400,000-Year-Old Teeth: Homo erectus and Denisovans, a Revealed Interbreeding

Teeth that reveal what DNA can no longer give

The fossils studied come from three Chinese sites: Zhoukoudian, made famous in the early twentieth century by remains nicknamed "Peking Man", Hexian and Sunjiadong. All date to around 400,000 years ago, far too old for DNA to be recovered from them.

The researchers therefore turned to proteins. Tooth enamel is the hardest tissue in the body, and its proteins survive far longer than DNA, which degrades beyond a few hundred thousand years in most contexts.

A molecular signature specific to East Asia

The six specimens share an amino acid variant never previously identified, a single-"letter" change in the protein sequence, never observed in any other hominin, living or fossil. This signature groups these Homo erectus together as a distinct group and settles an old debate: the Hexian fossils do belong to this species.

A second variant, however, is not specific to Homo erectus. It also appears in the Denisovans, an archaic group known mainly from a cave in Siberia. This variant is found in living humans, at about 21% in the Philippines and about 1% in India.

Interbreeding, a rule rather than an exception

The most likely scenario is that populations of East Asian Homo erectus passed this variant to the Denisovans through interbreeding, before the latter passed it on to the ancestors of present-day populations in Southeast Asia and Oceania. This transfer of genetic material from one species to another is called introgression.

The finding goes beyond this specific case. Humans outside Africa carry about 2% Neanderthal DNA, and Papuans as well as Aboriginal Australians add 2 to 5% Denisovan ancestry. Every human lineage studied genomically shows traces of mixing.

A function still unknown

The functional consequences of the variant inherited from Homo erectus remain unknown. Other introgressed genes have, however, played an adaptive role, and certain Denisovan variants present in Papuans appear to influence immune function.

The study also opens the way to examining other vanished populations, such as Homo floresiensis, nicknamed "the hobbit", found on the island of Flores, or Homo luzonensis, which occupied the Philippines. None of these groups has yet yielded DNA or molecular data.

A new picture of human evolution

The image of a single trunk with separate branches is gradually giving way to that of a braided river, made of channels that merge and split and constantly exchange their waters. The vanished lineages have left their mark in our genomes.

To explore these questions further, enthusiasts can visit the fossil and cast collections of the Muséum national d'histoire naturelle in Paris, as well as the exhibitions devoted to human evolution in major natural history museums.