The Quest for Adam

Jean-Jacques Hublin. Archaeology. Volume 52, Issue 4, July/August 1999.

Fighting in China following the Japanese attack on Pearl Harbor in December 1941 resulted in a paleoanthropological disaster. The largest and most complete collection of human fossil remains—unearthed at Zhoukoudian, near Beijing—vanished after being entrusted to a platoon of U.S. Marines on its way to the harbor of Tianjin. This amazing series of specimens, today assigned to the species Homo erectus, had been unearthed by the first large-scale paleoanthropological excavation, in which a team of Canadian, Swedish, and Chinese scientists removed thousands of cubic feet of sediments to uncover fossil hominids. In many respects, the Zhoukoudian project foreshadowed research on human origins in the second half of the century. Since then, finds in Africa, the Middle East, and Europe have greatly increased the body of fossil material available for study. The cave site of Sima de los Huesos in Spain, under excavation since 1988, has yielded more than 80 percent of all known Middle Pleistocene post-cranial fossil human remains. The result of this explosion of evidence is that we have a much clearer view of human evolution than we did 50 years ago. It is no longer a chain in which new finds can be set, one link after another. It is now a puzzle of many species, a complex bush, most of whose branches became extinct.

From Angel to Ape

Convincing the publicand even some scientists, that humans were part of the animal world was one of the most difficult achievements of the twentieth century. The clearest expression of the resistance to this notion is how humans and living apes are separated in zoological classification. Long after their close relationship had been demonstrated, chimpanzees, gorillas, and orangutans were still assigned to a separate family, Pongidae, while humans and their ancestors were placed in their own family, the Hominidae. This classification is still widely used today.

As late as the 1970s, the two families were considered to have evolved separately for a very long time, perhaps 20 to 28 million years. In 1966, however, Vince Sarich and Allan Wilson of the University of California, Berkeley, analyzed the molecular evidence for the relationships among the apes and the age of divergence between apes and humans. Proteins made by our body have a precise design specified by our genes. Mutations that accumulate with time result in gradual changes in this design. If one assumes that these changes occur at a regular speed, the degree of similarity between proteins in different species reflects the degree of kinship between them and can be used to establish a “molecular clock” for their past evolution. Applying these principles to blood proteins, Sarich and Wilson concluded that the divergence between chimpanzees and humans could not have happened much before 5 million years ago. This contradicted the prevailing view and caused heated debate. But their results have been confirmed by subsequent molecular studies, especially of DNA. These show that chimpanzees and gorillas are much more closely related to humans than they are to orangutans. In fact, chimpanzees share more than 99 percent of their genetic material with humans, even though it is packaged in their chromosomes in a different way. In 1992, Tim White of the University of California, Berkeley, and his collaborators discovered Ardipithecus ramidus in Ethiopia. This species, which seems to be very close to the point of divergence between chimpanzees and human ancestors, dates to 4.4 million years ago, perhaps confirming the genetic evidence.

From Ape to Human

The discovery ofAustralopithecus by Raymond Dart in 1924 at Taung, South Africa, was met with skepticism by a scientific establishment unwilling to accept his view that the fossil represented a human ancestor. Then, the big-brained skull found at Piltdown, England, in 1911-1912, seemed to be a much more acceptable ancestor. But by World War II the work by Robert Broom, G.W.H. Schepers, and John T. Robinson at South African sites such as Sterkfontein, Kromdraai, Makapansgat, and Swartkrans had confirmed Dart’s interpretation. And soon after, Piltdown man was proved to be a forgery.

