The Sima de los Huesos hominin, previously thought to belong to an ancient human species known as Homo heidelbergensis, is now reported to be an early member of the Neanderthal lineage.

500,000 to 400,000 years ago (Middle Pleistocene), archaic humans split off from other groups of that period living in Africa and East Asia, ultimately settling in Eurasia, where they evolved characteristics that would come to define the Neanderthal lineage.

Several hundred thousand years after that, modern humans settled in Eurasia, too. They interbred with Neanderthals, but even then showed signs of reproductive incompatibility. Because of this, modern humans eventually replaced Neanderthals.

The degree of divergence between Neanderthals and modern humans over such a short period of time has surprised scientists. Why did Neanderthals differentiate so quickly from other early hominins? What pattern of changes did Neanderthals undergo?

To answer these questions, scientists have needed an accurate picture of European populations around 400,000 years ago, the early stages of the Neanderthal lineage.

This has been challenging, however, because the European fossil record—an important tool for answering these questions—is isolated and dispersed, consisting of remains from disparate timelines. Samples at the Sima de los Huesos site in Atapuerca, Spain, however, are different.

“What makes the Sima de los Huesos site unique is the extraordinary and unprecedented accumulation of hominin fossils there; nothing quite so big has ever been discovered for any extinct hominin species – including Neanderthals,” said Prof Juan-Luis Arsuaga from the Complutense University of Madrid, who is the lead author of a paper published in the journal Science.

“This site has been excavated continuously since 1984. After thirty years, we have recovered nearly 7,000 human fossils corresponding to all skeletal regions of at least 28 individuals. This extraordinary collection includes 17 fragmentary skulls, many of which are very complete,” added study co-author Prof Ignacio Martínez from the University of Alcalá, Spain.

These skulls belong to a single population of a fossil hominin species, named the Sima de los Huesos hominin.

Some of them have been studied before, but seven are presented anew, and six are more complete than ever before.

With these intact samples at their fingertips, the anthropologists made progress characterizing defining features.

Their work has helped address hypotheses about Neanderthal evolution, specifically the accretion model hypothesis, which suggests that Neanderthals evolved their defining features at different times, not in a single linear sweep.

“For decades the nature of the evolutionary process that gave rise to Neanderthals has been discussed. An important question in these debates was whether the ‘neanderthalization process’ involved all regions of the skull from the beginning, or if, on the contrary, there were various stages in this process that affected different parts of the skull at different times,” Prof Martínez said.

The researchers’ skull samples showed Neanderthal features present in the face and teeth, but not elsewhere; the nearby braincase, for example, still showed features associated with more primitive hominins.

“We think based on the morphology that the Sima people were part of the Neanderthal clade, although not necessarily direct ancestors to the classic Neanderthals,” Prof Arsuaga said.

The Sima de los Huesos hominin is part of an early European lineage that includes Neanderthals, but is more primitive than the later Pleistocene variety.

Critically, many of the Neanderthal-derived features the researchers observed were related to mastication, or chewing.

“It seems these modifications had to do with an intensive use of the frontal teeth. The incisors show a great wear as if they had been used as a third hand, typical of Neanderthals,” Prof Arsuaga said.

The study suggests that facial modification was the first step in Neanderthal evolution. This mosaic pattern fits the prediction of the accretion model.

“One thing that surprised me about the skulls we analyzed is how similar the different individuals were. The other fossils of the same geological period are different and don’t fit in the Sima pattern. This means that there was a lot of diversity among different populations in the Middle Pleistocene,” Prof Arsuaga said.

Indeed, other European Middle Pleistocene Homo sapiens do not exhibit the suite of Neanderthal-derived features seen in this fossil group.

Thus, more than one evolutionary lineage appears to have coexisted during the European Middle Pleistocene, with that represented by the Sima de los Huesos sample being closer to Homo neanderthalensis.

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J. L. Arsuaga et al. 2014. Neandertal roots: Cranial and chronological evidence from Sima de los Huesos. Science, vol. 344, no. 6190, pp. 1358-1363; doi: 10.1126/science.1253958