung heroes in a host of modern technology applications. Although you may rarely hear about these elements, the good news is that they’re actually pretty common on Earth’s surface. Yes, they are “rare” compared to the most common elements. But compared to many of the elements that have been known since ancient times, calling them “rare” is a bit of a misnomer.

Cerium, which most people, even today, have never heard of, is more abundant than copper. The top three most abundant members of the group are more common than tin or lead. And the entire group (with one radioactive exception) is more abundant than silver—and far more abundant than gold. And they’re very useful, even though they are not chemical divas like gold. For example, an estimated 50 to 100 grams of cerium are used in every one of the millions of catalytic converters in vehicles around the world. IN catalytic converters, cerium isn’t the catalyst, but rather part of the heat-resistant oxide mineral that supports it.

Or consider hybrid and electric vehicles, with their big batteries sometimes made of nickel-metal hydride. Well, that “metal” in the battery is mostly lanthanum, with over 20 pounds used in each electric vehicle. We could call them nickel-lanthanum-hydride batteries! They save space and weight, and they’re about twice as efficient as traditional lead-acid batteries. In fact, ‘hidden earth’ elements might have been a better label for what are still referred to as “rare earth metals” As this name implies, they’re not truly rare, they’re just hidden.

The Goldschmidt geochemical classification of these oxygen friendly elements makes them lithophilic, literally “rock-loving” elements that combine readily with oxygen. (And the f-block of ‘rock-loving’ elements is squarely located in between other groups of ‘rock-loving’ elements on the table— groups 1, 2, and 3. as well as groups 4 and 5.) As the Earth differentiated, these lithophile elements chemically preferred to float with the lighter-weight silicate minerals, oxides, and sulfides near the surface. while iron and the “iron-loving” elements largely sank into the core.

So, finding a mixture of rare earth metals here on Earth isn’t such a chore. The bigger challenge is separating them from one another. In fact, it happened more than once that some of the greatest chemical minds of their time thought they had produced a pure sample of a new element, only to later discover that their creation was merely another mixture of the rare-earth elements.

Unlocking the Hidden History of DNA (L7 "Microbes Manipulate Us, Viruses Are Us")

The completion of the Human Genome Project in the early 2000s led to two shocking realizations. First, biologists realized that less than 2% of the DNA in the human genome actually codes for the proteins that make and run the human body. That 2% figure looks even crazier when you combine it with the second shocking realization: that the name Human Genome Project was something of a misnomer.

It turns out that 8% of our genome is not human at all. A full quarter billion base pairs of our DNA are simply old virus genes that got inserted long ago and never weeded out. Some rogue virus infiltrated our distant ancestor’s sperm or egg cell, which produced a baby. And the lucky virus got to hitch a ride around indefinitely in all of that baby’s descendants, including us today. Put the two findings together, and our “human” genome might look like it’s four times more virus than human.

So how is this possible? Let’s go back to 1958, when Francis Crick of double helix fame published his central dogma of molecular biology. It states that DNA produces RNA, and RNA produces proteins, in that order. But starting in the 1960s, scientists discovered that nature cares little for dogmas. It turns out that certain viruses, including HIV, can manipulate DNA in heretical ways.

Unlike everything else in nature, there are many viruses that use RNA instead of DNA to store genetic information. Viruses that use RNA instead of DNA include influenza, coronavirus, measles, polio, rabies, Ebola, and many strains of the common cold. Because RNA is typically singled stranded, it’s far less stable than the double helix of DNA. As a result, the genomes for RNA viruses never get very large. Polio is just 7,000 bases, influenza around 14,000. Even the COVID 19 coronavirus, which is at the upper limit of stability for an RNA virus, is only 30,000 bases.

But among RNA viruses, there’s also a more devious group, including HIV, that infect a cell differently. Most viruses don’t work this way, but these special RNA viruses can coax the cell into turning the virus’s RNA back into DNA. This means running the central dogma of molecular biology backwards, which is why we call these viruses the retroviruses. Even more scary, the retroviruses then trick the cell into splicing that new viral DNA into the cell’s own genome

In short, these retroviruses fuse their virus genetic material with the cell’s non virus genetic material. They show no respect for the line that we would prefer to draw between “their” DNA and “our” DNA. Retroviruses re zone our genetic neighborhoods to make room for themselves and never leave. This is the world of microbes, where viruses can manipulate the DNA of animals for their own ends. In fact, as we’ll see, some Machiavellian microbes can even manipulate the minds of animals to make us do their own bidding.

