400,000-Year-Old Teeth Tie Homo Erectus to Denisovans, Living Asians
Proteins from 400,000-year-old Homo erectus teeth in China point to interbreeding with Denisovans and a trace in people living in Southeast Asia today.
Proteins pulled from the enamel of 400,000-year-old Homo erectus teeth found in China suggest this early human species interbred with Denisovans, and that a small piece of that shared biology later reached people living today in the Philippines, Papua New Guinea and other parts of Southeast Asia. The study, led by Qiaomei Fu of the Chinese Academy of Sciences, was published in the journal Nature.
The finding challenges the long-held view of Homo erectus as an evolutionary dead end that left no descendants. It was described for The Conversation by Sally Christine Reynolds, an associate professor in hominin palaeoecology at Bournemouth University, and republished by ScienceDaily.
Reading proteins where DNA is gone
DNA does not survive in fossils this old. Tooth enamel, the hardest tissue in the body, holds on to its proteins far longer, and proteins carry a record of the genes that made them. Fu’s team studied six teeth from three Chinese sites: Zhoukoudian, famous for the “Peking Man” fossils dug up in the early 20th century, plus Hexian and Sunjiadong. All of them date to around 400,000 years ago.
All six specimens carry an amino acid variant that has never turned up in any other hominin, living or extinct. An amino acid variant is a single swapped building block in a protein chain. That shared marker groups the East Asian fossils together as one population and settles an old argument over whether the odd-looking Hexian fossils belong to Homo erectus at all.
A second variant that points to Denisovans
The teeth share a second variant that is not unique to Homo erectus. It also shows up in Denisovans, an archaic human group known mostly from a cave in Siberia. The matching genetic variant appears in living people at a frequency of 21% in the Philippines and about 1% in India, a spread that fits entry into modern humans through Denisovan ancestry.
The likeliest explanation, Reynolds writes, is a two-step handoff. Homo erectus groups in East Asia passed the variant to Denisovans by interbreeding, and Denisovans later passed it to the ancestors of modern Southeast Asians and Oceanians. Scientists call this movement of genes from one species to another introgression. What the variant does in the body is still unknown.
What this means for the human family tree
Homo erectus is widely seen as the first hominin to leave Africa, reaching Eurasia nearly two million years ago, and it spread farther across the map than any other human ancestor. The new result adds it to a growing list of lineages that mixed with others.
People outside Africa carry roughly 2% Neanderthal DNA. Papuans and Aboriginal Australians carry another 2 to 5% Denisovan ancestry, and some West African populations show signs of an archaic lineage nobody has identified. A 2019 review in the American Journal of Physical Anthropology counted at least three separate gene transfers from Denisovan-like groups into the ancestors of Southeast Asians and Oceanians, some as recent as 20,000 years ago. Some Denisovan-derived variants in Papuan genomes appear to affect immune function.
Reynolds suggests a braided river as a better picture than a tree: many channels that split, rejoin and swap water over time.
Other lost relatives could be tested next
Several other archaic groups in the region have never given up any molecular data. Homo erectus may have survived in Indonesia until perhaps 100,000 years ago. The small-bodied Homo floresiensis, nicknamed the “hobbit,” was on the island of Flores when modern humans arrived, and Homo luzonensis lived in the Philippines.
Reynolds argues that if enamel proteins hold up for 400,000 years, the same method might show whether those species also left traces in the people who came after them. The interpretation of a single shared variant rests on one study, and further work will be needed to confirm the route the variant took.
Source: ScienceDaily
