Yunxian skull 2. Photo: Gary Todd
After digital reconstruction, a nearly one-million-year-old Yunxian skull reveals an unexpected transitional form between early and modern humans.
Digital reconstruction of a heavily distorted skull fossil from China suggests that it belongs to an early branch of a group of hominins known as the Homo longi clade, which would be the sister group of modern humans (Science, 26 September).
Much of what is known about human evolution is based on fossil skulls. Fossil evidence shows, among other things, that during the Middle Pleistocene (773,000 to 129,000 years ago) multiple Homo lineages with divergent physical characteristics existed alongside one another. The Yunxian skulls from China date to nearly one million years ago and display a remarkable mosaic of primitive traits and features reminiscent of modern humans. Nevertheless, the distortion and compression of one of the skulls, Yunxian 2, has meant that scientists have been debating the relationship of these fossils to Homo sapiens for over thirty years.
Brow ridge
Using CT scans, researchers have now established that the damage to Yunxian 2 consists primarily of fractures and displacement of fragments, rather than plastic deformation. After digital correction, the skull resembles the early hominin Homo erectus in some respects – a thick brow ridge and a low, elongated braincase – but lacks other typical features of that species. The large brain volume and narrow inter-orbital distance of Yunxian 2, by contrast, are reminiscent of H. sapiens and other later Homo lineages.
Sister group
Given the combination of traits from earlier human species and more derived characteristics, the authors conclude that Yunxian 2 represents a transitional form between H. erectus and later Asian fossils. Further analysis shows, in their view, that most of these fossils – including the Yunxian skulls – share a common ancestor and moreover form the sister group of the sapiens clade. This sister group, which they term the longi clade, most likely also includes the enigmatic Denisovan: a relatively recently recognised human species that, like the Neanderthal (Homo neanderthalensis), has left traces of its DNA in present-day populations and to which scientists were only able to attribute a skull fossil a few years ago.
The Yunxian skulls from China date to nearly one million years ago
Although science previously regarded Neanderthals as the closest relatives of modern humans, the research team now puts forward a different scenario: according to them, H. sapiens diverged from H. neanderthalensis not 0.5 to 0.7 million years ago, but around 1.38 million years ago. The split between H. sapiens and the longi clade occurred later, at around 1.32 million years ago according to the researchers, which would mean that this group of hominins is the closest to modern humans.