American Scientist

Why are some discoveries welcomed, whereas others are received with skepticism? I am prompted to ask this by recent developments in paleoanthropology. On the face of things, the story is an old one: International team finds startling new fossil human, oldest of its type in the region; experts agog. The catch is that the new find now being hailed merely echoes an earlier one in the same place, by many of the same researchers—but the early find was received with a "wait and see" attitude, if not outright disbelief. What makes the difference?

The original find, in 1991, was a primitive human mandible or jaw found at the then newly discovered fossil site of Dmanisi in the Republic of Georgia. A joint German-Georgian team of scientists and students excavated there for some months, recovering beautiful fossils of extinct species like saber-toothed cats, elephants and rhinos, along with some crude stone tools. On the last day—similar episodes are so common that the Last Day Find is practically a cliché—Antje Justus, a German graduate student, freed a partial skeleton of a saber-toothed cat from the sediments in her area of the dig. Lying directly underneath the extinct cat was the fossilized jaw of a primitive human, with a complete set of teeth. This was the find everyone had been hoping for all summer long.

In that moment, Dmanisi was transformed from being an interesting site to being one of major significance for human origins. Although the jaw itself could not be dated directly (as is often the case), its inferred age was impressive. The most recent record of the extinct animals found at Dmanisi turned out to be about 1.2 million years ago, while the fresh-looking lava that lay underneath the fossil-bearing sediments was estimated to be about 1.8 million years old, according to preliminary radiometric dating. That meant that the owner of the Dmanisi mandible lived in the interval between 1.2 and 1.8 million years ago, making it the earliest evidence of Homo erectus from the Eurasian continent by a significant margin.

The first I heard of the find was in December of 1991, when Justus, paleontologist Leo Gabunia of the Republic of Georgia National Academy of Sciences, and dig director David Lordkipanidze of the Georgia State Museum traveled to a conference on Homo erectus at the Senckenberg Museum in Frankfurt, Germany. Gabunia and Justus gave a joint presentation briefly describing the site, the fauna, the tools, the jaw and the preliminary dates. They generously brought the original fossil with them, so that colleagues could examine it firsthand during the workshop portion of the conference.

I knew most of the conference participants, but Gabunia, Justus and Lordkipanidze seemed to have come out of nowhere, speaking of a site I couldn't find without an atlas. Gabunia is a quiet, silver-haired man who spoke in French so clear that even I understood the jaw's anatomy. Justus put the find in context, speaking in articulate English and looking even younger than she was. Lordkipanidze fell somewhere in the middle in terms of age and personality; his English was excellent, his enthusiasm palpable, and he was obviously knowledgeable. If they were even half right in what they were saying, this was a very important new find indeed.

Separated at Birth?

Like everyone else, I was eager to look at the jaw. I knew the African erectus specimens well, for my husband, Alan Walker, had co-directed the dig at Nariokotome, Kenya, which had yielded the most complete known skeleton of Homo erectus a few years before. (In 1991, as now, some researchers would call the Nariokotome and other early African specimens Homo ergaster to distinguish them from their presumed descendants in Eurasia, the fossils originally dubbed Homo erectus.) Whatever you wanted to call them, the specimens from Dmanisi and Nariokotome needed close comparison. When Gabunia put his fossil next to the cast of the Nariokotome jaw that Alan had brought, there was an almost visible spark of recognition. The two jaws were not just similar; they might have come from twins. Impulsively, Alan gave Gabunia the Nariokotome cast to take home with him, knowing there was none in the Republic of Georgia.

From then on, I was convinced the new jaw was Homo erectus and probably a very old one. The morphology, the date, the fauna were all right. No other Eurasian site had yielded hominids (human ancestors) anywhere near as old as the Dmanisi jaw; most were less than half a million years old. Only in Africa were there hominids dated to more than 1 million years, and the oldest Homo erectus (or ergaster, for those who preferred that term) in Africa was about 2 million years ago. By about 1 million years, Homo erectus had massively expanded its geographic range and was found in Java, somewhat later in China and later still in Europe. What propelled Homo erectus out of Africa into such a stunning dispersal? And why was there a time lag of almost 1 million years between the species' first evolutionary appearance in Africa and its invasion of Eurasia? It was an intriguing mystery.

In 1989, my husband and I had tackled this problem in a paper published in the Journal of Human Evolution. We interpreted Homo erectus's expanded brain size, increased body size and powerful strength relative to those of previous hominids as evidence that Homo erectus had a strikingly different diet from its predecessors. Earlier hominids were largely or exclusively vegetarian; we hypothesized that Homo erectus was the first efficient, regular hunter in human evolution. Only consistent access to very high-quality food would have enabled erectus mothers to bear and raise offspring with such nutritionally expensive characteristics.

Diets and Dates

To test our hypothesis, we made a prediction based on the energetic and ecological "rules" that govern the animal kingdom. If Homo erectus had indeed undergone a dietary shift from a plant- to an animal-based diet, then the ecological consequences of becoming a predator should be visible in the fossil record. The most obvious repercussion of this dietary shift would have been a density dilemma. Predators must be much more rare (less densely distributed across the landscape) than their prey. Violate this principle and you, as predator, risk starvation. There are two basic solutions to this problem. First, predators may, if time permits, evolve smaller bodies that require fewer or smaller prey. Second, the predators may lower their population density if they greatly expand their territory, in which case their depredations are spread across a wider range of prey populations. Could we see one of these solutions in the fossil record? Yes, gratifyingly, we could, for by spreading out of Africa across the Old World, Homo erectus had behaved just exactly as a newly predatory species ought to.