Major Transitions in Evolution (L11 "The Egg Came First — Early Reptile Evolution")

Welcome to the lecture where we answer the age-old question of, “which came first the chicken or the egg?” by using evolution. As we’ll see, understanding why the egg came first turns out to be an excellent example of how a deep time perspective can answer many previously unsolvable mysteries.

Why was the development of enclosed eggs so important? The simple answer is that it opened more options for tetrapods to live their entire lives on land and reproduce on land. Furthermore, they could better protect their eggs when these were laid on land. For example, at the time egg-laying evolved, egg predation pressures on land were probably quite low. The only other tetrapods that could’ve eaten eggs would’ve been amphibians and, because amphibians were mostly restricted to the water, they would’ve had to work a whole lot harder to get up on land and find eggs. A fair number of insects today are egg predators, especially ants, and insects were certainly around during the Carboniferous period. But none of the insect groups known to prey on amniote eggs had evolved yet in the Carboniferous. In other words, terrestrial environments would’ve offered a clear adaptive advantage to any tetrapods that could cut themselves off from those water bodies. The development of extensive forests just before the evolution of amniotic eggs suggests that terrestrial ecosystems were likely a driving factor in amniote evolution.

This timing demonstrates yet another probable relationship between tetrapod evolution and land plants. First tetrapods themselves evolved into the shady forest that held so much food and shelter. Then, these forests perhaps became patchy and water bodies became more separated from one another. As that happened with global climate change during the Carboniferous period, natural selection would’ve favored those tetrapods that could still maintain body moisture and reproduce without shade or water. Once freed from the water, this new lineage of tetrapods can move into and evolve in a wide variety of environments on land. They could face new selection pressures that sorted out their genes in ways that produced the many new clades of anapsids, synapsids, and diapsids during the Permian and Triassic periods from about 300–200 million years ago. That’s also a broad answer to the chicken and egg question.

The first amniotes producing eggs date from at least 310 million years ago, possibly much earlier, while the first bird ancestor, as we will see, dates from about 150–145 million years ago. That puts the gap between eggs and birds at over 150 million years. That’s a span that also includes two major extinction events and that’s only about halfway to the chicken. From the first birds all the way to jungle fowl and then to the first actual chickens is a second almost equally enormous interval. This is deep time popping up in an almost mindboggling way. The first eggs predate the first chickens by about a third of a billion years. Think about that one and how prepared you’re going to be to answer the chicken-or-egg riddle next time it’s posed to you. .

Biochemistry and Molecular Biology: How Life Works (L18 "How Plants Make Carbs and Other Metabolytes")

Caffeine is biochemically a methylxanthine, and that's a purine compound, similar to the bases adenine and guanine in DNA and RNA. Indeed, caffeine is derived from the purine nucleotides.

Let's remember that nucleotides contain a base, in this case, adenine or guanine, a sugar, and one or more phosphates. Both adenosine monophosphate (AMP) and guanosine monophosphate (GMP) can give rise to a related molecule, xanthosine monophosphate, XMP. Now, XMP loses its phosphate group to become xanthosine, which contains the base xanthine and a ribose sugar. From there, 3 methyl groups CH3 are added, and the sugar is lost, giving us caffeine.

Caffeine binds adenosine receptors in the brain. This isn't terribly surprising, given that caffeine is structurally similar to adenosine. But what does that do? It turns out that adenosine normally accumulates during our waking hours and binds to those brain receptors.

Adenosine receptors filling up with adenosine is a signal to our body to slow down and rest. This is why you get sleepy after a long day. When you sleep, adenosine levels go down, emptying the receptors and eventually leading to wakefulness, except maybe on Monday morning.

Caffeine binds to those adenosine receptors, but it doesn't make you sleepy. And because adenosine can't bind when caffeine is occupying its receptors, you stay alert. If you drink coffee regularly, however, the brain compensates by making more adenosine receptors. And when that happens, you need to drink more coffee to fill up the adenosine receptors and stay awake compared to someone who didn't start drinking coffee. You may have noticed this...