The australopithecines were divided into two groups: a gracile form named Australopithecus africanus and a robust one called Australopithecus or Paranthropus robustus, the latter not much bigger but having a much stronger masticatory system (jaws, teeth, and muscle attachments related to chewing). For Robinson, the two forms were so different that they represented not just two different species but two different genera. Louis and Mary Leakey’s discovery at Olduvai, Tanzania, in 1959 of the famous Zinjanthropus (Paranthropus) boisei showed that robust australopithecines had lived outside southern Africa as well. Some anthropologists, like Phillip Tobias of the University of the Witwatersrand, felt that only the gracile australopithecines could be directly ancestral to humans. They opposed proponents of the “single species” hypothesis, an influential concept in the late 1960s and early 1970s that held that humankind was never represented by more than one species at the same time. Its adherents believed that gracile and robust australopithecines were just females and males of the same species. Richard Leakey’s later discovery of well-preserved specimens of Paranthropus boisei at Lake Turkana helped resolve the vexed interpretation. The fossils he found, of males and females, did not vary to the degree that the South African Australopithecus africanus and Paranthropus robustus fossils did, confirming the existence of at least two species of australopithecines.

The exploration of Ethiopia’s Omo Valley starting in 1967 under the leadership of F. Clark Howell of the University of Chicago, Yves Coppens of the Centre National de la Recherche Scientifique in Paris, and Richard Leakey provided a chronological and environmental framework for deposits ranging from 4.1 to 0.8 million years ago. It was followed by the investigation of sites French geologist Maurice Taieb had found along the Awash River in eastern Ethiopia. In the fall of 1973, Coppens and Donald Johanson, then at Case Western Reserve University, organized an expedition to a site known as Hadar where, the following year, they found 40 percent of a female hominid skeleton, Australopithecus afarensis, which came to be known as Lucy. It is now known to be 3.3 million years old. In 1975, partial remains of 13 more individuals were found at the same site. Variation in size and shape of the Hadar material was initially taken to be evidence of two species, but this interpretation was highly debated. Some light was shed on the matter by Mary Leakey’s work at Laetoli, south of Olduvai. There, between 1974 and 1981, she uncovered a series of 3.7-million-year-old fossils, mainly jaws, that were analyzed by Tim White, as well as a series of human footprints left in volcanic ash. Taken together, the Hadar and Laetoli material was recognized as being from a single species, A. afarensis, that had strong sexual dimorphism, or differences in size and shape. This new species was considered ancestral to both later australopithecines and Homo.

These lower branches of the human family tree have since become even bushier. A well-preserved skull found at Lake Turkana by Alan Walker of Pennsylvania State University in 1985 proved that a robust australopithecine existed at least 2.5 million years ago. This specimen, known as the “black skull,” was assigned to a species that had already been described, Australopithecus (Paranthropus) aethiopicus. If it is ancestral to P. robustus and P. boisei, it shows that a robust lineage was already distinct at this time. Alternatively, several robust australopithecine species may have emerged independently and at different periods. A new Australopithecus, recently found in Chad by Michel Brunet of the University of Poitiers, could represent a new species (A. barhelghazali) or a western version of A afarensis. In any case, it clearly shows that the australopithecines were not restricted to southern and eastern Africa. The recently identified A. garhi in Ethiopia provides an even more complex picture of the evolution of this group. To date, the earliest known australopithecine is the 4.2- to 3.9-million-year-old Australopithecus anamensis, identified in Kenya by Meave Leakey.

Most paleoanthropologists see the earliest australopithecines as the direct ancestors of later humans, through some gracile species, and of an extinct lineage, the robust species. Contrary to a popular view, Lucy and her kin were not scaled-down models of ourselves running across the savannas. In many aspects, including social organization, australopithecines were probably much closer to apes than to humans. They were small-brained, primarily vegetarian, and preserved some ability to climb in trees. While Dart imagined them as early tool and weapon makers, it is still debated whether stone artifacts can be securely assigned to any of their species. Their remains in South African caves were carried their by predators, suggesting they were more often prey than hunter. They do display a very spectacular human feature—bipedalism—which likely emerged alongside the open landscapes that developed with drier and more seasonal climates, as indicated by environmental evidence at this time. However, their gait was very different from that of modern humans and they never lived very far from trees.

