Oldest human ancestor DNA sequence

DNA from humans ancestors alive 400,000 years ago have had their mitochondrial DNA sequenced.

For the first time, researchers have extracted and sequenced the mitochondrial DNA from an incredibly ancient human ancestor, and it's offering up a whole new way of looking at human evolution.

The findings published this week in the journal Nature come from a team at the Max Planck Institute for Evolutionary Anthropology in Leipzig in Germany.

The ancient fossils come from Spain, at a site called Sima de los Huesos, the "pit of bones", which must be one of the most difficult places to work: to reach it the scientists had to crawl for hundreds of metres through a network of tiny dark tunnels, then drop down a 13m vertical shaft.

But it's because of their inaccessible location that the bones have been so well preserved. Humidity in the cave system is close to saturation and the temperature is a constant 10.6 degrees C, year round.  That no-one else had ventured that far into the cave, meaning the remains had not been disturbed, was also a factor.

Advances in sequencing techniques mean it's now possible to reconstruct mitochondrial DNA from such ancient, degraded samples. Previously it was only possible to get such ancient DNA from samples preserved in permafrost.

Based on their physical shape, it was thought that the fossil bones came from Neanderthals, or at least a close relative. But the DNA sequence revealed the fossils are in fact closely related to another, highly mysterious group of early humans called Denisovans, which until now were only known from a fragment of DNA in a 41,000 year old finger bone from Asia. These were thought to be the Asian "cousins" to European Neanderthals.

Now the question is how did the Denisovan DNA end up in hominins in Spain?

There are various possible explanations. The "pit of bones" fossils could represent a group distinct from both Neanderthals and Denisovans. Another more likely scenarios is that they could be related to the ancestors of both Neanderthals and Denisovans. Or, it could be a case of inbreeding between "the pit of bones" people and another group of as-yet unidentified early humans that brought Denisovan DNA in from Asia.

There are problems with using mitochondrial DNA - for example, it's now know that interbreeding did take place between Neanderthals and modern humans, but there is no sign of that left behind in our mitochondrial DNA today.

What researchers really need now is to sequence some nuclear DNA from these "pit of bones" people. That could help clear up this muddled picture of early human evolution.