Big History: The Big Bang, Life on Earth, and the Rise of Humanity (L48 "Humans in the Cosmos")

Early in this course I said that one way of thinking about big history might be to imagine a community of people, old and young, around a campfire. As I say this, I'm actually thinking of some wonderful holidays that my wife and I spent with our children and friends of ours who also had children. And we spent them at a lake in Australia, north of Sydney. Each evening we would build a fire, and because all children are natural pyromaniacs, we would sit around it till late at night, and they'd poke sticks into it. And eventually we could look up and see a wonderful starlit southern-hemisphere sky. I'm sure you can think of similar occasions. And it's not at all hard to imagine that most people perhaps throughout most of human history have had similar experiences.

Now, we can imagine that the youngest people in the group start asking questions, they start asking questions about the meaning of life and about where things come from. So they say: Why are there so many people in the world, for example, or, How big is the world really? And imagine that we try to give them the best and most intelligent answers we can. Our answers might take the form of the story we've been following throughout this course, but played in reverse. So here's how it might sound if I told this to an adult audience.

Let's begin with human history. Today's society is the largest and by far the most complex human society that has ever existed. Today there are more than 6 billion humans. And though they live in distinct societies with different states that are often in conflict with each other, all these communities are linked through trade, travel, and modern forms of communication into a single global community.

This modern global community, which is the world we live in today, was created very recently, just during the last 300 years. About 300 or 400 years ago, human beings, initially in some parts of the world and then eventually throughout the world, crossed a sort of threshold. Human societies became more interconnected, and as commerce was becoming more important than ever before, people in some regions began to innovate faster than ever before.

Now, these changes are often described as the Industrial Revolution. What they did was to lay the foundations for today's vast and rapidly changing societies by suddenly introducing a whole new wave of new ways of dealing with the environment and getting energy.

For several thousand years before this, most people had lived in the large, powerful communities that we describe as agrarian civilizations. They had cities with magnificent monumental architecture. They had powerful rulers. And these state systems were sustained by large populations of peasants who lived in the countryside and produced most of society's resources. Innovation was much slower than today, and that's why things tended to change much more slowly. The pace of history was slower. And there were far fewer people than today. Two thousand years ago, for example, there were probably only about 250 million people on Earth.

Where did the agrarian civilizations come from? Well, the first agrarian civilizations appeared in regions such as Mesopotamia, Egypt, and China. And they appeared about 5,000 years ago. During the previous 5,000 years, an increasing number of humans lived in small, relatively independent communities of small farmers that were governed by local chiefs. But there also existed many people who lived not by farming, but by foraging, that is to say, by gathering the resources they needed as they migrated through their territories.

Agriculture appeared about 10,000 or 11,000 years ago. Before the appearance of agriculture, all human beings were foragers. They all lived like foragers (or hunter-gatherers). What agriculture did was to greatly increase the amount of resources that human communities could extract from a given area. And the result was it led to rapid population growth, and eventually that led to the creation of the very large human communities that were the first agrarian civilizations. So that's why the appearance of agriculture counts as the most important threshold in history before the appearance of the modern world. For the 200,000 years or so before agriculture, all humans had lived as foragers. That means they lived in small, family-sized nomadic communities of perhaps less than 50 people for the most part, sometimes with links with their neighbors. For most of this time, there were very few humans on Earth probably little more than the numbers of great apes in the world today. So that's a period of about 200,000 years.

The first members of our species, Homo sapiens, appeared about 200,000 to 300,000 years ago. We don't know exactly when, but we're pretty sure they appeared somewhere in Africa. Their appearance counts as the most important threshold before the appearance of agriculture. What made these first humans different from all other animals and what accounted for their ability to explore so many different environments was their ability to exchange and store information about their environments. Humans could talk to each other. And they could exchange information with a speed and efficiency that no other animal could match. And that means they could store information. In addition, unlike any other animals, they could ask about the meaning of existence. Humans were probably storytellers from the very beginning of their history.

Now that's a brief history of how human societies developed into the remarkable global community of today. But how were the first human societies created? Humans after all are living organisms. So to explore the origin of humans, of our species, we must describe the history of living organisms. And that's the next stage in this story.

Our species evolved in the same way as all other species, by natural selection: Tiny changes in the average qualities of each community slowly accumulated over many generations, until the nature of each species slowly changes. And this is the process that created the huge variety of species today. Our ancestors evolved from highly intelligent bipedal ape-like ancestors known as hominines. The first hominines appeared about 6 million years before the appearance of Homo sapiens, our species.

