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SMITHSONIAN INTIMATE GUIDE TO HUMAN ORIGINS

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Chapter One: THE CLUES

The winds of the Sahara desert sandblast the Earth. They scour rocks, stripping an inch-thick layer away every year. But not everything the winds touch is so easy to dislodge. Fossils of long-dead animals and plants resist, even as the soft sandstone surrounding them vanishes. Gradually the fossils emerge into sunlight after millions of years in darkness. Paleontologists try to rescue them during this fleeting moment of exposure. Before long, even the toughest fossil will melt away under the sandblasting winds, lost to science forever.

One team of paleontologists regularly visits a particularly bleak region of the Sahara called the Djurab, in northern Chad. Some of the scientists are French, some Chadian. Their tents are sometimes buried in sand; sometimes they have to wear ski masks to protect their faces from the scouring winds. In addition to the threats nature offers, they face human ones as well. Private armies bury land mines in the area, and sometimes they hold up the paleontologists at gunpoint.

The scientists search for fossils among sandstone formations that turned to rock six to seven million years ago, along the shores of an ancient lake. Today the Djurab sees hardly a drop of rain, but six to seven million years ago, the paleontologists have discovered, fish once swam, along with crocodiles and frogs. Pythons slithered through the lakeside grasses, while turtles clambered in the mud. Trees and bushes formed a thick collar around the lake, attracting elephants, pigs, hippos, rodents, and monkeys. Giraffes and horses grazed; big cats hunted; hyenas scavenged. Dung beetles rolled balls and termites built galleries.

In 2001, a young university student named Ahounta Djimdoumalbaye found a peculiar skull exposed by the winds. It was smaller than a horse, bigger than a monkey. As Djimdoumalbaye and other members of the team scraped it out of the sandstone, its broad dome emerged, crowned by a ridge over two forward-facing eye sockets. Its face was flat and its front teeth were relatively small. These telltale clues revealed it to be part of our own close evolutionary family, more kin to humans than to chimpanzees or any other mammal. Paleontologists call such fossil species hominids. And this particular hominid, which the scientists dubbed Sahelanthropus tchadensis, is the oldest one yet found. It may belong to one of the earliest species that evolved after our lineage branched off from that of other apes.

Digging in the Sahara is not the only way scientists gather clues about human evolution. Some of the most important ones have been discovered in suburban Maryland, where robots at the National Human Genome Research Institute analyze DNA.. Instead of fossil bones, our genome hides genetic traces of our history, such as silenced genes that have not been able to produce proteins for millions of years. By comparing the human genome to those of other animals, scientists can trace the 600-million-year history of our brains. They are also isolating genes that changed dramatically in just the past 100,000 years, as our ancestors acquired the gifts — such as language — that make us uniquely human.

In addition to studying genomes and fossils, scientists can find clues about where we come from in other ways. MRI scans can give hints as to how the circuits of our brains were rewired over the past few million years. Supercomputers can re-create the shambling gait of our ancestors before they walked upright. But just as important as all this technology are the long, patient hours that primatologists spend watching our closest relatives — the chimpanzees and other apes. In their careful observations come new insights into the origins of human culture.

When you consider all of the ways scientists can study human evolution today, it’s mind-boggling to think about how little Charles Darwin had to work with when he first began to think about our origins. Fossil spear-points and other stone tools had been known since the 1600s. Three years before Darwin published The Origin of Species, German quarry workers shoveled up a skullcap and a few other bones of Neanderthals. Today, most researchers consider Neanderthals a separate species of human that became extinct 28,000 years ago. But at the time, naturalists were baffled by the Neanderthal’s almost-humanness. It seemed to be about the same size as a grown man, but its skull displayed a low, thick brow ridge. Thomas Huxley, one of Britain’s leading naturalists, decided that Neanderthals were not far beyond the range of variation found in living humans and made them members of our own species. “In no sense, then, can the Neanderthal bones be regarded as the remains of a human being intermediate between Men and Apes,” he wrote in his 1863 book Man’s Place in Nature.

For fossils, that was about it. Genomes were unknown — DNA itself had yet to be discovered. Yet Charles Darwin still managed to recognize some of the basic processes that have shaped our species. Like other animals, humans pass on certain traits to their children. In each generation, these traits vary from one person to the next. Some people are tall; others, short: some are heavy; some, thin. Like other Victorians, Darwin knew very well that not all people manage to pass on their traits to the next generation. (Three of his ten children died before the age of ten.) If a trait helps a person survive and reproduce, it may, over time, become more common among our species. In other words, humans are just as subject to natural selection as any other species.

Darwin believed that selection could cause a new species to branch off from an existing one. He saw these branches forming a tree of life, with new limbs continually budding off as old limbs were pruned by extinction. Humans, Darwin concluded, had emerged from this same tree of life. The closest branches to our own produced our closest living relatives — chimpanzees, gorillas and orangutans.

