Mostly because the answers tend to get hung up on one trait that differs from our closest great ape relatives: our upright stance, the shape of our toes, the size of our brains. Millions of years of separate evolution has, of course, resulted in considerable divergence in all manner of traits. It makes no sense to elevate one particular one to some kind of special status - the one thing that makes humans … human.
Questions about what separates us from other animals also carry some unfortunate baggage. The belief that there is something inherently special about humans and the way we arose is more suited to creation mythologies and religious doctrine than to a scientific, testable view of the world.
The notion of special creation, and those perversions of evolutionary thinking that defend humans as exceptions, tend to come pre-fitted with taxonomic chauvinism of the following sort :.
Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
The shard of evolutionary thought that most gets under the skin of religion is the notion that humans are not created special, are not made in any divine image. Owen lectured the British Association in that human brains bore special structures - such as the hippocampus minor - lacking in apes. Here, Owen claimed, lay the evidence for human uniqueness. 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.
He witnesses an operation on a child—and this was in the era before anaesthetics—and he fled the operating theater, vowing never to return. He didn't succeed at that either, but he did find his direction in life, reviving his childhood interest in nature. 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. 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 conversation with.
And Darwin fit the bill perfectly. 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.
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 mammals of South America? This cluster of 13 isolated islands lies 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 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 Darwin. And to see them in their native habitat, blending against that black rock, just as Darwin described it, is an absolute thrill. They are as black as the porous rocks over which they crawl. He was a year-old collector, collecting, really, almost at random, any kind of plants, any kind of animals, any kinds 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. But I found it very difficult to keep my balance.
But the local people knew something else about the tortoises. 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. And these differences depended on which islands they lived on.
Why would there be slightly different birds, slightly different species, on different islands, all in one part of the world? They too differed from island to island.
His brain began racing. He starts this process he describes as "mental rioting," just a stream of consciousness where he's jotting down—note after note after note—thoughts as they occur to him.
Originally, there must have been just one type of finch on 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. Why would the Creator bother with making slightly different finches for each of these different islands that all looked alike? 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? So, again, here was more evidence that species changed. Somehow these ancient giants must have been transformed into the smaller creatures we see today.
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. 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? 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. But, but the idea that all of us have, have fish in our family tree, I think it's amazing. And what it meant was, if you go far enough back in our family tree of humans, you'll come to fish. If you go far enough back in the family tree of birds, you'll come to dinosaurs. So that creatures that don't look anything at all like each other are actually deeply connected.
No one came close to having this idea before Darwin. Beginning with a common ancestor, over time, across generations, species could change dramatically. Some might add new body features, others might drop them.
Ultimately one type of creature could be transformed into something utterly different. It's a process Darwin called "descent with modification. But it all begged a question: why? What was making creatures change? Darwin needed clues. And he found them in a very surprising place. That love affair still continues today, especially among scientists like Heidi Parker at the National Institutes of Health.
We have sizes that range from something the size of a groundhog, up to dogs like Zeppie, here, who can get to be the size of mule deer. Through a careful process of selection, dog breeders mixed different dogs with different physical traits to create new forms. They could select for individual traits, such as size or shape, and they could actually change the look of their breed.
It was created by mixing greyhounds for speed, with terriers, used to hunt small game. And then it hit Darwin. Was there a similar form of selection going on in nature, but without human interference? Could natural selection explain the great diversity of life? He took something very familiar and comfortable, for example, animal breeding, and explained that the same sort of thing was going on in nature, just at a little bit different pace and with no human guide.
It was then that Darwin took a completely fresh look at nature. The Victorian view of nature was sentimental—lambs lay down with lions—but Darwin's travels on the Beagle led him to a different view.
For Darwin, nature was savage. Every creature was locked in a desperate struggle for survival, ultimately ending in death. And sometimes it's not just there's a lot of death, but it's very unpleasant death. And this brutal battle, this war of nature as Darwin described it, was actually a creative process. For instance, some could handle extremes of climate.
Others were brilliantly honed killing machines, perfect for catching the available prey. Still others were perfect to evade those who might be hunting them. But how did this harsh view of nature explain the finches on the Galapagos, where Darwin observed that that the birds on different islands had different beak shapes? Somehow those different beaks must be helping the finches survive.