The hominines, in turn, were descended from the mammals known as primates. They were related most closely of all to the great apes, such as the chimpanzees and gorillas. The primates as a group were tree-dwelling mammals, they had large brains, they had dexterous hands, and they had stereoscopic vision. These are all the things you need to live in trees. And as a group, they had appeared about 65 million years ago.

The primates were mammals. Mammals were a type of animal that had first evolved about 250 million years ago. The mammals, in turn, were descended from large creatures with backbones whose ancestors had learned to live on the land about 400 million years ago. All amphibians and reptiles are also descended from these ancestors.

Large multi-celled organisms, like ourselves, had only been around since about 600 million years ago. That's the first time, during the so-called Cambrian era, that you get very large animals made up of billions of individual cells. Before that, all living organisms on Earth were single-celled. Most would have been invisible to a human eye. The first living organisms on this planet seem to have appeared as early as 3.8 billion years ago. That's just 700 million years after the formation of our Earth. They were the remote ancestors of all living creatures on Earth today, including you and me.

What's remarkable is the speed with which they appeared, just 700 million years after the creation of our planet, and our early planet was not a very hospitable place for life. So the speed with which they appeared suggests that life is likely to appear in our Universe wherever the conditions are right, and that means wherever we find planets that are bathed in the light from nearby stars, but far enough away for liquid water to form, because water in liquid form provides an ideal environment for complex chemical reactions like those that formed living organisms.

The other crucial ingredient, of course, is a rich mixture of chemicals. So it seems that wherever you get a hospitable environment for life in our Universe, it's very probable that life will appear. So the formation of life itself as we move back in time is the next most important threshold before the appearance of our species, though we've also seen lots of minor thresholds as life evolved.

Life, in turn, was only possible where the conditions were right. So to understand the appearance of life, we need to understand the appearance of planets, of stars, and of chemical elements. That takes us to the history of geology and into astronomy.

Our Earth was formed about 4.5 billion years ago. And it was formed along with all the other planets, moons, and asteroids and comets of our solar system. All of them were formed as a byproduct of the processes that created our Sun. What happened was that debris, leftover bits and pieces, if you like, orbiting the Sun smashed together within the Earth's orbit and slowly accumulated into larger and larger lumps until eventually they all aggregated into a single large lump, which was our early planet.

The formation of our planet, therefore, is the next important threshold in our story as we move back in time. Now, how common was this process? Well, solar systems may have formed countless billions of times in the history of the Universe. But this happens to be the only solar system whose origins we know much about at present.

Our planet, like the living organisms that inhabit it, is made up of many different chemical elements. In fact, in our body, you can probably find traces of elements from across the periodic table. So neither our planet, neither our Earth, nor you and me could have been formed if the chemical elements had not been manufactured. They were manufactured in the violent death throes of large stars, in supernovae, or in the dying days of other stars. We don't know when the first stars died and scattered new elements into space. But it's very probable that it happened within 1 billion years of the creation of the Universe. So this new threshold, the creation of chemical elements, takes us back more than 12 billion years, close to the beginnings of the Universe.

Since then, billions upon billions of stars have died, scattering new elements into interstellar space. Now obviously, stars could not have died if stars had not been born. Stars were born ”like our Sun”as clouds of gas, clouds of matter collapsing under the pressure of gravity heated up in their centers until eventually hydrogen began to fuse, and at that point they lit up. They turned into stars. Star formation has continued ever since the first stars appeared” that's why, as I said in an earlier lecture, there are probably more stars in the Universe than there are grains of sand on all the beaches and deserts of our Earth.

The first stars may have been formed more than 13 billion years ago quite soon, within 200 or 300 million years of the origins of our Universe. So as we move back in time, the creation of stars counts as one more great turning point in the story. So we've looked at human societies, life, the creation of planets and stars. And that takes us back to the beginning.

Obviously nothing could have existed if the Universe itself had not been created. The Universe, we now know, was created about 13.7 billion years ago. Stars were formed from great clouds of hydrogen and helium atoms which, like the force of gravity itself, were created at the moment of the creation of our Universe.

This is the first turning point of all, the first threshold of all and in many ways it's the most mysterious. Our Universe began as a tiny, hot, expanding ball of something that popped out of nothingness like an explosion and the explosion, which cosmologists call the big bang, has continued ever since. You and I and the planet we live on are simply part of the debris. That's the very beginning of the story. And we can't go further back in time.

