An ancient skull found recently in the Hualongdong region of eastern China dates back to the late Middle Pleistocene, a key period for evolving humans. The unique skull could signal a previously unknown type of hominin.
The 300,000-year-old skull, which came from an individual known as HLD6, was discovered in 2019. However, the study about it came out just last month. Its lower jaw particularly intrigued researchers, since it differs from every known taxonomic group. It had features of both modern humans and Pleistocene hominins.
The ancient skull. Photo: Xiujie Wu/National Research Center on Human Evolution
The mandible looks much like our own, with a triangular shape and a clear bend. However, it does not seem to have a chin, which makes it similar to that of the Denisovans. Homo sapiens and Denisovans are distinct hominin groups that both evolved from Homo erectus.
“HLD6 does not present a true chin but has some weakly expressed traits that seem to anticipate this typically H. sapiens feature,” said Maria Martinon-Torres, author of the study.
The skull does not fit the evolutionary path of any known hominin. The research team initially theorized that this could be because it belonged to a 12- or 13-year-old. But later comparisons with both mature and juvenile hominin skulls from the same era showed that this was not the case.
This suggests that there was another lineage of hominins in this time period, a hybrid between modern humans and Denisovans. The rest of the HLD6 skull fits the theory. Sections of it resemble Homo sapiens, while its skull cap and chin have more primitive features.
The authors wrote that the face seems largely modern while the jaw and top of the head are more primitive.
The virtually reconstructed HLD 6 skull. Image: Wu et al., Journal of Human Evolution, 2023
Several hominin fossils found in China from his period have also not fit into a clear lineage. They were labeled as anomalies, advanced forms of Homo erectus, or early forms of Homo sapiens. Now it seems they might fall into a new category.
Other studies have also suggested that this is a fourth lineage, lying outside of Homo sapiens, Neanderthals, and Denisovans. A 2014 study analyzed the genomes of Neanderthal remains in Europe. This also suggested a fourth lineage of hominin, but there were no fossils to back up the claim.
HLD6 could be the missing link in the evolution of humans. But more research is needed before scientists can conclusively add a new branch to our family tree.