The Hunt for Adam

The ancestry of our owngenus, Homo, and its relationship with the australopithecines has been one of the most disputed topics of the past half-century. In this battle the Leakey family has played a central role, supporting the extreme view that the true human lineage had a very old and completely separate development from the australopithecines. A crucial moment of the quest for the oldest humans was the discovery in 1960 of Homo habilis in the same levels at Olduvai that produced a robust australopithecine, the famous Zinjanthropus (Paranthropus) boisei. This new species was published in 1964 by Louis Leakey Phillip Tobias, and John Napier of the Royal Free Hospital School of Medicine, who placed it at the head of our own genus. Many scholars promptly rejected this interpretation, partly because of the specimen’s small brain capacity.

Richard Leakey’s discovery of a skull at Koobi Fora on Lake Turkana in 1972 was a major step toward the acceptance of Homo habilis. Known by its inventory number, ER 1470, the skull displayed a larger cranial capacity than the Olduvai H. habilis. Initially it was assumed to be older than 2.6 or even 2.9 million years, but the overlying volcanic deposits were later dated to nearly 1.9 million years ago. Following the Koobi Fora discoveries, the earliest Homo was reinterpreted as two species. Homo rudolfensis, exemplified by ER 1470 and a mandible found near Lake Malawi, is a large-brained form which parallels the robust australopithecines in the scale of its masticatory system. H. habilis includes the Olduvai fossils and ER 1813, another Koobi Fora specimen; its braincase is smaller, but it has a more human dentition. Their common ancestor could be a Homo species older than 2.5 million years. Some specialists have argued that the two forms could be males and females of one species, but this would imply a huge sexual dimorphism and does not explain the morphological differences.

Like the australopithecines, early Homo do not fit into a linear chain but rather into a complex and probably only partially known bush of species. Some anthropologists still support the concept of a genus Homo rooted some 4 million years ago or more and unconnected with the australopithecines, but the paleontological evidence currently suggests that some 2.5 million years ago or so, probably in relation to increasing aridity, australopithecines gave birth to several lineages. One, Paranthropus, adopted a new vegetarian diet requiring intensive chewing. Another, the Homo lineage, developed some degree of meat-eating. This behavior, associated with the ability to hunt small mammals, also exists, to a lesser extent, in chimpanzees. It is probably not a coincidence that the first stone tools were produced at this time.

Out of Asia

In the years following World War II, anatomist Franz Weidenreich prepared a detailed description of the missing Zhoukoudian fossils using casts made before the specimens were lost. At this time, fossil hominids immediately predating Homo sapiens and Neandertals were called Pithecanthropus erectus for Javanese discoveries and Sinanthropus pekinensis for the Zhoukoudian specimens. In 1950 biologist Ernst Mayr, following the proposal of geneticist Theodosius Dobzhansky, emphasized that the differences between the Javanese and Chinese fossils were superficial. In Mayr’s view, H. erectus was a unique human species just preceding H. sapiens, which included Neandertals.

In the next decade, specimens assigned to Homo erectus were discovered outside of Asia. North Africa was the first to yield them, especially in the site of Ternifine (now Tighenif) in Algeria, where Camille Arambourg of the Musée National d’Histoire Naturelle in Paris and Robert Hoffstetter of the French Centre National de la Recherche Scientifique excavated from 1954 to 1956. Telanthropus, discovered in South Africa in 1949, was reassigned to the new taxon. A braincase was found at Olduvai in 1961. With the discovery at Koobi Fora of two nearly complete specimens in the 1970s. H. erectus was shown to encompass a huge span of time, from 1.8 million to 300,000 years ago, and a vast territory covering most of the Old World.

This led some anthropologists to reconsider the status of this species and the emergence of Homo sapiens. In 1983, at a conference in Germany, several speakers stressed anatomical differences between the African and Asian material and proposed limiting the species Homo erectus to the Asian populations. The oldest African specimens, like those of Lake Turkana, were assigned to a different species called Homo ergaster. Unfortunately the Asian material remains poorly dated. It is still debated whether the first human settlements in Asia are nearly 1.8 million years old or only 1.0 million. In 1996 Carl Swisher of the Berkeley Geochronological Center dated H. erectus on Java to fewer than 50,000 years ago. If confirmed, this controversial dating would support the concept of a conservative Asian lineage that was not part of the origins of H. sapiens and survived even after the emergence of fully modern humans.

