Chris Stringer: The tip of the iceberg in understanding of human history

The remarkable recovery of an ancient mitochondrial DNA lineage in a human fossil from Denisova cave in Siberia could really be the tip of the iceberg in terms of understanding human origins, particularly in this region of the world. Asia covers a large area and we have a pretty poor idea of who was there or how they were related to the rest of the human family.

We already have a wide collection of human fossils in Asia, yet we have little idea of how they are related to one another, or to fossils from other parts of the world. This is the first real line of evidence we can use to try to relate some of these people to us, to the Neanderthals and to Homo erectus, the first species to emerge from Africa about 1.75 million years ago.

The suggested divergence date of the Denisova lineage, around one million years ago, appears to be too late for it to be a descendant of the first H. erectus to migrate out of Africa, and too early for it to be a descendant of Homo heidelbergensis, the possible ancestor of the Neanderthals and of modern man, if that species originated only about 650,000 years ago.

However, the species Homo antecessor ("Pioneer Man"), known from sites at Atapuerca, Spain, was known to exist approximately 0.8 million to 1.2 million years ago, closely matching the age estimate for the origin of the Denisova lineage. Nonetheless, recent research has suggested that H. antecessor evolved from early Asian H. erectus, in which case its origins would also be too ancient, compared with the mystery Denisova lineage.

The first question to ask is whether the origin of the Denisova lineage, the "coalescent estimate" of between 779,000 years ago and 1.3 million years ago, is accurate. This is a complex issue as the calculation depends on several assumptions, and is in turn based on another estimate, that the chimp and human lineages split at about 6 million years ago. If the Denisova lineage is in fact younger, it could represent an early off-shoot of the H. heidelbergensis lineage, after all.

Certain enigmatic Asian fossils dated between 250,000 and 650,000 years ago, such as Narmada in India and the Chinese fossils at Yunxian, Dali and Jinniushan, have also been considered as possible Asian derivatives of H. heidelbergensis. So they too are potential candidates for this mystery non-erectus lineage.

However, there are other, younger, fragmentary fossils such as the Denisova ones themselves, and partial skulls from Salkhit in Mongolia and Maba in China. These have been difficult to classify. This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia.

Another intriguing question is whether there might have been overlap and interaction not just between Neanderthals and early moderns in Asia, but also between either of those lineages and this newly recognised one at Denisova. The distinctiveness of the mitochondrial DNA so far suggests little or no interbreeding occurred, but we won't know till we have further data from other parts of the nuclear DNA from the fossils.

The writer is head of human origins at the Natural History Museum in London