Neanderthals and modern humans parted company earlier than thought

Tooth study pushes back the last common ancestor by around 400,000 years. Andrew Masterson reports.

A selection of molars recovered from the Spanish cave site.

Aida Gómez-Robles

The last common ancestor of Neanderthals and modern humans lived much earlier than previously thought, a new dental analysis reveals.

Pinpointing the time at which the two hominin lineages diverged is hotly debated by researchers. In recent years a tentative consensus emerged, based on studies of ancient DNA, which indicated that Neanderthals and the ancestors of Homo sapiens went their separate way about 400,000 years ago.

Agreement was cautious, however, and subject to challenge. In 2017, for instance, a study of European hominin remains suggested that the split possibly occurred earlier, around 600,000 years ago.

Now a new study, centring on hominin remains found in a Spanish cave, push the likely diversion date back even further, to around 800,000 years ago.

The research is the work of anthropologist Aida Gómez-Robles from University College London, UK, and is published in the journal Science Advances.

Gómez-Robles’s approach took advantage of the fact that while tooth sizes vary quite broadly within hominin species, tooth shape, at least in the matter of molars, tends to be much more homogenous, and changes only slowly.

She took a very close look at the teeth of some of the 28 sets of hominin remains discovered in a cave known Sima de los Huesos (SH), in Spain – a site that has been subject to continual exploration since it was first discovered in 1984.

Establishing the age of the Spanish remains was for many years a controversial process, but a 2014 study employing luminescence dating techniques and palaeomagnetism conclusively established that the cave occupants lived about 430,000 years ago.

The date, for Gómez-Robles, was highly significant. Analysis of the hominins’ teeth revealed that they were “unexpectedly derived toward the Neanderthal condition”.

It followed, therefore, that the Sima de los Huesos hominins must have existed after the last common ancestor of Neanderthals and modern humans.

Not only that, Gómez-Robles writes, they must have lived substantially after the divergence, unless they were, at least in the matter of dentition, for some reason subject to “exceedingly high evolutionary rates”.

Such a possibility, she concedes, cannot be completely discounted on the existing evidence. She presents two possible scenarios in which it may have arisen.

First, she suggests, the Spanish population may have been isolated and subject to developmental pressures not found among other populations.

And second, the Spanish cave dwellers may not have been, strictly speaking, members of either the Neanderthal or modern human lineage, but rather hybrids resulting from an earlier bout of interbreeding (possibly with added contributions from Denisovans).

However, she concludes, “the simplest explanation of the results presented in this study is that Neanderthals and modern humans diverged before [800,000 years ago], which would make evolutionary rates for the SH dentition roughly comparable to those found in other species.”

The findings are unlikely to settle the divergence debate once and for all, particularly as they sit at odds with DNA-based dating attempts. Gómez-Robles fully acknowledges this state of affairs, and suggests that much more research is necessary – and many more fossils need to be found – before the matter can be resolved.

“The discrepancies between the dates at which clear Neanderthal and modern human affinities are observed in the hominin fossil record may seem to indicate differential evolutionary rates in both lineages, which would affect the inferences made through the present study,” she notes.

“However, they may simply reflect the incompleteness of the fossil record, particularly for the modern human lineage.”

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