The discovery of stone axes in the same sediment layer as cruder tools indicates that hominins with differing tool-making technologies may have coexisted.

The axes, found in Kenya by Christopher Lepre, a palaeontologist at Columbia University in New York, and his team are estimated to be around 1.76 million years old. That's 350,000 years older than any other complex tools yet discovered.

The finding, published August 31 in Nature, includes another important discovery: the hand axes, usually associated with the emergence around 1.5 million years ago of Homo erectus as the dominant hominin species, were found alongside primitive chopping tools that had already been in use for at least a million years.

"This supports the idea that the two earliest stone-tool manufacturing techniques and traditions were, at least sometimes, utilized contemporaneously," says palaeoanthropologist Briana Pobiner at the Smithsonian Institution in Washington DC.

Chip off the old block
The hand axes, which have a distinctive, carefully made oval shape, are part of the Acheulian technology — those tools thought to have been developed around 1.6 million years ago. The more primitive tools, typically chunks of stone with crudely-chipped edges, belong to the earlier Oldowan toolkit. Because H. erectus is often associated with Acheulian tools, Lepre and his colleagues suggest that the hand axes they found might have been made by H. erectus, and the Oldowan tools by the less cognitively-capable Homo habilis.

The proposal could help clear up confusion over palaeontological observations of hominin departures from Africa. H. erectus is often thought to have been mentally and behaviourally most capable of migrating, but early fossil sites in Eurasia almost always reveal hominins with Oldowan toolkits, or no tools at all.

Lepre's findings indicate that multiple hominin groups with differing tool-making abilities could have dispersed to Eurasia, and that H. erectus either didn't go first, or didn't take Acheulian tools with them. "This really nicely explains the fossil sites that we have been finding in Georgia and the Far East that just don't have the Acheulian tools," says palaeoanthropologist James Adovasio at the Mercyhurst Archaeological Institute in Pennsylvania.

"From a stone-tool perspective, all the evidence points to the idea that Oldowan hominins were the first to leave Africa," says Lepre.

Which species is responsible for the spread of Oldowan tools is still open for speculation, however. "This does not necessarily point to Homo habilis, nor does it rule out the possibility that an Oldowan-using Homo erectus group might have been involved," Lepre says.

"It might have been that a single species was capable of making both kinds of tools but that factors like what raw materials were available and what tasks needed to be conducted with the tools governed which types of tools were made," says Pobiner.

This article is reproduced with permission from the magazine Nature. The article was first published on August 31, 2011.