So that's a quick summary of the story we've told in this course. The story I've just told is a highly condensed summary of the best modern scientific attempts to understand origins, to understand how everything around us was created. It's our best shot at explaining origins, just as every traditional creation story also represented the best attempt, given the available knowledge, to answer all the fundamental questions about origins.

Outsmart Yourself: Brain-Based Strategies to a Better You (L6 "The Myth of Multitasking")

It’s often said that the greatest power of the human brain is that it can perform many different processes in parallel. You open your eyes, and your brain processes incoming visual information—you don’t have to choose to do so. While you’re at it, you also touch, hear, taste, and smell. You do all of these things at the same time, in parallel—the processing just happens.

The same thing seems to happen for many actions that we perform. You can walk across the room while searching your pockets for your keys, while also having a conversation, while also pausing to say hello to a friend as they pass by. You do all of these things while, of course, continuing to see, touch, taste, hear, and smell. The human brain has billions of neurons, hundreds of thousands of circuits, and they can process information in parallel. It’s an amazing thing. Modern computers are, in some respects, much faster and more accurate than the human brain in terms of sequential operations, but those artificial computers are just starting to scratch the surface of this amazing parallel processing thing that the human brain performs.

In this lecture, I’m going to argue that you should try to limit the number of things that you try to do at the same time. My primary tip will be to explore the thrill of doing one thing at a time—single tasking or monotasking as it’s sometimes called. I’ll also argue that there are hidden costs to so-called multitasking, both short- and long-term problems that emerge when you try to do more than one thing at a time.

We often engage in one primary task—say, writing something—while also engaging in a secondary task and a tertiary task as well. For instance, I might answer the phone when it rings and talk with a coworker. My computer periodically makes a beep, indicating that an e-mail has arrived. A little pop-up window appears, indicating who the e-mail is from and what the subject of the message is. So, I’m also monitoring this incoming information and making decisions about whether or not I should stop writing and respond to it. So, it’s a good thing that the human brain is so good at multitasking, because our modern world demands it. On the surface, especially, it seems far more efficient to do multiple things at once. To not do so would be to waste our natural ability. All we need to do is develop the requisite expertise, and perhaps we can do the work of two or more people. Thank you, technology.

There’s a problem, however. We feel like we can do multiple tasks at the same time. There’s actually a feeling of pleasure that many people describe associated with multitasking. It can be invigorating to push your mind and body up near its maximum capacity for processing information. The problem is, when we carefully assess people’s performance during multitasking, significant reductions in performance are found. In some cases, the drops in performance can be really big. The drop in performance is bad, but perhaps the most troubling thing is how unaware we are of the drop. We can feel like we’re doing our best work while actually performing pretty badly.

Think about the typical task of writing something while also monitoring your incoming e-mail. You’re thinking about the topic of your writing, thinking about the global structure of the document you’re composing, you’re thinking about what to say next, and then composing that next sentence. This is a pretty engaging task that pulls from a variety of different brain resources. While you’re doing that, there is that telltale bong from your computer that means an e-mail has arrived. You feel like you’re continuing to write while you glance up and read that message. You decide the e-mail can wait and continue writing. It feels like you’re doing those two things at the same time, but what you actually do is stop the thought processes that go with writing, you switch to thinking about the e-mail, and then return to the writing. That switch takes time, and it turns out it requires a substantial amount of brain resources to accomplish.

Perhaps the most publicized application of this research in the real world has come in the domain of driving while using a cell phone. I hopefully don’t have to tell you that it’s a bad thing to drive while texting or talking on a phone, but, just in case, it is a bad thing to do. The extra risk of being involved in a car accident associated with using a cell phone while driving is even a little larger than the risk associated with driving while legally intoxicated. If you wouldn’t drive drunk, you should certainly not drive while using your phone.

How does the cell phone create this problem? There are some obvious things that occur to most people. When you’re reading and typing text messages on a phone, you have to look away from the road, at least for a few seconds at a time, right? If something happens up in front of your car during those few seconds that you’re looking at your phone and not looking at the road, there’s no way you can react. If the driver in the car in front of you slams on his brakes, you won’t even start to react until you look back and see that car’s brake lights. Even when you talk on a handheld cellular phone, you usually have to look down to dial the number or select the contact and hit send.

