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From Eternity to Here
Paleoanthropology at the Millennium

Charles Darwin's great idea, that all forms of life on Earth have descended with modification from earlier species, has provided a framework for understanding the rich human fossil record that has been excavated during the past century and a half.

Fossils, however, tell only part of the story. While they reveal what some of our ancestors looked like, how they walked, and what they ate, they do not tell us when they lived, or in what sequence they came to inhabit different parts of the world. For that information, we need reliable dating methods. Furthermore, we need genetic methods to tell us which primate species was our closest relative, and to which continent we should look for our oldest and our most recent ancestors.

As the twentieth century dawned, our understanding of human evolution was dawning too, but with a great deal more darkness than light. Many early discoveries were made by European men whose personal circumstances enabled them to dabble in fossil-finding. In 1856, Johann Fuhlrott, a schoolteacher at Eberfeld, Germany, recognized the first Neandertal bones as being those of ancient humans. In 1868 Louis Lartet, a geologist and son of solicitor and prehistorian Edouard Lartet, discovered anatomically modern skeletons at Cro-Magnon, in the Dordogne region of France. In the 1890s Eugene Dubois, a Belgian physician working in the Dutch East Indies, found early human fossils in Java that he called Pithecanthropus erectus (erect apeman) but that are now named Homo erectus.

By comparing the skeletons of various large primates, Darwin and his paleontologist friend Thomas Henry Huxley deduced that we were most closely related to the African apes, and therefore they thought the human line had originated in Africa. But most anthropologists in the first half of this century looked to Asia for the source of humanity. After all, China boasted the oldest civilization, and the gibbon, an Asian ape favored by Dubois as a human ancestor, sometimes walked upright as we do. Most European intellectuals considered Africa the Dark Continent, too primitive and backward to have given rise to big-brained masters of the universe like us.

In 1924 Raymond Dart, an Australian who had done his medical training in England, was professor of neuroanatomy in Johannesburg, South Africa. One day, as he was dressing to be best man at a wedding, workers from the limestone mine at Taung brought him a box of fossil-containing rocks. He was so excited to see a small skull in the box that the bridegroom had to pull him away to the ceremony. The Taung child's skull had an ape-sized brain, small human-like canine teeth, and anatomical indications that it walked upright. Dart called it Australopithecus africanus (African southern ape) and thought it an ancestor intermediate between apes and modern humans. Like Darwin, he suspected that the human lineage had originated in Africa. Now he was confident he had the evidence.

The little Taung skull, however, was mostly discounted by the anthropological establishment of Europe. Human ancestors were assumed to have a large brain, which was a major reason why the phony Piltdown skull, "discovered" by Charles Dawson in 1912, was so widely accepted.

Many additional fossils of australopithecines, most of them adults, were subsequently excavated in South Africa, and the pelvic bones confirmed that this genus was bipedal. Walking upright on two legs, rather than having a large brain, emerged as the earliest human characteristic in the fossil record. The designation "human," like every aspect of evolution, is debatable. Some anthropologists use the word "human" only for the genus Homo. Still others reserve that label for modern Homo sapiens. We choose to apply the term interchangeably with "hominid" to refer to all members of the lineage from australopithecines onward to the present day.

Australopithecine fossils took on new significance when they could be dated accurately. The classical method estimated the age of fossils of extinct animal species found in association with the human fossils. Some of these animal fossils were also found elsewhere in strata whose dates were known. It was assumed the dates are the same. Clearly this method is not very precise. The first radioactive dating method, Carbon-14, developed in the 1940s and 1950s, cannot date materials older than 50,000 years, and the australopithecines were much older than that. A breakthrough occurred at Olduvai Gorge in Tanzania during the 1960s, when potassium-argon dating, a new radioactive technique that works for fossils millions of years old, was applied to the sediments in which Louis and Mary Leakey had found hominid fossils, such as the famous "Zinjanthropus," a robust australopithecine.

