An ancient skull, warped and damaged by the ravages of time and degradation, may have just altered our understanding of the history of modern humans.
Using careful 3D scanning and digital reconstruction techniques, a team of researchers from China and the UK has rebuilt the damaged artifact, discovering exactly where it fits on the hominid family tree.
It's not the skull of a modern human ancestor, but that of a closely related human. Even so, its age pushes back the timeline for the divergence between the ancestor of Homo sapiens and its close relatives, suggesting that the origin of our species is several hundred thousand years older than we thought it was.
Related: 1930s 'Dragon Man' Finally Gives Elusive Ancient Human Species a Face
Two skulls, known as Yunxian 1 and Yunxian 2 (sometimes catalogued as EV 9001 and EV 9002), date back to around a million years ago.
Their survival at all is somewhat miraculous – few hominid remains of that vintage exist on the fossil record. However, time and fossilization are not kind, and the skulls were discovered (in 1989 and 1990, respectively) heavily damaged and distorted.
This made identifying their position on the hominid family tree particularly challenging. They were given the placeholder name "Yunxian Man" after the contemporaneous name of the district in which they were found.
However, in the decades since the skulls were excavated from the calcareous rock in which they had lain for millennia, archaeological tools have improved dramatically. Now, to study a fossil, scientists don't have to damage it further; instead, they can conduct high-resolution 3D scanning and perform their analysis using digital tools.
Of the two skulls, Yunxian 2 was the least distorted, so the researchers chose it as the best option for a digital reconstruction. They were able to effectively reverse the damage sufficiently to catalog and describe its features and determine how it fits into hominid history.
Interestingly, this analysis revealed that the skull has a mixture of both older and newer traits. It has a thick brow ridge and a long, low braincase, characteristics also seen in earlier hominids, such as Homo erectus. The base of its skull is broad, and its forehead is flat.
On the other hand, it also exhibits traits seen in later hominids. The size of its braincase was larger than that of Homo erectus (although still smaller than that of Homo neanderthalensis or Homo sapiens). It also lacks the strongly angled rear skull region seen in Homo erectus, and the shape of its face is flatter.
This mixture of traits suggests that the Yunxian Man is a link between Homo erectus, which came before, and the hominid species that were yet to come. The researchers believe that it fits in the Homo longi clade, a sister group to Homo sapiens, and the lineage to which the Denisovans are thought to belong.
The morphological traits of the skull, and its age, suggest all these species diverged from each other much sooner than we thought. According to previous estimates, modern humans and Neanderthals diverged around 500,000 to 700,000 years ago.
According to the new calculations, however, the splits occurred all within a very short timeframe of each other, starting around 1.38 million years ago, with Neanderthals peeling off first.
The Homo longi and Homo sapiens clades then diverged approximately 1.32 million years ago. Subsequently, Homo longi developed distinct traits around 1.2 million years ago, and Homo sapiens around 1.02 million years ago.
These are the points at which diagnostic traits first appear in the fossil record. For this reason, these later dates are what archaeologists call the origin point of a clade – which means Homo sapiens can now be traced back to over a million years ago.
"This changes a lot of thinking because it suggests that by one million years ago our ancestors had already split into distinct groups, pointing to a much earlier and more complex human evolutionary split than previously believed," physical anthropologist Chris Stringer of the Natural History Museum in the UK told The Guardian.
"It more or less doubles the time of origin of Homo sapiens."
This dramatic revision to the human timeline will require close scrutiny. If validated, however, the findings could help resolve some of the biggest mysteries about hominid evolution, including the "muddle in the middle", a confusing mess of puzzling fossils dating back a million years.
The research has been published in Science.