Phone and carmakers have addressed this problem; they’ve created hands-free cell phones. Problem solved, right? Unfortunately, no.

It is true that a fully hands-free system can enable you to keep your eyes on the road the whole time, but several studies have found that the increased accident rate stays almost as high with hands-free cell phones as it is with handheld cell phones. What’s going on here? Why should the hands-free phone be almost as bad, even when your eyes can stay forward at all times? The problem? Multitasking.

The experiments we’ve discussed here apply very specifically to this situation, in at least two important ways. First, when it comes to processing sensory information and making a discrete decision about it, the human brain is limited to one decision at a time. No matter how expert you are at the other pieces of performing the task, the decision part remains a single-task bottleneck. When you’re pondering the statements of someone on the other end of a phone or text exchange, you’re making a lot of decisions. Should I respond now or keep listening? What should I say? Should I mention our last conversation or not? Lots and lots of decisions.

And every time you’re making one of those decisions, you’re not able to make visuomotor action decisions about driving the car. Should I hit the brakes? Should I change lanes? Those decisions have to wait until the bottleneck is freed up. We get very good at alternating between two or more tasks, but the switching always introduces a little delay. And at 60 or 70 miles per hour, a little delay can translate into the difference between avoiding a collision and having an accident.

NOTE: This excerpt is part of a terrific example of over-learning that feels fun and empowering for the entire field, thanks to intensely well-planned use of visuals throughout, including appropriate photographs, highlighting, graphics, and on-camera demonstrations.

Understanding the World's Greatest Structures from Antiquity to Modernity (L24 "Strategies for Understanding Any Structure"

Now that you’ve learned all the major principles of structural mechanics and examined many of the world’s greatest structures in terms of those principles, you should be able to analyze any structure you come across—great or humble—just as we have in this course. In this lecture, you have the chance to test your newfound analytical skills as we look at one last group of great structures.

Perhaps the most straightforward approach to understanding a new structure is by direct comparison with structures you’ve already seen.

For example, the dome of the U.S. Capitol can be understood through direct comparison with the dome of St. Paul’s Cathedral in London. Like Saint Paul’s, the Capitol dome uses a 3-part configuration: nonstructural outer and inner shells concealing a parabolic structural dome. Both work the same way; the only significant difference is that St. Paul’s structural dome is brick, while the Capitol’s structural dome is an open iron framework. In this difference, we can see the influence of the iron-framed dome of the Paris Grain Market, built about 100 years after St. Paul’s and about 50 years before the Capitol.

Similarly, when we encounter the spectacular new Tokyo Sky Tree, we should recognize it as a descendant of the Eiffel Tower. The Sky Tree, at 2080 ft, is the world’s tallest tower. Its overall shape and truss construction reflect the same response to wind load that Eiffel used in Paris, but the Sky Tree goes further, with a reinforced concrete core and a uniquely varying cross-section: triangular at the base, for stability, transitioning to circular at the top, for decreased wind resistance.

But what happens when you encounter structures in unfamiliar categories? Perhaps you can draw analogies with different types of structures that nonetheless carry load in the same way.

For example, we haven’t discussed dams in this course, yet you can see at a glance that the Hoover Dam is just an arch turned sideways, holding back a wall of water the same way that the Pont-Saint-Martin’s arch carries the weight of the stone above it.

What about tunnels? I hope you can now see the Chunnel, that 31-mile tunnel linking England and France, as just another interesting variation on the arch. Completed in 1994, the Chunnel actually consists of three passages: two rail tunnels and a service tunnel. All three were excavated with immense tunnel boring machines, shown here.... The tunnel lining consists of arc-shaped precast concrete segments placed around the entire perimeter of the excavation, as shown here. If you’re thinking that these segments look a lot like the stone voussoirs of a Roman arch, congratulations: You are learning to see and understand structure. The tunnel lining of the Chunnel works exactly like an arch, except its principal loading is soil pressure, which is exerted inward around the entire perimeter of the tunnel lining, like this; and so the tunnel lining needs to be a full circle, rather than the semicircle of a traditional arch.

In that sense, the Chunnel is also quite similar to the Treasury of Atreus, that ancient Mycenaean corbelled dome, built underground with its circular layers of stone held in place by soil pressure.