Olduvai, unlike the South African cave sites, preserved distinct layers of lava from nearby volcanic eruptions. Lava contains the naturally radioactive element potassium-40, which decays to the inert gas argon-40. When lava flows, argon diffuses into the atmosphere, but as the lava cools, argon is trapped in a solid matrix, and the clock starts ticking. Argon accumulates over time, and its ratio to potassium increases. This rising ratio enables scientists to measure ages ranging from thousands to billions of years for the volcanic layers and, hence, the fossils trapped between them.

On the basis of the traditional faunal dating, the Leakeys' "Zinjanthropus" was thought to be about 700,000 years old. However, the argon/potassium ratio of the Olduvai layer in which "Zinjanthropus" was found indicated an age of nearly two million years, almost three times as ancient as expected.

Paleomagnetism, another big step forward in dating techniques, came out of the new science of plate tectonics. For reasons unknown, Earth's north and south magnetic poles have reversed many times during the past four billion years. Iron crystals behave like tiny compasses that point to the magnetic poles. After being incorporated into the Earth's crust, they retain their orientation. Thus the history of north-south reversals is recorded in Earth's iron-bearing sediments. The pattern of reversals can be measured with a magnetometer, and this pattern defines a worldwide time scale.

Before the new time scale can be applied, the magnetic reversals must be dated by the potassium-argon method. As luck would have it, the original calibration was carried out at Olduvai Gorge, where the lava layers dated by radioactivity also contained remnant magnetism.

Many important hominid sites, such as those in South Africa and Europe, have no volcanic lava, but they do retain paleomagnetic patterns, which can be matched with the worldwide time scale. South African dates for australopithecines range in age from one million to more than three million years old, similar to the potassium-argon ages for these fossils in East Africa. On this basis, Dart's Taung specimen is thought to be about two million years old.

A new "molecular clock," more revolutionary and controversial than the radioactive one, was applied to human evolution in the 1960s by Vincent Sarich and Allan Wilson at the University of California at Berkeley. They compared blood proteins of humans, apes, and monkeys. The serum albumin of humans, chimpanzees, and gorillas differed from each other by only one percent, while all three differ by six percent from the albumin of New World monkeys.

The fossil record indicated that New and Old World primates (including apes and humans) diverged about 30 million years ago. Sarich and Wilson therefore deduced that humans, chimpanzees, and gorillas must have diverged from each other one-sixth of 30 million, or five million years ago-a much more recent divergence than most experts had anticipated. Furthermore, these results pointed, as Darwin had, to an African genesis.

Many paleoanthropologists at the time were convinced that fragmentary fossils from Pakistan called Ramapithecus represented human ancestors more than 15 million years old, implying an ape-human separation in Asia at 20-25 million years. The Sarich-Wilson results were strongly resisted for many years, but eventually, more complete fossils of crania and limb bones convinced everyone that Ramapithecus was an arboreal ape, not a hominid. This was an important triumph for the new field of molecular anthropology.

High-tech methods are now available to determine an individual's age at death from the rate and pattern of growth of its teeth. Glenn Conroy and colleagues reexamined Taung using a computerized tomographic (CT) scanner, a device which makes detailed x-ray images for diagnosis of human diseases, by "looking inside" the bones. The Taung specimen had all its baby teeth, and its first permanent molars were just erupting. When Dart had compared the teeth with those of modern children, he estimated the Taung child had died when it was only six years old. But CT scans of the dense fossil jaws showed a fast pattern of tooth formation, like that of a chimpanzee. The first chimpanzee molar erupts between three and four years of age, so the Taung child is now thought to have been about three and a half years old when it died.

The study of australopithecine growth and dental development, together with the molecular data, implied that this early group of human ancestors shared a recent evolutionary history with African apes. From 1960 onward, Jane Goodall's observations in the field reinforced this idea. She saw chimpanzees using tools, catching and killing small animals, communicating with each other by human-like gestures and facial expressions, and forming lifelong social bonds with relatives and friends-behaviors formerly thought to be uniquely human.

