One question. Why is there such a stunning diversity of life? One answer. Evolution.
Charles Darwin's brilliant theory that explains how species adapt and change. It's been called the best idea anyone ever had. But there's one big problem. The problem.
How does it actually work? Now extraordinary science is answering that question. It is uncovering the hidden mechanisms inside creatures'bodies that can explain astonishing transformations. Like how birds can evolve from dinosaurs.
Why a fish was once your ancestor. And above all, what makes us human. Right now on NOVA, you'll find out what Darwin never knew.
Major funding for NOVA is provided by the following. The tree of life on Earth is one of stunning diversity. 9,000 species of birds. 350,000 kinds of beetles.
28,000 types of fish. 2 million living species. And counting.
And we are just one of them. Music. But why is there such an amazing variety of animals?
Why are there so many types of fish? So many different species of beetle? How did this extraordinary profusion of life on Earth come about?
Today, we celebrate the man who would ultimately answer that question. Charles Darwin. He was born 200 years ago and it is 150 years since he published the work that has become the bedrock of our understanding of life on earth.
What Darwin wanted to understand was how you get this extraordinary diversity of life on earth. He was spot-on. He really nailed it. Darwin's theory of evolution, his account of why species adapt and change, has been called the best idea anyone ever had. But even Darwin admitted that his work was incomplete.
Vast questions were still unanswered. And the biggest question was how. How did evolution take place? He didn't know any of the mechanics of that process. He didn't understand the physical forces that would actually change the way species appeared.
But today, we can answer the questions that Darwin could not. We can look under the hood of evolution and see exactly how this mysterious process gives rise to such astounding diversity. What's incredible about this time and from a scientific perspective is we're going to be able to understand that diversity.
And that just adds to the excitement. It doesn't demystify it, it makes it all the more magical. And this is the magic and mystery of evolution. Over eons of time, a single species gives rise to many. An ancient fish evolves to become the ancestor of all four-limbed animals, even us.
And one species, our own, divides us. ...develops a large and uniquely complex brain, enabling us to dominate the planet. This is the search for the answers to what Darwin never knew.
Darwin began his love affair with nature when he was a child. Just like many of his modern followers, including evolutionary biologist Sean Carroll. I developed my interest in animals the same way I think most biologists did, which is either going out in the backyard or going to zoos.
Anytime I got a chance, I'd flip over logs and look for salamanders and snakes and frogs and things like this, and I was just fascinated with their patterns. and behavior. So it was with the young Charles Darwin. Young Charles liked the traits around the outdoors.
He loved to collect beetles and things. He was a completely ordinary kid, and he didn't like school. In fact, he was such a poor student that his father, a rather successful physician and a pretty imposing figure, was worried about Darwin's direction in life. So his father packed him off to Edinburgh, the finest medical school in Europe, to become a doctor.
But young Charles was just too squeamish. And he's really horrified by medical school. He witnessed an operation on a child, and this is in the era before anesthetics. And he just fled the operating theater, vowing never to return. Next, his father sent him to Cambridge to study for the clergy.
He didn't succeed at that either. But he did find his direction in life, reviving his childhood interest in nature. Darwin starts on his path to his divinity degree and he starts to mature as a student. He becomes more serious about some subjects, particularly natural history, and he learns a lot more about botany and about geology and these things.
He's becoming a pretty solid field scientist. His reputation as a naturalist gained him a spectacular invitation. Charles was offered a place on the British Navy ship, the HMS Beagle, whose mission was to survey the waters around South America. Now, the captain of the Beagle wanted a well-educated scientific person aboard and a dinner companion, somebody to share a conversation with.
And Darwin fit the bill perfectly. And so, Charles Darwin set off on a fateful voyage that would revolutionize our understanding of life's great diversity. The voyage of the Beagle took nearly five years. It wove its way from the Cape Verde Islands and along the coast of Brazil.
It was in Argentina that he made his first important discovery. Early on in the voyage, Darwin found some amazing fossils. He dug up some skulls, some jaws, some backbones of what turned out to be giant mammals.
Now, these were clearly extinct. And Darwin began to ponder what was the relationship of those fossils to the living animals of South America. Music One port of call on Darwin's voyage proved more important than all the others.
The Galapagos. This cluster of 13 isolated islands lies 600 miles off the coast of Ecuador in the Pacific Ocean. These islands are home to unusual animals found nowhere else on Earth.
Penguins that live at the equator and swim in warm water instead of the frigid seas of the South Pole. Giant tortoises that weigh up to 600 pounds. Iguanas, huge lizards that swim and dive in the sea.
Everywhere else, they dwell only on land. Traveling for the first time in the Galapagos, Sean Carroll is seeing the same creatures that so intrigued him. Darwin. Of all animals, I think these marine iguanas are the greatest symbol of the Galapagos, what I most wanted to see here.
