Chinese researchers analyzing enamel proteins from Homo erectus teeth dating to about 400,000 years ago have identified two mutations in the ameloblastin protein that are absent in modern humans, one of which has not been observed in any other primate. A second ameloblastin mutation found in the Homo erectus samples is also present in Denisovans and in a small fraction of modern humans, the researchers reported.

The team used microscopic samples of Homo erectus teeth from three sites in China, each dating to roughly 400,000 years ago, to analyze ancient proteins. Researchers first analyzed animal teeth from the same sites to confirm that their process could isolate and identify enamel protein fragments.

Five Homo erectus tooth samples and one Denisovan sample from the Harbin site in China were examined. Depending on the Homo erectus individual, the team obtained between six and eleven fragments of different enamel proteins, and the Denisovan individual from Harbin yielded a similar number of fragments.

The Harbin Denisovan individual carried one copy of the Homo erectus variant of ameloblastin and one copy matching the modern human variant. The shared ameloblastin mutation appears in DNA sequences of modern human populations in India and in the Philippines, including over 20 percent of some native populations in the Philippines.

Researchers estimate that the genomic region around the ameloblastin gene likely contains highly archaic Homo erectus DNA. The team favors the explanation that Denisovans interbred with Homo erectus, and that modern humans inherited Homo erectus DNA through later interbreeding between Denisovans and modern humans' ancestors.

Ryan McRae, a paleoanthropologist, said the findings connect present-day humans to deep ancestry. "This traces who we are now back to our ancestors in a really cool and exciting way, using new methods," McRae said. He also offered an alternative interpretation of the shared genetic signal. "It could be that H. erectus is actually just an ancestor to the Denisovans, who inherited those genes over time," he said.

Researcher Qiaomei Fu said further evidence would be required to resolve the question. "We really need to get more DNA," she said.

Earlier studies obtained enamel protein data from a Denisovan individual near Taiwan and an archaic human individual from Spain. Researchers have previously obtained amino acid sequences from tooth enamel protein fragments that are 2 million years old.