And there's a reason for that; they use their beaks as tools. Now, if you think of the type of tool you would want to crush a seed that's very tough, but is the food that you really like, you'd want a beak like this, which is the type of beak the ground finch has. But on another island, the available food isn't seeds but flowers. CLIFF TABIN: On the other hand, if you wanted to get into narrow spaces to get pollen and nectar, that are very hard to get at, you wouldn't need a big, strong beak, you'd need a probing beak.
And this pattern was repeated across the Galapagos. It seems that the finches' beaks had altered to fit the diet of each particular island. And that was how one original type of finch had been transformed into many. But how had these changes come about? Here Darwin had another clue. He could see it in his own family. As every parent knows, no two children are ever exactly the same.
Charles looked different from his brother Erasmus, even though they shared the same parents. Charles's children looked a bit like him and his wife Emma, but they, too, looked different from each other. That was something he called "variation. In any generation, the animals in a litter are never quite the same. And in the wild, such a tiny variation might make all the difference between life and death.
Two penguins, for instance, might differ a tiny bit in the thickness of their blubber, a big factor if you live in extreme cold. In a harsh climate, the environment will select who will live and who will die. And slowly, Darwin suggested, over many, many generations, these tiny variations would allow the fit to get fitter, and the unfit would vanish. These variations accumulate and eventually new species branch off. This is evolution by natural selection. It is one of the keys to how new species are formed.
And so, in , after years of painstaking research, Darwin finally published his masterwork, On the Origin of Species. It is still impossible to overstate its importance. It shook people up, it changed the way people thought. In its place, Darwin provided proper scientific theory, based on facts and observation. It's a vision of how evolution by natural selection works. His ideas largely stay intact today. He didn't actually know how it worked.
What was happening inside a creature's body that makes it change? But now, at last, modern science is providing the answers, through a hidden mechanism that Darwin knew nothing about. They really are. They're probably eaten by foxes and coyotes and rattlesnakes and owls. Its best hope for survival is camouflage. Not surprisingly, its fur matches the color of the Pinacate rocks. But in some sections of the desert, the environment is different.
Ancient volcanoes erupted, and now the desert is a patchwork of dark lava and light rock. But of course a light mouse on dark rock is easy pickings. So something has happened that Darwin might have predicted. The mice now living on the dark rocks have evolved darker fur. Those that stayed on the light rocks remain light. Nachmann was fascinated. How had this happened? To find out he first needed to catch some mice. So, with Sean Carroll, he visits a line of traps he set the previous night.
But Nachmann can now do something that Darwin never could; he can look inside the animals' D. It has taken our understanding of how creatures evolve and develop to a level that Darwin could never have dreamed of. It's a perfect system for storing the vast amounts of information that's necessary for building all kinds of creatures.
That helix is, in turn, made up of four smaller molecules, called by the letters G, A, T and C. Lined along each D. Many genes get translated into proteins, and these proteins make the stuff of our bodies. One protein makes hair; another makes cartilage; others make muscle.
Another gives us freckles. Another gives us dimples. But D. When a baby is conceived, the fertilized egg receives half its D. It's why we look a bit like our parents, but also different. Without mutation, everything would stay constant, generation after generation. Mutation generates variation, differences between individuals.
An A, for instance, can be replaced by a G or a C by a T. This can cause minute changes that no one is even aware of. But when mutations occur in the cells we pass down to our children, they can cause big changes, like turning a light-colored mouse dark. Well, mutations are neither good or bad. Whether they are favored, or whether they are rejected, or whether they're just neutral, depends upon the conditions an organism finds itself in.
So, for the pocket mouse, a mutation that caused the mouse to turn black, that is good if they're, you're living on black rock.
It's bad if you're living out in the sandy desert. Back in the lab he began the painstaking business of comparing the genes of the two types of mice, trying to pinpoint any differences. But then, in one gene, he found something. There were four places where the sequence of As, Ts, Cs and Gs were different.
