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Skulls with mix of traits shine light on human evolution

This is Skull 17 from the Sima de los Huesos site in Sierra de Atapuerca, Spain.
[Image © Javier Trueba / Madrid Scientific Films]

Researchers have analyzed the biggest collection of ancient human fossils ever recovered from a single excavation site. Their study in the 20 June issue of the journal Science sheds light on the origin and evolution of Neandertals, an extinct species of human.

The researchers identified both Neandertal-like features and features associated with earlier humans in the fossils. This mixed pattern supports a theory of Neandertal evolution that suggests Neandertals developed their distinctive jaws and teeth and other fatures separately, at different times, not all at once as some scientists have thought. In other words, human evolution didn't proceed through a slow process of change with just one kind of human, or hominin, quietly evolving towards the classic Neandertal.

Having this new data from the Sima de los Huesos site, as the Spanish cave site is called, has allowed scientists to better understand hominin evolution during the Middle Pleistocene, a period in which the path of hominin evolution has been controversial.

About 400 to 500 thousand years ago, in the heart of an era called the Pleistocene in which ice repeatedly cover big stretches of land, ancient humans lived in Africa and East Asia. Some split off, ultimately settling in Eurasia (land including both Europe and Asia). There, they evolved characteristics that would come to define the Neandertal line.

Several hundred thousand years after that, modern humans—who had evolved in Africa—settled in Eurasia, too. They interbred with Neandertals, but breeding wasn't always successful because the two groups had changed over time. Because of their differences, modern humans—whose differences helped them be more successful—eventually replaced Neandertals.

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

To answer these questions, scientists have needed an accurate picture of European populations around 400,000 years ago, the early stages of the Neandertal lineage. This has been challenging to obtain, however, because the European fossil record is incomplete.

Samples at the Sima de los Huesos site, however, are different, representing a unique bunch of hominin fossils from a single species, and including 17 skulls, many of which are very complete. Some have been studied before, but seven are presented anew here, by a team led by Juan-Luis Arsuaga, Professor of Paleontology at Madrid's Complutensis University.

Studying these intact samples, the researchers observed a pattern: There were Neandertal features in the face and teeth, but not in other parts of the skull. Many of the Neandertal-like features were related to chewing.

The work of Arsuaga et al. helps address hypotheses about Neandertal evolution, specifically the accretion model hypothesis, which suggests that Neandertals evolved their defining features at different times. This study suggests that facial changes were the first step.