The skulls of Yunxian 1 and 2 were distorted after millennia underground.

Credit: Alamy

The Yunxian 3 skull is half-buried in an upright position. Researchers have uncovered the forehead, including the brow ridge and eye sockets, as well as the top, back and left cheekbone of the skull. It is not yet known whether teeth or a lower jawbone are attached to the skull, says Gao Xing at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, who is leading the excavation.

Vialet says that the Yunxian 1 and 2 skulls share some features with older Javanese fossils, and others with younger Homo erectus fossils from mainland Asia. Like the Javanese fossils, they are large, big-brained skulls. But she says that they are less heavily built, a characteristic that usually indicates a more modern individual.

Researchers have found Homo erectus remains at more than a dozen sites across China. Vialet says that the ancient humans at Yunxian could be the ancestors of some of these populations, but their skulls bear distinct features that set them apart.

For example, fossils from around 700,000 years ago that were discovered in the Zhoukoudian cave system in suburban Beijing — known as the Peking Man Site — have a prominent sagittal keel, a crest that runs along the midline of the skull for the attachment of strong jaw muscles. The Yunxian skulls all seem to lack this feature, says Vialet.

Variable fossils

Yameng Zhang, a palaeoanthropologist at Shandong University, says that the Homo erectus fossils found in China are highly variable and researchers don’t know why. It could be that each population evolved independently in Asia. Or they could have been the result of multiple waves of expansion out of Africa, he says. “More complete Chinese H. erectus like Yunxian 3 are crucial to answer this question.”

Vialet says that the Yunxian 3 skull should be compared with Chinese as well as European hominin fossils, such as the 1.4-million-year-old face from the Sima del Elefante cave in Atapuerca, Spain, discovered in July. She is currently comparing Yunxian 2 with European hominin fossils, and says that the Yunxian people could be more similar to European populations from the middle Pleistocene epoch than they are to later specimens from China.

If the Yunxian 3 skull has teeth, especially molars, they could be useful for discerning evolutionary relationships with other early humans, says Clément Zanolli at the University of Bordeaux, France.

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Archaeologists work to uncover the secrets of ancient humans at the site in Yun county, in the central Chinese province of Hubei.

Photo: Weibo

An age-old question

Once the Yunxian 3 skull is excavated, probably within the next few months, dating it will be an important task. Several techniques have been used to estimate the age of Yunxian 1 and Yunxian 2 at between 800,000 and 1.1 million years.

Wei Wang, a geochronologist at Shandong University, says that hominin fossils in China are often more difficult to date than fossils in Africa, because China lacks volcanic sediments that can be reliably dated by measuring the amounts of radioactive isotopes in the rock.

Jean-Jacques Bahain at the National Museum of Natural History in Paris dated sediments collected from the Yunxian site using electron spin resonance and uranium series dating3. This requires a close comparison between values taken from the fossil and the quartz in the sediment. But he says that the samples he measured weren’t collected at the same time and location as the Yunxian 1 and 2 skulls.

The discovery of Yunxian 3 therefore represents a unique opportunity to collect sediment samples from the ground that the skull sits in, he says.

Small animal fossils surrounding the Yunxian 3 skull are slowing the extraction process, according to Gao. Bahain says that such specimens could help to pinpoint the age of the Yunxian 3 skull, and also connect it to early human remains elsewhere in China that have been found with prehistoric fauna.