Read more: “Instant Expert: Fossils“
In the past decade, techniques for retrieving DNA from ancient remains – whether petrified, frozen, mummified or otherwise preserved – have undergone a technological leap, giving us a new window onto prehistoric worlds. Now we can find out the eye colour of Ötzi the Ice Man, trace the movements of mammoths around the globe, and discover whether our ancestors interbred with other hominins. As with any evidence from fossils, interpreting the messages in ancient DNA can prove difficult.
Take the much-publicised case of Neanderthal DNA. In 1997, Svante Pääbo of the Max Planck Institute for Evolutionary Biology in Leipzig, Germany, and colleagues announced that they had extracted 379 base pairs from mitochondrial DNA in a Neanderthal arm bone. Comparing the mtDNA with the corresponding sequences from almost 1000 modern humans, they concluded that Neanderthals contributed nothing to modern human mtDNA. By 2008, fragments of mtDNA from 16 other Neanderthals had yielded similar results. That same year, a complete Neanderthal mtDNA genome consisting of 16,565 base pairs seemed to seal the issue. There was no evidence of interbreeding between Neanderthals and modern humans.
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Then, in 2010, Pääbo and colleagues published a draft sequence of the Neanderthal’s nuclear genome – 4 billion nucleotides – based on three individuals. When they compared this with the genomes of five modern humans, they found that people of non-African origin had inherited between 1 and 4 per cent of their genes from Neanderthals. They interpreted this as evidence that the two species interbred around 45,000 years ago, shortly after early modern humans left Africa.
Not everyone agrees, however. Some researchers have argued that the evidence can be interpreted in other ways that would uphold the conclusion that humans and Neanderthals did not interbreed (¿ìè¶ÌÊÓƵ, 24 March, p 34). Larger samples would help resolve the ambiguity in the evidence.
The fossil record will always be open to interpretation, but the more we learn about analysing ancient DNA the better we will become at extracting information from it. With such advances, these are exciting times for the study of fossils, particularly those that hold secrets about our own evolution.