Prof Chris Stringer, Natural History Museum
Traditionally, the evidence to reconstruct our evolutionary history has come from the prehistoric evidence of artefacts and fossils. But we also have an evolutionary history within us, locked up in the genetic code of our DNA. The last decade has witnessed remarkable developments in our ability to study that record, even in the fossils themselves.
DNA research 25 years ago demonstrated that a particular form of our genetic material (mitochondrial DNA – mtDNA) could be traced back to a female ancestor who lived in Africa about 150,000 years ago, so-called ‘mitochondrial Eve’. This provided strong support for the idea that Homo sapiens had evolved recently on that continent. But none of this research could be conducted on ancient materials such as the human fossils themselves. Then, in 1997, mtDNA was recovered from the type specimen of the species Homo neanderthalensis – the skeleton found in the NeanderValley in Germany in 1856. And in the last decade this has led on to a whole new field called palaeogenomics, which has allowed the reconstruction of whole genomes from two extinct humans: the Neanderthals and the Denisovans.
The former group was already well known from a range of fossils across Europe and western Asia, with evidence that they went physically extinct about 30,000 years ago. The latter group was completely unknown until teeth and a finger bone found in 2008 in DenisovaCave, southern Siberia, were analysed, producing a genome that is related to, but distinct from, that of Neanderthals (CWA 41).
Most of the fossil and genetic data up to 2010 pointed to an origin of the modern form of Homo sapiens, with our rounded skulls and complex behaviour, in Africa by 150,000 years ago. There was then a dispersal to the rest of the world beginning about 60,000 years ago, when other forms of humans outside Africa, such as the Neanderthals, were seemingly completely replaced.
But the latest genomic comparisons of fossil and modern humans suggest that living populations outside Africa actually contain about a 2% input of Neanderthal DNA, from at least one episode of ancient interbreeding, while populations in Australia and New Guinea contain an additional component (about 4%) of Denisovan DNA, from at least one further and more localised interbreeding event.
To learn more about the genes involved we need to compare the ancient genomes with those of living apes and humans, in order to pick up any differences. Distinct mutations in the melanocortin gene of Neanderthals and Denisovans suggest that some Neanderthals had red hair (CWA 27), while one Denisovan was a girl with dark colouring in her skin, eyes, and hair. Other comparisons show up areas where the Neanderthals and Denisovans share the same coding as apes, while modern humans share new mutations in regions known to code for features in the skeleton, the brain, and in personality.
Since the fossil genomes have only been available in the last few years much research remains to be done, but we are likely to learn some of the fundamentals of what made a Neanderthal, what made a Denisovan, and what makes a modern human a modern human. The effects of the Neanderthal and Denisovan DNA in living humans are still largely unknown, although most does not seem to be functional. However, there are suggestions that some differences in immune systems today could be due to a Neanderthal or Denisovan inheritance in non-Africans. This would not be surprising since more ancient populations outside Africa would have evolved immunities to local diseases. By interbreeding with the natives, modern humans could have rapidly acquired valuable defences against diseases that would have been completely new to them.