The most spectacular Homo erectus discovery to date was made at Lake Turkana, where Kamoya Kimeu of the Kenya National Museum found a small skull fragment in 1.53-million-year-old sediments near the Nariokotome River. Between 1984 and 1989, after moving 1,000 tons of sediments, Alan Walker and Richard Leakey found the rest of the skeleton, the remains of a boy who would have reached six feet in adulthood. Its proportions were not intermediate between Homo habilis and Homo sapiens, but almost completely modern. Until then, the H. erectus body had been poorly known. In the march of fossil hominids popularized by many books, its place was occupied by a short and stocky silhouette, but the Nariokotome boy was slender and reminiscent of the modern Africans living in the same area.

The emergence of the first Homo erectus (or Homo ergaster) was an anatomical revolution, probably related to major behavioral changes. They were the first genuine humans, fully independent of trees. With a modern gait and a hairless, sweating skin cooling their body, they were highly mobile and well-adapted to the burning sun of the African steppe and savanna. They were also the first humans to spread out of Africa.

The Garden of Eden

In 1951, F. Clark Howellre analyzed the available Neandertal material. He suggested that modern humans could have originated from “generalized” forms of Neandertals existing in the Middle East, but not from the more specialized late Neandertals who inhabited Western Europe between 80,000 and 40,000 years ago. This group would have been replaced by modern invaders from the east. Attention focused on Skhul and Qafzeh, two caves in the Levant where fossil hominids had been discovered in the 1930s. In 1965 Bernard Vandermeersch of the University of Paris renewed excavations at Qafzeh, and made an impressive series of discoveries including burials with human remains that appeared essentially modern. Like the European and Middle Eastern Neandertals, they were associated with Mousterian (Middle Palaeolithic) stone tools, but the associated fauna indicated that they were older and not intermediate in age between Neandertals and Upper Palaeolithic modern humans. Only with the use of new isotopic dating methods in the early 1990s, which yielded dates around 100,000 years ago, was the early age of the modern humans at Qafzeh and Skhul fully accepted.

The Middle Eastern evidence proved the contemporaneity of two different hominids, once more contradicting the single species hypothesis. European Neandertals could not be ancestral to modern humans. This also contradicted the “polycentric model” for modern human origins developed by Carleton Coon of the University of Pennsylvania in his 1962 book The Origin of Races. Following views expressed by Weidenreich on the eve of World War II, Coon assumed that physical differences between living groups of humans had existed for a long time and were the result of separate regional lineages evolving through Homo erectus and Neandertal phases. Modern humans, he believed, appeared independently in different geographical areas. This concept has recently been supported by Milford Wolpoff of the University of Michigan and Alan Thorne of the Australian National University in the form of the “multiregional continuity” model, which, however, allows for some genetic exchanges between regional lineages.

By contrast, William White Howells of Harvard University, who studied cranial variations in modern populations in the early 1970s, was impressed by the strong homogeneity of living humans He claimed that all modern humans have a rather recent common origin and that they replaced archaic humans such as Neandertals. In the early 1980s, Gunter Brauer of the University of Hamburg proposed an African origin for modern Europeans, foreshadowing the conclusion of geneticists Allan Wilson, Rebecca Cann, and Mark Stoneking. Working at the University of California, Berkeley, in 1987, they produced a tree diagram showing the affinities of mitochondrial DNA, a rapidly evolving genetic material which is transmitted only through the maternal line. This tree indicated that people of African origin display the greatest variation in their mitochondrial DNA, while all other modern populations were rooted within the African variation, their common ancestors probably only some 200,000 years old. This study had an incredible impact, dubbed the “African Eve” hypothesis, it even made the cover of Newsweek. Though its methodology-especially how the African origin of the common modern human ancestors was determined—was criticized, the Berkeley group’s work was the starting point of a number of similar studies, all of them leading more or less to the same result. African populations demonstrate the largest genetic variation, the common ancestor of all modern people is 200,000 to 100,000 years old, and modern humans spread around the globe after a recent exodus from Africa. Nowadays this has become a new orthodoxy and most discussion focuses on the possibility of some hybridization between modern invaders out of Africa and local archaic populations, including Neandertals in Europe and possibly Homo erectus in Asia. The claim that the ca. 25,000-year-old remains of a boy found this past December at Lagar Velho, Portugal, have both modern human and Neandertal characteristics is a case in point.

