Archaeologists have uncovered teeth in China that don’t appear to belong to any known species of Homo—not quite modern humans, not quite Neanderthals. The findings, published in the American Journal of Physical Anthropology, suggests that the teeth may belong to hybrids of known populations or even to a whole new species of human ancestors we never knew about.

From about 340,000 to 90,000 years ago, Homo neanderthalensis resided in Europe and western Asia, while anatomically modern humans (Homo sapiens) were living in Africa. Meanwhile, a mysterious group of extinct human relatives called Denisovans were present in Siberia. While the "hobbit," Homo floresiensis, showed up in Indonesia 95,000 years ago, the evolutionary picture of the genus Homo remains incomplete due to the dearth of East Asian hominin (that’s us and our ancestors) fossils from the late Middle to the early Late Pleistocene.

In 1976, hominin dental samples—nine teeth from four individuals, BBC reports—were recovered from the early Late Pleistocene site of Xujiayao in northern China. Now, an international team from the Chinese Academy of Sciences and Spain’s National Research Center on Human Evolution (CENIEH) have re-analyzed the fossils, measuring the size and shape of various dental features, from the roots to the crown to the grooves between the cusps.

"Teeth are like 'landscapes in miniature,'" study author María Martinón-Torres of CENIEH tells the BBC. "Each of those slopes, grooves, valleys define a pattern or combination of features that can be distinctive of a population."

They found a mix of primitive and derived features: They’re different from archaic and recent modern humans, and they have some features that are common (but not exclusive) to the Neanderthal lineage. Furthermore, the fossils have retained some primitive structures that have previously been found in East Asian hominins from the Early and Middle Pleistocene, despite how they’re geologically younger.

These findings indicate the existence of a population in China that’s contemporary with Homo sapiens and Neanderthals, but of “unclear taxonomic status.” Perhaps these features were the result of interbreeding between different Homo species, or perhaps there’s a primitive hominin lineage that survived late into the Late Pleistocene in China. "What we have seen is an unknown group for us," Martinón-Torres says. But she cautions: “We cannot go further to say it's a new species because we need to compare it to other things."

Images via Centro Nacional de Investigación sobre la Evolución Humana