Science & technology | Peopling of the New World

The first humans in America may not have been Homo sapiens

Pre-prehistoric man

WHEN did the first human beings arrive in the Americas? Though there are arguments about the details, the consensus is that it was around 15,000 years ago, when retreating glaciers at the end of the last ice age permitted travellers from Asia to cross what is now the Bering strait but was then dry land.

This makes sense. The evidence suggests that, recent migrants from Africa and their progeny aside, people now alive in Asia, Australia, Europe and the Americas are descended from a handful of Africans who left the continent of their birth about 70,000 years ago. This fits nicely with the conventional date for America’s colonisation, by giving time for the heirs of these African émigrés to make it to eastern Asia, ready for the hop to the New World when conditions permitted.

What, then, to make of a discovery, reported in this week’s Nature, by Thomas Deméré of the San Diego Natural History Museum and his colleagues? They have just dated an archaeological site found in California in 1992, which seems to be a place where human beings used stone tools to dismember a mastodon, a now-extinct type of elephant. Unfortunately for existing theories, the age Dr Deméré and his associates have come up with for this site is 130,000 years—a time when Homo sapiens was confined to Africa.

The Cerutti mastodon site, as the place is known, is near San Diego. It is named after its discoverer, Richard Cerutti, who is one of Dr Deméré’s co-authors on the paper. It contained a lone mastodon skeleton and five large cobblestones. The whole area appears to have been buried more or less intact by sediment from a stream. The cobbles are far larger than any other stones in the sediment. Marks they bear, and fragments found nearby that seem to have flaked off them, suggest they have been used as tools.

Two of the tools seem to have been anvils, and three hammerstones. Their purpose, judging from the condition of many of the mastodon’s bones, which have been shattered in ways that suggest they have been hit hard and deliberately, and fragments of which (such as the two detached femur heads pictured above) are clustered around the putative anvils, was to break those bones. This might have been done to extract the nutritious marrow inside, or to use the bone-fragments themselves to make further tools. Indeed, Dr Deméré and his colleagues have conducted experiments on a modern elephant skeleton that help confirm this interpretation.

All of this was interesting 15 years ago, when the site was discovered. The stone tools found are similar to those used over 1m years ago in Africa, by Homo erectus, an ancestor of Homo sapiens, and dissimilar to the precisely crafted tools of the Clovis culture typical of other early-human discoveries in North America, a fact which has long been a source of speculation about the true nature of the Cerutti mastodon site. Unfortunately, no organic material remains in the bones, so they cannot be radiocarbon-dated.

It is this lack of a reliable date which the new paper addresses. A second attempt, made a few years ago using a method called optically stimulated luminescence to examine some of the site’s sediment, hinted that it was at least 60,000 years old. Dr Deméré and his colleagues therefore brought a third technique, uranium-thorium dating, to bear on the matter. They used this to date fragments from several of the mastodon’s bones. All agreed it had died about 130,700 years ago, give or take 9,400 years. If the cobbles at the site really are stone tools, then, the history of America’s colonisation by early man will have to be rewritten.

A mammoth conclusion

There were indeed human beings outside Africa 130,000 years ago, but they were not Homo sapiens. Europe was populated by Neanderthal man, Homo neanderthalensis. Parts of Asia were inhabited by a recently discovered (and, as-yet not formally named) species called the Denisovans. Fossils of Homo erectus are known from China, Indonesia, India and Georgia—and though most of these remains are clearly older than 130,000 years, some researchers believe the species was still around then. On top of all these widespread species, moreover, the island of Flores, also in Indonesia, was home to a type of dwarf human, Homo floresiensis, only a little after the period in question.

The date Dr Deméré has come up with is propitious, too. It coincides with the last interglacial warm period before the present one—a time when a crossing from Asia to America would not have been blocked by sky-high walls of ice. That the arrival of modern man in the New World was anticipated by more than 100,000 years, by an earlier species, is by no means unlikely.

Concluding that the Cerutti mastodon site was a butcher’s shop does, though, depend on the five cobblestones in question actually being tools. By itself, a 130,000-year-old skeleton proves nothing. Dr Deméré’s arguments that the fragmentation patterns of the mastodon’s bones and the ways those bones are gathered around the putative anvils both indicate deliberation, and that the flakes from the cobbles were caused by hammerstones hitting those anvils, are persuasive, but not probative. Settling the matter would require some bones from early humans themselves to turn up.

These findings do, however, shine a spotlight on claims of greater antiquity than 15,000 years that have been made in the past for a few other sites in the Americas, notably the Calico Hills, also in California, and Pedra Furada, in Brazil. Nothing unarguably as old as the Cerutti mastodon skeleton has yet been unearthed in these places, but the dating of that skeleton should prompt renewed investigation, and also a search for other possible sites.

As to the fate of any pre-aboriginal Americans, that would be pure speculation. Suffice to say that the Neanderthals, the Denisovans and the “hobbits” of Flores did not long outlast the arrival of Homo sapiens in their respective necks of the woods. Any cousins these species did have in the Americas would be unlikely to have fared better.

This article appeared in the Science & technology section of the print edition under the headline "Pre-prehistoric man"

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