What we could not explain or understand was the troublesome time lag before the geographic expansion took place. Other colleagues suggested that perhaps Homo erectus was confined to Africa until some technological breakthrough occurred, the favorite being the invention of the Acheulian tool culture. We didn't like this idea much, for there is no obvious functional property of Acheulian tools that makes them superior to the earliest Oldowan tools, but we had no better alternative.

Thus, when Gabunia and Justus presented their finds at Senckenberg, every listener in the audience knew that the Homo erectus lineage hadn't gotten out of Africa until 1 million years ago. Many scholars concluded that the Dmanisi jaw was not Homo erectus but a later species of Homo and figured that the date was wrong. What should have made us all suspicious was that proof that Homo erectus was confined to Africa prior to 1 million years ago was nothing more than a flimsy absence of evidence of the species in Eurasia.

I wonder, too, whether the identity of those presenting the work contributed to the general skepticism. Had three of the well-known and highly respected leaders of the field announced the Dmanisi finds, the general response might have been more favorable. As it was, Justus was only a student, and students are notoriously prone to oversell the importance of their finds; Gabunia and Lordkipanidze were mature scientists, but they had no reputation in paleoanthropology as far as Western Europeans and Americans were concerned.

Instead of focusing on the new Dmanisi material, most of the participants at the Senckenberg conference got drawn into a flaming debate over the taxonomic status of Homo erectus started by Milford Wolpoff of the University of Michigan, Alan Thorne of the University of Canberra and their colleagues. They argued forcefully that Homo erectus had no validity as a species and should be eliminated altogether. All members of the genus Homo, from about 2 million years ago to the present, were one highly variable, widely spread species, Homo sapiens, with no natural breaks or subdivisions. The subject of the conference, Homo erectus, didn't exist. It was a radical suggestion.

Tempers flared and voices grew loud. One European, shocked by the vehemence, said quietly to me that, in his country, such insults would be resolved with pistols at dawn.

Skulls

Although the Dmanisi jaw and its significance were largely overshadowed in 1991, excavations continued. In the summer of 1999, David Lordkipanidze sent word that there was something new and special from Dmanisi: "Skulls," he said enigmatically. We waited eagerly for more information. In May of 2000, a wonderful new paper appeared in Science by Gabunia and a host of colleagues, including Justus, Lordkipanidze and the German researchers who had worked with them from the beginning. Enlarging the team were two Americans—Carl Swisher III, a dating specialist, and Susan Antón, an expert on the skull of Homo erectus—and Marie-Antoinette de Lumley, a renowned archaeologist from the Laboratoire Museum National d'Histoire Naturelle.

Skulls it was. The paper announced two new skulls of Homo erectus from Dmanisi, one very complete and the other a partial skull missing the face. Anatomically, these specimens were very similar to the older, African specimens like Nariokotome, which the Dmanisi team called Homo ergaster, meaning that ergaster was no longer a strictly African form. The antiquity of Dmanisi was now firmly established at 1.7 million years, based on state-of-the-art radiometric and paleomagnetic studies by Swisher and colleagues; the date was supported by additional study of the faunal material. Finally, more than 1,000 stone artifacts excavated from Dmanisi confirmed that the tools were part of the Oldowan (or Mode I) culture.

This time, the new Dmanisi discoveries were widely hailed by the media who garnered many catchy quotes from major figures in paleoanthropology. "Fossil signs of first human migration are found," The New York Times cheered. "This has doubled again the age of humans in Europe, or at least at the gates of Europe," declared Giacomo Giacobini of the University of Turin, echoing an endorsement by Ofer Bar-Yosef of Harvard University, who has visited Dmanisi. "As soon as Homo erectus evolves in Africa, they're out," remarked Walker to a reporter. Ian Tattersall of the American Museum of Natural History added, "these guys [were] moving very, very fast." The million-year time lag simply evaporated, leaving in its place an impressively rapid outward dispersal from Africa.

Why are the claims for Dmanisi accepted now, when they were not in 1991? For one thing, the evidence itself is stronger. Skulls are more readily identifiable to species than are jaws when hominids are at issue. Bringing new experts onto the team, well known for their work in dating, morphology and archaeology, has also enhanced the credibility of the work.

Since 1991 the field's focus has shifted. The move to eliminate Homo erectus is largely defunct, and many anthropologists use Homo ergaster as an informal shorthand for "the earliest part of the evolving ergaster/erectus lineage." Moreover, the simple dichotomy that once linked early Africa-Oldowan and contrasted that complex with the late Eurasian-Acheulian has been dismantled. In 1994, Carl Swisher and colleagues produced evidence that the Javan Homo erectus sites may range in age from as much as 1.8 million years to roughly 50,000 years, making them both younger and older than previously thought. Scrappy fossils that may be Homo erectus have been found in China, too, dating to about 1.9 million years. Thus, "early" no longer implies "African." Similarly, though Oldowan tools once suggested great antiquity, they too have been found at younger sites, such as Gran Dolina at Atapuerca, Spain, some 780,000 years ago, whereas the oldest Acheulian sites in Eurasia are now as old as 1.5 million years. A whole series of finds and analyses has contributed to a new paradigm that makes the Dmanisi finds more palatable.

This episode offers an important lesson about how science is done. When we scrutinize a colleague's work, we try to make an objective judgment. We evaluate the work against the holy grails of Replicability and Causality, but these are largely unattainable goals, at least for those working with fossils. Like most scientists, we tend to accord an extra dollop of credibility to studies conducted by colleagues known to have done reputable work.

But should the work of the young or the less known be held to higher standards than that of the great matriarchs and silverbacked males of the field? Skepticism is a cheap stance to adopt, for it is easier to cast doubt than to substantiate, especially if new techniques and new paradigms must be forged along the way. Science is a process of discovery, not confirmation. Let us allow for the occasional, delicious surprise that makes us rethink all we thought we knew.

© Pat Shipman