Scientists have pieced together the oldest human DNA yet recovered after extracting it from the thigh bone of an individual who died 400,000 years ago.
The research throws up a new riddle in the story of human evolution, since it shows that the ancient Spaniard was related to early humans from Siberia.
The DNA belonged to a type of human whose fossils have been found in large numbers at La Sima de los Huesos – the “bone pit” – at Atapuerca in northern Spain.
The ancient cave site has yielded at least 28 skeletons. Although the species has been classified as Homo heidelbergensis, it also bears traits typical of Neanderthals.
Scientists were able to sequence almost the complete genetic code, or genome, from the creature’s mitochondria, tiny powerhouses in cells that generate energy and have their own DNA.
Mitochondrial DNA, or mtDNA, is only passed down from mothers and can be used to track lineages.
The study revealed a surprise. Instead of an anticipated link with the Neanderthals, researchers found genetic similarities with the Denisovans, an enigmatic eastern Eurasian group from Siberia.
About 40,000 years ago, Denisovans co-existed with Neanderthals and early modern humans, and the three may have interbred.
The findings, from a team led by Dr Matthias Meyer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, appear in the latest edition of the journal Nature.
Commenting on the discovery, British expert Professor Chris Stringer, from the Natural History Museum in London, said: “This unusual finding could be due to at least two different scenarios.
“One is that the mtDNA is derived from an ancient population, ancestral to both the La Sima fossil population and Denisovans, which has since been lost in lineages in Africa and western Eurasia.
“A second is that ancestral species interbred in Eurasia, passing over distinctive mtDNA which may have been lost later in the Neanderthal lineage, yet retained in the Denisovan branch.
“Either way, this new finding can help us start to disentangle the relationships of the various human groups known from the last 600,000 years.
“If more mtDNA can be recovered from the Sima ‘population’ of fossils, it may demonstrate how these individuals were related to each other, and how varied their population was.”