Million-year-old skull discovery rewrites the story of human evolution

A deformed human skull discovered over thirty years ago in central China is now upending what researchers believed they understood about our origins. The fossil, named Yunxian 2, was extracted from the Hanjiang River terrace in Hubei Province in 1990.

Its distorted form has prevented scientists for years from definitively positioning it on the human family tree. Now, due to the recent scanning and digital reconstruction, the skull, according to scientists, may rewrite the history of modern humans by hundreds of thousands of years.

When initially found, Yunxian 2 was presumed to be an early human ancestor like Homo erectus, the lineage that is well known for migrating out of Africa and walking on two legs with a human-like body. The age of the fossil—between 940,000 and 1.1 million years—appeared to be consistent with that designation. Yet fresh examination indicates something much more unexpected.

The reconstructed cranium through digital means showcases a combination of traits that don't match nicely into a single species. It contains the primitive characteristics like the heavy brow ridge, wide base and palate, and elongated low braincase. It also lacks the steeply sloping back of the skull of Homo erectus and the occipital "bun" of the Neanderthals. Its face juts forward like that of Homo erectus but less, with broad cheekbones and a nose that juts outward.

Measurements put its brain capacity at roughly 1,143 cubic centimeters—larger than earlier Homo erectus but smaller than later Neanderthals and modern humans. In short, Yunxian 2 seems to be a link between earlier and later species, a snapshot of human evolution in action.

To determine where Yunxian 2 belongs, researchers compared its shape with more than 170 other modern and fossil human skulls. Using geometric morphometrics, which maps thousands of shape differences between bones, they positioned Yunxian 2 with Asian fossils such as Dali, Jinniushan, and the giant Harbin skull.

These include what scientists have now dubbed the "longi clade," in honor of Homo longi, or "Dragon Man." This clade includes the mysterious Denisovans—extinct humans recognized mostly from DNA extracted from a Siberian cave—and is the evident sister group to Homo sapiens.

The study estimates that the longi clade split from the line that would eventually give rise to modern humans around 1.32 million years ago. That would make Yunxian 2 one of the group's oldest known members, close to the common ancestor of our species and our closest relatives.

For decades, scientists have grappled with the so-called "muddle in the middle"—the Middle Pleistocene period between 770,000 and 126,000 years ago, when the fossil record shows a confusing range of hominins. Skulls unearthed in as far-flung places as South Africa, Greece, Ethiopia, and the Philippines all show a jumble of traits. Some were designated as new species, but others were not classified.

By placing Yunxian 2 and other Asian fossils in the longi clade, researchers argue that many of those mysterious remnants are nothing other than different branches of three major groups: Homo sapiens, Homo longi, and Neanderthals. If that is the case, then these three major lineages would have shared the world for nearly a million years—far longer than has been thought.

"Human evolution is a tree," said Prof. Xijun Ni of Fudan University, who led the research along with Chris Stringer of the Natural History Museum in London. "This tree had many branches, and there were three large ones which co-existed at the same time for almost a million years. So this is an amazing result."

What made this possible was technology. The Yunxian skulls were severely crushed when they were first found, so early reconstructions were distorted and hard to decipher. By making high-resolution CT scans and utilizing digital modeling, researchers digitally uncrushed the fossil, realigned pieces, and even 3D-printed copies. The process revealed features that had been hidden for decades.

The team validated their results by applying a number of models and attempting different ways of reconstructing the skull. In every case, Yunxian 2 still arrived at the same result: it does not belong to Homo erectus but instead falls in a separate line that likely gave rise to Denisovans.

Although the findings were hailed as a breakthrough, not everyone is leaping to rewrite the textbooks. Dr. Aylwyn Scally, an evolutionary geneticist at the University of Cambridge, urged caution. "One has to be particularly careful about the time estimates," he said. "Even with the most amount of genetic data, it's very difficult to put a time when these populations could have co-existed to within 100,000 years, or even more."

However, the study concurs with genetic evidence showing that humans were splitting into different lineages earlier than has been thought. The earliest Homo sapiens fossils in Africa are about 300,000 years old. If Yunxian 2 is actually an early representative of the Homo longi clade, then our own species could have emerged about a million years ago.

If correct, the implications are massive. The split between modern humans and our sister species would have happened far earlier than previously believed. That would leave Homo sapiens, Neanderthals, and Denisovans potentially existing on the planet together for up to 800,000 years. Such extensive overlap would have created ample opportunity for encounter, migration, and inter-mixture.

Chris Stringer, a researcher who has spent decades studying human origins, described the discovery as a dramatic twist. "From the moment we got the result, we couldn't believe it," he said. "But we did it again and again … and we're now confident of the result."

The reconstruction of Yunxian 2 gives scientists a clearer picture of one of the most baffling ages of human evolution. By showing what modern humans and their direct predecessors may have split more than a million years ago, it reshapes our knowledge of how we originated.

This revised timeline can account for perplexing fossils that could not be conveniently categorized, and it supports the case for Asia being a main stage of human evolution, rather than just Africa and Europe.

To you, this research highlights that our ancestry is not a straight line but a branched, interlinked history. It illustrates how quickly new technology can rewrite the past, and it suggests that many more discoveries—maybe even million-year-old Homo sapiens fossils—are waiting to be made.

Research findings are available online in the journal Science.

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