In the ranks of prehistoric humans, Neanderthals were our closest relatives.

We were so close, in fact, that our species interbred with theirs. Tracing back our lineages, there must have been a last common ancestor of Homo sapiens and Neanderthals sometime in prehistory. (Related: "Geno 2.0 Can Reveal How Neanderthal You Are.")

But who was this mystery human?

Picking out direct ancestors in the fossil record is tricky. To figure out when the last common ancestor of Homo sapiens and Neanderthals lived, paleoanthropologists have been sifting through both genetic and anatomical evidence.

In the last five years, anthropologists have used DNA to reconstruct the evolutionary history of humans. Researchers have suggested a range of dates for when the last common ancestor of our lineage and Neanderthals could have lived. (Related: "Last of the Neanderthals.")

The dates range from more than 800,000 years ago to less than 300,000, with many estimates in the neighborhood of 400,000 years ago. According to some studies, this time frame would seem to match that of the extinct species Homo heidelbergensis, which has been found in Africa, Europe, and possibly Asia.

But this may not be so. A new study theorizes that the last common ancestor of H. sapiens and Neanderthals lived longer ago than previously expected, with fossil evidence yet to be uncovered.

What's new?

In a study published on Monday in the Proceedings of the National Academy of Sciences, George Washington University anthropologist Aida Gómez-Robles and colleagues turned to teeth to test what had been gleaned from genetics.

Using a collection of 1,200 premolars and molars from a variety of prehistoric humans, the researchers pinpointed specific landmarks on the teeth. The landmarks were then used to reconstruct the tooth shapes of our common ancestors at critical points in evolutionary history.

The logic behind this method, Gómez-Robles says, is that "the most likely dental shape of an ancestral species is an intermediate shape between the one observed in both daughter species." In the case of H. sapiens and Neanderthals, the last common ancestor of both lineages would be expected to have teeth with a shape and anatomy in between those of the two species.

With that hypothetical shape in mind, Gómez-Robles and coauthors compared what was expected against fossils found so far.

"If a fossil species is very similar to the expected ancestral morphology, then that species is a plausible ancestor," Gómez-Robles says, though she stresses that such a match is a possibility rather than definite proof of ancestry.

Why is it important?

Estimates based on DNA show that the last common ancestor of H. sapiens and Neanderthals lived around 400,000 years ago. This made H. heidelbergensis, a widespread species alive at the time, seem like a good candidate for that ancestor.

The new study contradicts this idea. The tooth reconstruction of the last common ancestor of humans and Neanderthals created by Gómez-Robles and colleagues doesn't match the teeth of H. heidelbergensis.

In fact, the researchers found that none of the human species living during the time predicted by genetic data fit the tooth pattern generated by the new study. More than that, "European species that might be candidates show morphological affinities with Neanderthals," Gómez-Robles says, which hints that these humans were already on the Neanderthal side of the split.

This suggests that the last common ancestor of H. sapiens and Neanderthals lived sometime earlier, perhaps as far back as one million years ago.

What does it mean?

Paleoanthropologists have yet to find our last common ancestor with Neanderthals. Tracking this elusive human will require going back to museum collections and continuing searches in the field.

From the new study's results, Gómez-Robles says that "we think that candidates have to be looked for in Africa." At present, million-year-old fossils attributed to the prehistoric humans H. rhodesiensis and H. erectus look promising.

This critical window of human prehistory in Africa is still cloudy. "There are not so many African fossil remains dated to one million years ago," Gómez-Robles says, and those that have been found are often attributed to H. erectus.

But do they really belong to this species? There may be an as-yet-unknown human hiding in the mix, and this human may be key to solving the puzzle of when our ancestors split from Neanderthals.