The Neandertal Mystery

The Neandertals arethe best documented of the archaic populations modern humans replaced. Since the first discoveries in the mid-nineteenth century, some 200 sites in Europe and the Middle East have produced fossil remains of these humans and their ancestors. While in Ernst Mayr’s view the Neandertals were part of the species Homo sapiens, an increasing number of scholars have revived the term Homo neanderthalensis and the notion that Neandertals were a different species. In the 1970s a reexamination of their anatomical features led to a better definition of Neandertals and demonstrated that they did not develop in Africa or the Far East, but were essentially a European group which expanded toward the Middle East and Central Asia at some time during its evolution. It became obvious that many of the oldest European specimens should be assigned to “Pre-Neandertals.” I supported the view that the Neandertal lineage could be identified at least 450,000 years ago, and this ancient pedigree was later confirmed by the discoveries at Sima de los Huesos at Atapuerca, Spain.

In 1997, for the first time, the structure of a small fragment of mitochondrial DNA extracted from a bone found in the Neander Valley in 1856 was analyzed. On average it proved to differ more from any similar piece of modern mitochondrial DNA than two modern similar pieces differ from one other. The study’s authors reject the notion of continuity between Neandertals and modern populations, estimating that the isolation and separate evolution of the Neandertal lineage are several hundred thousand years old.

A striking aspect of paleoanthropology during the past 50 years has been the growing emphasis on the biomechanics, diet, growth, and development, and demography of extinct hominids. This is especially true for Neandertals, for whom we have a large enough sample to examine these topics. More than for any other hominids, we are able to reconstruct the soft tissue of these large-bodied and cold-adapted hominids. Histological studies of their bones have been used to determine individual ages at death. Measurements of the isotopic ratios of carbon and nitrogen have shown that they were highly carnivorous, with a diet similar to the wolves and hyenas found at the same sites.

In recent years the issue of the Neandertal extinction has attracted more scrutiny. It is now well established that some 40,000 years ago Europe was occupied by modern humans bearing new technologies. These humans had body proportions similar to those of modern Africans and contrasting with those of the Neandertals. The European record is detailed enough to allow us to combine biological and cultural evidence to understand the scenario leading to the final replacement of one population by another. How long did the coexistence of the two groups last? How did they interact? Did they interbreed? These questions, not definitively answered, are the Neandertal mystery.

New Perspectives

During the Past 50 Years paleoanthropological research has shifted from Europe and Asia to Africa. This continent appears more and more as the demographic and evolutionary core of human development. We were originally tropical primates who adapted to a dry and open environment. From this vast continent several waves of colonists spread into Eurasia and eventually to Australia and the Americas. More and more, these movements are being related to environmental changes.

Fossil hominids are increasingly considered in a biological perspective, as members of extinct populations. Paleoanthropologists are paying more attention to their individual variation, biological adaptation, pathology, and physiology. Nowadays the understanding of their evolution takes into consideration the ecological changes as well as the interaction between cultural and biological factors.

The relations between some branches of our family tree are still debated, but today we have a much sharper—albeit much more complex—image of hominid evolution than the naive picture of the late 1940s. Our family tree is no longer a linear series of species but a dense bush with many dead branches. The quest for the famous “missing link” has become vain simply because we no longer deal with a simple chain.