Not until the 1980s did methods emerge that were sufficiently precise to determine which two of the three lineages-human, chimpanzee, and gorilla-were closest to each other. Direct comparisons of the DNA of the three groups showed humans and chimpanzees to be the most closely related pair. This was a surprise. To the naked eye, chimpanzees and gorillas look and act much more alike, but in evolutionary analysis the eye often deceives.

Three-quarters of a century after Raymond Dart's initial discovery, we have numerous australopithecine fossils two to four million years old, widely distributed through South and East Africa, and one recently found in Chad, in north central Africa, dated at three million years. The oldest australopithecine, A. anamensis, dated at four million years, was discovered by Meave Leakey near Lake Turkana in Kenya (The Lastest Leakey Search). Limb bones show that it, too, was bipedal. Australopithecines were a very successful group, comprising perhaps as many as ten species. Except for walking on two legs, they were quite chimp-like in overall size, brain capacity, and dental development.

As the australopithecines in Africa were found to be so ape-like, questions swirled again about the ancestry and geographical origin of our genus, Homo. The fossil species called Homo erectus, first discovered by Dubois in Java about a hundred years ago, with more specimens unearthed by Davidson Black in China during the 1930s, seemed to support the theory that the human line had originated in Asia. These fossils had a brain two-thirds the size of modern humans, prominent brow ridges, sloping foreheads, and big teeth.

Mary and Louis Leakey, working in East Africa, discovered and named a smaller-brained new species, Homo habilis (handy man), and they were able to give it a geological date of 1.8 million years. The oldest fossils assigned to the genus Homo from Malawi and Ethiopia are more than two million years old. It seems that our genus as well as our lineage had an African origin.

The fossil evidence and geological dates in East and South Africa were increasingly indicating that two different hominid species-early Homo and Australopithecus-were living at the same time, 1.8-1.9 million years ago. This contradicted the popular "single-species" thesis held by some anthropologists, the simple and simplistic notion that human evolution had progressed along a straight line, one species at a time.

If Africa is the cradle of humankind, as the evidence now overwhelmingly suggests, when did hominids first leave Africa? It was a big surprise when geochronologists in Berkeley recently dated Homo fossils in Java at 1.8-1.9 million years, dates almost double the antiquity previously estimated! The new dating method was argon-40/argon-39, a refinement of the potassium-argon technique. In this method, the specimen to be dated is irradiated, which converts normally stable potassium-39 to argon-39. The ratio of the two argon isotopes can be measured more precisely than the ratio of potassium to argon.

Archeological sites have yielded stone artifacts and occasional bones revealing a hominid presence over 1.5 million years ago in Soviet Georgia, Israel, and southern Spain. Ancient hominids were established in China by a million years ago. These widely distributed populations very likely represent several different species of the genus Homo rather than the single species, Homo erectus, but the fossil record is too fragmentary at present to provide a detailed anatomical evaluation.

All this activity in Asia and Africa has tended to put Europe in the background, a demotion from its presumed central position in the early half of the twentieth century. However, Europe is making a comeback, as anthropologists pursue the question: When did hominids first arrive on the European continent? Southern Spain may soon claim the oldest hominid site at a village named Orce, where excavations are uncovering artifacts twice as old as those from the present record-holder in northern Spain. Orce contains a long sequence of deposits, with stone artifacts in several localities and a paleomagnetic "fingerprint" 1.6 million years old. The identifying layer is the Olduvai subchron, a magnetic reversal identical to one originally established at Olduvai Gorge. Radioimmunoassay, a technique first applied to frozen mammoths in 1980 that uses radioactive antibodies to target fossil protein molecules, showed that fragments of suspected hominid bones at Orce did contain human albumin.

For many years, the Middle Pleistocene-from one million to 200,000 years ago-was known as the "Muddle in the Middle," because of a paucity of fossils and a lack of reliable dates. The Orce fossils from southern Spain and 800,000-year-old fossils from the Gran Dolina cave in Atapuerca have helped fill in the picture. Human occupation in Europe is now estimated at about 500,000 years ago at sites such as Mauer in Germany, Petralona in Greece, Boxgrove in England, and Arago in France.

