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.