When a mouse is born with these mutations, its fur grows dark. And that means it can survive on the dark rocks when others would not. Here was a clear example of evolution and natural selection at work. Scientists can now pinpoint a range of examples of evolution in action. The Colobus monkey can see in color because of a mutation in one gene; it can now tell nutritious red leaves from tough old green ones.
A genetic glitch gave this Antarctic fish a potent antifreeze in its blood, so it can survive in the icy waters when others cannot. So powerful was this link between genetic mutation and evolution that an idea took hold: to understand how evolution works, all you need to do is compare creatures' genes. Identify all the genes, identify all the differences, and you could explain the differences between, say, a mouse, and monkeys and humans. All three billion letters of our D. In parallel, the D.
Surely this would be a quantum leap in our understanding of how different life forms evolved? With this came another idea: that complex animals like us would have many more genes than simpler ones. You might think that would require a whole lot more genetic information. Just how big would our genome be compared to other life forms? It's down to something like or 23, protein-coding genes in a human genome. The simple nematode worm has about that same number.
And there are plants that have considerably more genes than the glorious human genome. Huge though the breakthrough had been, the genetic revolution had opened up a whole new set of puzzles. As a solution to the mystery of how evolution works, genes and their mutations were only part of the story.
There had to be something else, more subtle and more mysterious going on. The first tantalizing clues would come from those life forms that Darwin himself had studied, embryos. Look at these embryos. It is almost impossible to tell, just days after conception, which is the chicken, the turtle, the bat, the human. They look almost the same. Only as they grow, does it become clear which is which.
Darwin wondered, as scientists do today: how could they start out so similar and end up so different? And we have rediscovered what Darwin was talking about all along, that the embryo is where the action is, in terms of animal diversity.
It is the platform for diversity. NARRATOR: What fascinates modern biologists is that all these different animals don't just look the same, they are using virtually the same set of key genes to build their bodies.
The body-plan genes determine where the head goes; where the limbs go, and what form they take: whether they are arms, legs or wings. It is the same genes at work in every creature from the leopard to the peacock to the fruit fly, and yet they produce radically different results.
This has led scientists to a crucial insight about how animal bodies have evolved. It's not the number of genes that counts. They're large, expensive and then reproduce very slowly. To get data, we have to find the simplest examples of the phenomenon we want to understand. This fruit fly is dancing for sex.
A rapt female takes in the show. She's particularly besotted by the dark spots on the male's wings. The males of this species does a rather elaborate courtship dance where he displays these spotted wings in front of the female. Even in the remarkably brief chapter of the Origin in which he recruited the fossil record to his cause, Darwin was dubious:.
For Darwin was always ready to acknowledge what a seminal event his discovery during the Beagle voyage of the amazing South American fossil glyptodonts had been for him. And finding these extinct beasts in the very same place as surviving members of their family—something that implied the replacement of faunas by related ones—was a revelation to Darwin:.
Darwin in F. Darwin , p. Still, although his geological observations had made Darwin acutely aware of the transitory nature of everything he saw around him, he clearly felt very acutely the inadequacies of the fossil record for determining specific events.
But the Great Chain of Being, the idea that all living things were ranged in graded series, was nonetheless part of the ethos that suffused English society, and it was a notion from which Darwin found it difficult to disengage himself entirely. For it was not only a religious concept with a succession of forms leading from the most lowly pond scum, through mankind, the highest Earthly form, on up to the Angels and God above.
It had political and social dimensions as well. It is well-established that, long before he published On the Origin of Species , Darwin was fully aware that his theory firmly placed our species Homo sapiens as simply another product of the evolutionary process, among literally millions of others.
So, while the effective absence of a hominid fossil record before he published the Origin may have meant that Darwin could not have made extensive reference to it there if he had wanted to, we still need to ask if there are reasons beyond the admittedly powerful sociopolitical ones why he more or less ignored it in the post-Neanderthal times of The Descent of Man.
One reason for such neglect is, of course, the very specific monogenist agenda that Darwin was pursuing in that work. However, since many of the osteological differences between Homo neanderthalensis and Homo sapiens only emerge later in development, it is fully understandable that Huxley like everyone else at the time did not recognize it as such.