Apparently, the genus Homo arose in Africa and radiated into a number of different species that became widely distributed in Africa, the Middle East, and Asia. The picture, still developing, is complex, not one wave out of Africa a million years ago, as previously thought, but multiple dispersals starting about two million years ago.

Something happened in western Europe around 40,000 years ago that resulted in a new, more sophisticated tradition of tool-making, and soon after that, of cave art. For this place and time we have fossil evidence of fully modern humans, the Cro-Magnons, named for the site where they were first found in southwest France.

For most of the past century, the evidence of the fossils, the finely made tools, and the brilliant cave art was so compelling that it was natural to assume this period in Europe marked the origin of our species, Homo sapiens. However, there were disturbing bits and pieces of evidence suggesting that modern humans may have been around much earlier. One clue was a somewhat modern-looking mandible, unearthed in Mauer, Germany in 1907, which gave rise to the new species name, Homo heidelbergensis. This species has recently taken on new importance as the potential common ancestor of Neandertals and Homo sapiens.

In Africa, modern-looking fossil skulls from Omo-Kibish, Ethiopia and Klasies River Mouth, South Africa, over 100,000 years old, were at first thought to be much too old to be Homo sapiens, so the term "archaic Homo sapiens" came into the lexicon. As usual, there were several conflicting hypotheses trying to fit these diverse pieces of the puzzle together.

Then, in 1987, Allan Wilson, the "molecular clock man" of Berkeley, reentered the human origins fray with his students Rebecca Cann and Mark Stoneking. Their analysis of mitochondrial DNA indicated that Homo sapiens had originated in Africa about 200,000 years ago, not in Europe 30-40,000 years ago. More recent DNA data put the origin at about 150,000 years.

Some paleoanthropologists believe, contrary to the molecular evidence, that present Homo sapiens populations evolved independently in Africa, Asia, and Europe from the resident populations of Homo erectus, over a period of about a million years. This hypothesis is called "regional continuity."

Much of the recent research supports the mitochondrial DNA results. Gene frequencies, which include classical markers such as blood groups, together with various types of DNA sequencing-the Y chromosome, nuclear DNA, microsatellite DNA, and short tandem DNA repeats-all point to a recent African origin of Homo sapiens.

Analysis of the DNA and genes of living populations continues to be an active area of research and controversy. Debates about the meaning of these data are often intense, and are very reminiscent of those that greeted the molecular evidence that apes and humans split five million years ago, now almost universally accepted, and the chimpanzee-human connection, whose once-vocal critics have mostly gone silent in the past couple of years. In our estimation, the weight of the evidence is heavily in favor of an African origin of Homo sapiens within the past 150,000 years.

Despite the Eurocentrism of human origin studies in the first half of this century, it now appears that Europe 40,000 years ago was among the last places inhabited by our species, not the first. Our species, like our genus and the human lineage itself, arose in Africa much earlier than we once thought, and occupied most of the world's land masses, radiating first to the Middle East, then to Asia, Australia, and Europe. Humans arrived quite late in the Americas, about 15,000 years ago.

As the century draws to a close, we have a much more complete and complex picture of human origins and evolution than we had a hundred years ago. Much of that picture has been provided by precise scientific methods that were unknown in 1900, including radioisotope dating, paleomagnetism, CT scanning, and techniques that let scientists compare the basic molecules of life.

Plenty of questions remain to be answered in the twenty-first century. How many species of Australopithecus evolved in Africa? How many species of Homo inside and outside Africa? From which australopithecine group did Homo arise? From which Homo species and in which part of Africa did Homo erectus spring? So many questions long thought to be unanswerable have been at least partly resolved by the combination of new fossil data and new technology for analyzing them. We can look forward to a century of increasing insight into the mysterious and fascinating prehistory of the primate who stood up on two legs five million years ago and whose ancestors now stand astride the planet Earth.

Adrienne L. Zihlman is professor of anthropology at the University of California, Santa Cruz.

Jerold M. Lowenstein is professor of medicine at the University of California at San Francisco and chairman of the Department of Nuclear Medicine at California Pacific Medical Center in San Francisco.

Summer 1999

Vol. 52:3