Pushing back DNA sequencing: a Spanish cave bear

At the time, only 3 years ago, publication of the first full Neanderthal genome  seemed miraculous. Yet the apparent magic proved repeatable, including for an obscure but distinct group of extinct humans – the  Denisovans – known only from their DNA in a single pinkie bone. These advances astonished the world by showing that anatomically modern humans were capable of interbreeding with both groups; and did so that many people now living outside of Africa carry the genetic evidence. But the samples analysed for DNA were little more than 40 thousand years old. Older fossils of extinct animals have given up their genetic features, such as the wooly mammoth and a horse about 700 ka old, but only from samples frozen into permafrost at high northern latitudes.

The degradation of DNA over time seemed destined to limit palaeo-genetics, even when slowed down by natural freezing. The degradation breaks down any surviving genetic material into shorter and shorter fragments of the DNA molecule, ultimately to its atoms being recombined in new molecules of totally unrelated compounds through the chemical processes of fossilisation. Reassembling the fragments correctly becomes increasingly difficult the smaller they are. Few outside of a highly skilled specialists were optimistic of breaking the 100 ka barrier, even using frozen fossils. Unsurprisingly, having had such dramatic successes, the specialists continue to ride their luck and their ingenuity.

Excavations at the site of Gran Dolina, in Ata...

Excavations at Gran Dolina, in Atapuerca, Spain. (Photo credit: Wikipedia)

The cave complex of the Atapuerca Mountains  in northern Spain, whose sediments range in age from almost a million years ago to recent times, contain rich accumulations of human remains, including the pre-Neanderthal Homo heidelbergensis and H. antecessor dating back to more than 800 ka. If ever there was a magnet for archaeo-geneticists Atapuerca is definitely one. Moreover, physical anthropologists seem never to stop disputing their interpretations. Jesse Dabney of the now famous Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and co-workers from Britain, New Zealand, Spain and Australia are now beginning to report results. The first are from a cave bear (probably Ursos deningeri) known to be older than 300 ka (Dabney, J. and 10 others 2013. Complete mitochondrial genome sequence of a Middle Pleistocene cave bear reconstructed from ultrashort DNA fragments in one of its foreleg bones. Proceedings of the National Academy of Sciences, v. 110, doi/10.1073/pnas.1314445110). The bear’s mitochondrial DNA was pieced together from fragments as small as 50 base pairs, and shows its ancestry to bears (U. spelaeus) from the later Pleistocene that became extinct at about 28 ka.

reconstruction of a European cave bear (Ursus ...

Reconstruction of a European cave bear (Ursus spelaeus) (credit: Wikipedia)

It may be only a matter of time before human DNA emerges from the rich Atapuerca fossil hoard; indeed the authors strongly hint that they are working on that now.


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