And to see them in their native habitat blending against that black rock, just as Darwin described it, it's an absolute thrill. It's a hideous looking creature, of a dirty black color, stupid and sluggish in its movements. They are as black as the porous rocks over which they crawl. Darwin meticulously described the iguanas in his diary.
But he was far from the scientific authority he would become. The Darwin that arrived here was not the great theorist that we know today. He was a 26 year old collector, collecting really almost at random any kind of plants, any kind of animals, any kind of rocks.
He didn't even know the meaning of what he was collecting until much later. He was also fascinated by the giant tortoises, which allowed him to ride on their backs as they slowly lumbered around. I frequently got on their backs, and then, upon giving a few raps on the behind a part of the shell, they would rise up and walk away. But I found it very difficult to keep my balance.
Darwin measured the creature's extreme slowness, about four miles a day, he calculated. But the local people knew something else about the tortoises. They could tell which island any tortoise came from just by looking at its shell. Their shells differed depending on which island they lived on.
Some tortoises had shells shaped like a dome. Others had shells arcing over their heads like a saddle. Others differed subtly in color. or by how much the bottom of the shell flared out. Darwin had literally been sitting on a clue, a way to understand the great diversity of life.
But he didn't yet realize it. Instead, Darwin turned his attention to birds. The islands were full of what seemed to be a familiar assortment of species.
So he stuffed his collecting bag with what he thought were types of finches, grosbeaks, wrens and blackbirds. And then, after five weeks in the Galapagos, Darwin and the Beagle went to other ports in the Pacific and finally set sail for home. On board, he started to sort through the vast number of specimens he had collected on the five-year voyage.
But it was not until he returned to Britain that he was able to make sense of them. It began with a startling revelation. All the different birds he had collected actually were variations of a single type.
He learns that those birds he had collected on the Galapagos actually represent 13 different species of finch. What misled Darwin was that they looked radically different. Some had wide, tough beaks. Others had long, slender ones. And these differences depended on which islands they lived on.
Now why would that be? Why would there be slightly different birds, slightly different species, on different islands, all in one part of the world? Darwin now thought back to the Galapagos tortoises.
They too differed from island to island. His brain began racing. Thoughts are starting to crystallize, take shape in his mind, bit by bit, bit by bit.
He starts this process he describes as mental rioting. Just stream of consciousness. Where he's jotting down note after note after note.
Thoughts as they occur to him. And finally, they converge on this one idea. What Darwin now realized was that somehow, for some reason... Species change. Originally there must have been just one type of finch in the Galapagos.
But over time it had diversified into many kinds, with different beak shapes. The same for the tortoises. One type of tortoise must have turned into many kinds, with different shells, depending on which island they lived on.
With this great insight, Darwin entered dangerous new territory. The standard view at the time was that God had created every species, and that what God had created was perfect and could not change. But Darwin said, no, why would the creator bother with making slightly different finches for each of these different islands that all looked alike? The prevailing view just didn't make sense.
But this was only the beginning of Darwin's revolution. He turned his attention to the fossils he had collected in South America. One was of a giant sloth. Another was of a huge armadillo-like creature.
These animals were extinct. But little sloths still existed in South America. And so did smaller armadillos.
What could this mean? It dawned on him that they resembled each other. So what he had found in the ground were the buried ancestors of the living animals of South America.
So again, here was more evidence that species changed. Somehow these ancient giants must have been transformed into the smaller creatures we see today. But what Darwin would later find out took this idea of how species change into a completely new league. In Victorian times, scientists routinely studied life forms at the embryonic stage.
How these tiny forms develop from just a single cell into an entire creature. has long been seen as one of the wonders of nature. Watching a developing embryo is truly the most glorious miracle of nature. I mean, you know, no baloney. What Darwin learned from studying the embryos amazed him.
In snake embryos, you could see tiny bumps. The bony rudiments of legs. But these would never develop in the adult snake. Darwin wondered, were snakes somehow descended from animals with legs? He learned that whales, which have no teeth as adults, had them as embryos.
Those teeth disappeared before they were born. To Darwin, it had to mean whales were descended from creatures with teeth. But human embryos provided the most startling evidence. Under the microscope, tiny slits around the neck were clearly visible. Exactly the same structures were found in fish.
But in fish, they turned into gills. In humans, they became the bones of our inner ear. Surely, this showed that humans must be descended from fish.
It's an astonishing thought. I don't know about your ancestors, but mine included priests and, you know, the usual suspects. But the idea that all of us have fish in our family tree, I think it's amazing.
And so Darwin arrived at an astonishing conclusion, one that would become central to his understanding of the great diversity of life. Darwin had this amazingly bold idea.