And in any event, Huxley basically ignored it. The other Engis cranium was adult, and it was on a plaster cast of this specimen that Huxley based his analysis. The Engis adult clearly is a Homo sapiens and it is now known to represent a later burial into the Neanderthal deposits at the site—which means it is younger than those deposits. He recognized this cranium as that of a fully modern person, concluding that it:. He then continued to the Neanderthal skull, an altogether more interesting specimen, and to which he devoted much greater space.
Initially, he quoted extensively from Schaaffhausen who had declared that the bones:. Huxley finally proceeded to a detailed examination of the Neanderthal skullcap, again based on a plaster cast. He was amazed by the differences between the cranial contours of the Neanderthal and Engis crania, but he noted that:. Having established this philosophical baseline, Huxley proceeded to a long dissertation about variation in human skulls, eventually concluding that the key to comparison among them was provided by the basicranial axis, a line between certain points on the internal base of the skull:.
In which case:. Numerous observations lead me to believe that we must answer this question in the affirmative. One might object at this point that the basicranial axis had no relevance whatever to the Feldhofer Neanderthal, a specimen that totally lacked a skull base.
The important thing here, though, was that Huxley had managed to establish a graded series. And by superimposing the profile of the Neanderthaler onto an Australian skull, he contrived to convince himself that:. Yet, at the same time, the Neanderthal skullcap had held a large brain—larger, indeed, than the modern average.
As a result, he concluded that:. By this intellectual sleight of hand, Huxley dismissed the Neanderthal find as a mere savage Homo sapiens , essentially robbing the slender human fossil record then known of any potential human precursor.
Instead, in a move that was as radical in its own way as the alternative would have been, Huxley pushed the antiquity of the species Homo sapiens back into the remotest past and was moved to ask:. Was the oldest Homo sapiens pliocene or miocene, or yet more ancient? In still older strata do the fossilized bones of an ape more anthropoid, or a Man more pithecoid, than any yet known await the researches of some unborn palaeontologist?
Taken overall, this rather startling conclusion was not just a major shift away from the demonstrable morphology of the Neanderthal specimen—which in the same year had been branded a distinct species, Homo neanderthalensis , by the Dublin anatomist William King. It was also a considerable reversal of perspective for one who had been a convinced saltationist.
After all, when reviewing On the Origin of Species , Huxley had been moved to observe that:. In a very real sense, then, it is to Huxley that we can trace the exceptionalism that has dogged paleoanthropology ever since. For by employing anti-Darwinian reasoning in support of the conclusion that the Feldhofer fossil was merely a brutish Homo sapiens , Huxley had provided Darwin with just the excuse he needed not to broach the fossil evidence seriously in The Descent of Man.
Darwin could brush the crucial Neanderthal fossil off in passing because Huxley, in however non-Darwinian a spirit and however much in contradiction of his own principles, had given him license to.
There were, then, many reasons why Darwin should have been disposed in The Descent of Man to shrink from any substantive discussion of whether extinct human relatives might actually be represented in fossil form. None of this means, of course, that The Descent of Man has not exerted an immense influence on the sciences of human origins over the last century and a half. Just as it is easy for English speakers to forget how much they owe to William Shakespeare for the language they use daily, we tend to lose sight of the fact that much received wisdom in paleoanthropology has come down to us direct from Darwin.
It was Darwin who documented beyond doubt, in The Descent of Man , that all living humans belong to a unitary species with a single origin—which we now know, on the basis of evidence of which Darwin could never have dreamed, to have been around , years ago.
He also had the inspired hunch that our species originated in the continent of Africa—and again, this guess has been amply substantiated by later science. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation.
Virtually every section in the first part of the Descent of Man foreshadows an area of anthropology or biology that has independently flowered since; and in this way, Darwin wrote much of the agenda that was to be followed by paleoanthropology and primatology over the next century and a half. Darwin C. On the origin of species. London: John Murray; Google Scholar. The descent of man and selection in relation to sex.
London: John Murrayp. Darwin F, editor. New York: H. Schuman; Eldredge N. Darwin: discovering the tree of life. New York: Norton; Huxley TH. New York: Appleton;
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