Excavations of a complex of caves in the Sierra de Atapuerca in northern Spain have unearthed hominin fossils that range in age from the early Pleistocene to the Holocene1. One of these sites, the ‘Sima de los Huesos’ (‘pit of bones’), has yielded the world’s largest assemblage of Middle Pleistocene hominin fossils2, 3, consisting of at least 28 individuals4 dated to over 300,000 years ago5. The skeletal remains share a number of morphological features with fossils classified as Homo heidelbergensis and also display distinct Neanderthal-derived traits6, 7, 8. Here we determine an almost complete mitochondrial genome sequence of a hominin from Sima de los Huesos and show that it is closely related to the lineage leading to mitochondrial genomes of Denisovans9, 10, an eastern Eurasian sister group to Neanderthals. Our results pave the way for DNA research on hominins from the Middle Pleistocene.The authors argued at the time that Denisovans and the population from Sima de los Huesos shared a common ancestor, with a split of about 700 kya, and that both are more closely related to each other than either is to Neandertals. This strongly suggests that Neandertals are a later derivation from a common crown group that represented the progenitors of all three groups. The authors suggest that these findings are unexpected because of the incipient Neandertal traits found in the Sima collection, including occipital bunning in the back of the head and some mid-facial prognathism (it looks like the maxilla is puffed out). Arsuaga and others have stated previously that they thought all of the Sima de los Huesos material belonged to the Neandertal lineage. That, apparently, is not the case. As these authors point out, there may have been more than one evolutionary lineage roaming the valleys and forests of Europe.
Now the nuclear DNA has been sequenced and these results strongly suggest that the people represented by the SH population were, in fact, early Neandertals. From the paper by Meyer et al:
The nuclear DNA sequences of femur AT-5431 and the incisor show that they belonged to the Neanderthal evolutionary lineage, and the limited data available for the molar suggest that the same is true for this specimen. Thus, the results show that the SH hominins were early Neanderthals or closely related to the ancestors of Neanderthals after the divergence from a common ancestor shared with Denisovans. Although it is difficult to determine the age of Middle Pleistocene sites with certainty, geological dating methods1, as well as the length of the branches in trees relating the mtDNAs from femur XIII and an SH cave bear to other mtDNAs2, 12, suggest an age of around 400,000 years for the SH fossils. This age is compatible with the population split time of 381,000–473,000 years ago estimated for Neanderthals and Denisovans on the basis of their nuclear genome sequences and using the human mutation rate of 0.5 × 10−9 per base pair per year7
These authors now suggest that, in combination with the MtDNA results from last year, that the SH hominins are ancestral to Neandertals and that the MtDNA found in later Neandertals is derived relative to that in the SH hominins and that the SH hominins may have had several different MtDNA strands.
It appears that the evidence supports a model in which the classic Neandertals from France and Germany, which are dated to between 50 and 70 kya, represent a more stabilized, homogenous genome relative to that of earlier hominins. I have always been struck by the similarities between the Atapuerca 4 cranium and Petralona, from Greece, and this suggests that there may have been extensive migration between regions and considerable gene flow between groups (J. Lawrence Angel, again). We know that the range of Neandertals extended from the lower Iberian peninsula to Teshik-Tash, north of the Taurus Mountains and Okladnikov, in Siberia. It does not seem like a stretch that their precursors did the same thing.
Hat tip to Todd Wood.