Sometimes you’ll encounter a structure that superficially resembles one of our categories but is actually something else entirely. For example, the Qiancheng Bridge in China’s Fujian Province is a rainbow bridge, a style that dates to the 11th century A.D. Most people refer to these structures as arch bridges, but the structural system isn’t really an arch: there’s no lateral support at its base. In fact, it’s a rigid frame that gets its rigidity from this interweaving of transverse and longitudinal elements. Its members carry load in bending and axial compression combined, rather than in compression like an arch....

In fact, it is a frame, similar to the rigid frames that we saw in iron- and steel-framed buildings. It doesn’t look the same, because the modern rigid frames we examined in Lecture 18 get their rigidity from specially designed connections. The rainbow bridge gets its rigidity from this interweaving of transverse and longitudinal elements. But, like a modern rigid frame, its members carry load in bending and axial compression combined, rather than just in compression as an arch does.

By the way, the frame bridge is not just an ancient Chinese technology. Here’s a typical modern example: the Fahy Bridge over the Lehigh River in Bethlehem, Pennsylvania. Because we already know how rigid frames work in buildings, we can understand this analogous structure without too much difficulty

When you encounter a structure for which there aren’t any obvious analogies, you can return to the technique of analyzing the structural system we discussed in Lecture 9....

That said, you can gain many fascinating and rewarding insights about structures without any sort of formal analysis. Structures often communicate with us in clear and compelling ways simply through the shapes and proportions of their elements.

The 12 towers of London’s Millennium Dome tell us that they carry load in compression by their stout proportions and the orientations of the attached stay-cables. The array of cables radiating out from the towers tells us that they carry tension by virtue of their slender proportions.

We might call this the “language of structure,” a language that structural elements use to tell us how they work. If we can read the language of structure, if we can discern how members carry load based solely on their shapes and proportions, we don’t need to do a formal structural analysis; the members themselves tell us that the Millennium Dome, for example, is a cable-stayed building.

So my final recommended strategy for seeing and understanding structure is to learn to read the language of structure; to see the shapes and proportions of structural elements as subtle messages about their load-carrying purpose. In this language, there is no more interesting bit of structural vocabulary than the parabola.

There is something very special about the parabola, and its cousin, the catenary. In this course, we’ve seen that the parabola is a direct reflection of the underlying science that governs the behavior of many different types of structural elements. It’s the natural shape of a draped cable; it’s the shape of the thrust line in an arch; it’s the shape of the moment diagram for a uniformly loaded beam. So when we see the parabolic form in a structure, we’re almost always looking at an element that was optimally designed for load-carrying. We’ve seen the parabola in the main cables of great suspension bridges; in the vaulting of the Persian imperial palace at Ctesiphon; and in arches, from Eiffel’s Garabit Viaduct to Calatrava’s Campo Volantin Bridge in Bilbao.

But the parabola is found in more than just cables and arches. It’s also in the profile of optimally designed beams, like the graceful box-girders of the Raftsundet Bridge in Norway; and even in trusses, like Brunel’s Royal Albert Bridge at Saltash, a particularly rich example because we get two parabolas for the price of one: a top chord that works like an arch, and a bottom chord that works like a cable.

Now that you understand why parabolas are used in so many different kinds of structural applications, you’re going to start noticing them in all sorts of unexpected places.

For example, you’ll occasionally encounter them in the facades of buildings, as you can see in Marquette Plaza, the former Federal Reserve Bank Building in Minneapolis. Here, that parabolic shape on the facade corresponds to a steel tension element that works like a draped cable to support the upper floors of the building, allowing an open, column-free first-floor lobby.

Here’s that very same concept applied in reverse: the Broadgate Exchange House in London. That huge parabolic arch carries the weight of the building in compression; and because it’s supported only out at its ends, the arch system allowed the building to be constructed directly over a set of train tracks running below ground level. The tower of the cablestayed Denver Millennium Bridge is a compression member with a parabolic profile; and the tower of the Reggio Emilia Bridge in Italy is an immense parabolic tube.

I could provide hundreds more examples like these, but that might spoil all of your fun. Now that you know how to look for the characteristic shapes and proportions of the language of structure, you’re going to be amazed at what you find,

Let’s conclude this course with a look at my personal favorite structure, Pier Nervi’s Palazzetto dello Sport in Rome, built in 1957...