The uncanny similarities between humans and apes had been noted for over a century. In the 1690s, the English anatomist Edward Tyson performed the first dissection of a chimpanzee. In his 1699 book The Anatomy of a Pygmie, he wrote that the chimp and the human brain bore a “surprising” resemblance, commenting that “one would be apt to think, that since there is so great a disparity between the Soul of a Man, and a Brute, the Organ likewise in which ’tis placed should be very different too.”

Darwin mused for a long time about the similarities between humans and apes. He paid regular visits to the London Zoo to observe an orangutan named Jenny, taking notes about the similarity of her facial expressions to those of humans.  But for years Darwin kept his ideas about human origins to himself. He filled The Origin of Species with enough animals to stock the London zoo many times over. Whales, bears and flatworms parade through his pages, along with giant extinct sloths, armadillos and marine reptiles. But one species is glaringly absent : Homo sapiens. “Light will be thrown on the origin of man and his history,” was all Darwin would say.

Darwin knew that it would be hard enough to persuade his readers that animals or plants evolved. To add humans to evolution’s list of accomplishments might instantly turn them away. “I thought that I should thus only add to the prejudices against my views,” he later wrote.

He was probably right. Most of the attacks launched against The Origin of Species  sooner or later came around to the question of man’s place in nature. Richard Owen, the greatest British anatomist of the nineteenth century, thought he could refute evolution by finding a part of the brain that was unique to humans. Such a trait would set humans safely apart from other apes, and thus out of the grasp of Darwin’s natural selection. In 1860, a year after the publication of The Origin of Species, Owen attended the most famous debate over Darwin, at Oxford University, where Bishop Samuel Wilberforce delivered a furious attack on The Origin of Species. He ended his speech by turning to Thomas Huxley and asking him whether he descended from an ape on his mother’s or father’s side!

As the years passed, however, the attacks faded. Naturalists still debated about how evolution worked. Did life have a built-in direction along which it evolved, they wondered, or was it nothing more than the adaptation of populations to the randomly changing conditions they faced? But all sides increasingly agreed that life had indeed evolved. Seeing a consensus emerging, Darwin decided the time was right to put humans in his zoo. In 1871 he published The Descent of Man, presenting evidence that humans had evolved and suggesting that they were modified mammals descended from an ape-like African ancestor.

Today, scientists have far more evidence at hand than Darwin had to test ideas about human evolution. In many ways he has been splendidly vindicated. Fossils of hominids have been found dating as far back as six million years. For the first four million years of hominid ancestry, all known species lived in Africa, strongly supporting an African origin to our branch on the tree of life. But scientists have also discovered many things that would surprise Darwin. For example, Darwin thought that as the ancestors of humans evolved bipedalism, they evolved large brains at the same time. But fossils suggest that the first hominids to stand upright had brains one-third the size of a human’s. It took three million years before their descendants would evolve a brain approaching our own.

Despite recent insights into our origins, there is much we still don’t know. That’s not unusual in science. Physicists, for example, may have plumbed the atom but the fundamental nature of gravity still eludes them. And compared to scientists who study human evolution, physicists are drowning in information!  To understand how an electron works, they can run experiments over and over again, studying the behavior of trillions of particles. History, on the other hand, is an experiment that runs only once. Still, scientists can test hypotheses about evolution by surveying the different forms life has taken on its different branches. They can search for ancient relics carried down from one generation to the next in DNA. And they can study the fossils of extinct species.

Making matters worse, our species has a history that’s particularly hard to reconstruct. At first this might seem odd, since we belong to a very young lineage. The earliest evidence of life on Earth — a balance of chemicals in ancient rocks that could have been formed only by biological organisms — dates back 3.7 billion years. The common ancestor that humans share with chimpanzees lived only six to seven million years ago. If the history of life were squeezed into a single day, that common ancestor would have been born three minutes before midnight. The oldest fossils known to belong to our own species, Homo sapiens, date back 195,000 years — only five seconds before the end of the day.

While our hominid roots may not run very deep, the mysteries of our evolution remain vast. Part of the problem is that hominids are particularly rare in the fossil record. They never formed great herds, nor did they live in the sorts of places where fossils have a good chance to be preserved. The unique features of our species make the task even more challenging. Of all living animals, only humans can use full-blown language and only humans have true consciousness. Scientists are just beginning to understand how our brains make all this possible, yet these traits leave little mark on the fossil record.

While scientists will never recover a perfect picture of human evolution, their success in recent years makes it a safe bet that they will continue to make astonishing new discoveries for years to come. Perhaps Darwin said it best: “It has often and confidently been asserted, that man’s origin can never be known,” he wrote in The Descent of Man. “But ignorance more frequently begets confidence than does knowledge. It is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science.”

COPYRIGHT 2005 CARL ZIMMER

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