Dental evidence suggests Neanderthals and modern humans diverged from a common ancestor around 800,000 years ago — hundreds of thousands of years earlier than standard estimates. The finding could finally reveal the provenance of our shared ancestry, but some experts say the new evidence is unconvincing.
Archaeological and genetic evidence suggests Neanderthals were romping around Eurasia around 400,000 years ago, and that modern humans, Homo sapiens, emerged in Africa around 300,000 years ago.
These two groups of hominins — both types of humans — are descended from an unknown common ancestor. The timing and geographic location of their momentous evolutionary split is not known, but studies of skulls and DNA suggests it happened around 500,000 to 600,000 years ago.
The new research, published this week in Science Advances, suggests the divergence between Neanderthals and modern humans from our last common ancestor (LCA) happened no earlier than 800,000 years ago.
The lone author of the new study, anthropologist Aida Gómez-Robles from the University College London, reached this conclusion after analysing Neanderthal teeth dated to 430,000 years ago. The experts we spoke with, however, said more evidence is needed to bolster this claim.
The Neanderthal teeth used in the study were previously found in Sima de los Huesos, a Spanish cave that hosted hominins during the Middle Pleistocene.
The remains of nearly 30 individuals have been found at Sima, and they exhibit anatomical features which are very Neanderthal-like in nature. In fact, they’re so Neanderthal-like that scientists think these bones and teeth probably came from an early version of the Neanderthals.
The layer within which the remains were found was previously dated to 430,000 years ago. That means Neanderthals, with their distinct features, must’ve diverged from our LCA long before then. Evolution moves very slowly. But as the new research pointed out, the features seen in the teeth required more than just a few hundred thousands of years to appear.
“Any divergence time between Neanderthals and modern humans younger than 800,000 years ago would have entailed an unexpectedly fast dental evolution in the early Neanderthals from Sima de los Huesos,” said Gómez-Robles in a UCL statement.
“There are different factors that could potentially explain these results, including strong selection to change the teeth of these hominins or their isolation from other Neanderthals found in mainland Europe.
“However, the simplest explanation is that the divergence between Neanderthals and modern humans was older than 800,000 years. This would make the evolutionary rates of the early Neanderthals from Sima de los Huesos roughly comparable to those found in other species.”
The hominins at the Sima site had very small premolars and molars, which is consistent with Neanderthals. These small dental features likely evolved from the larger teeth of the yet-to-be identified LCA. For the study, Gómez-Robles analysed the teeth of different hominin species and used the resulting quantitative data to establish a baseline rate of dental evolution among hominins.
“The Sima people’s teeth are very different from those that we would expect to find in their last common ancestral species with modern humans, suggesting that they evolved separately over a long period of time to develop such stark differences,” said Gómez-Robles.
Our shared LCA with the Neanderthals is still not known, but this finding suggests the mystery species cannot be too much younger than 800,000 years old. The hominin species Homo heidelbergensis, which lived from around 800,000 to 300,000 years ago, is now an unlikely candidate, according to the new research.
Katerina Douka, an archaeologist at the University of Oxford who’s not affiliated with the new study, said the statistical and modelling analyses performed in the study was “very interesting”, but the conclusions relied on a single basic assumption: That the absolute date established for the Sima de los Huesos individuals is actually correct.
“However, we know that the age of Sima is not bulletproof and if the real age was younger, as young as 250,000 years for example, the divergence rates calculated in this study would be compatible with average evolutionary rates, and not at all controversial,” Douka explained to Gizmodo in an email.
Sharon Browning, a biostatistician from the University of Washington, felt that the new paper relied far too heavily on an extrapolation made from a single data point, that being the observed dental divergence. The paper, she told Gizmodo in an email, didn’t sufficiently consider all the other data, particularly DNA divergence.
“The author argued that uncertainty in mutation rates, for example, can affect the DNA divergence results. This is certainly true, to a point,” said Browning. “However even using the lower end of plausible mutation rates,” previous research from 2012 “found a Neanderthal-human split time of no more than 600,000 years ago,” she said.
Also, the DNA data available for the Sima individuals isn’t very complete, so even though their DNA might bear a resemblance to Neanderthals, it’s possible that this group interbred with some other unknown hominins, resulting in the observed dental differences, according to Browning.
This “is just one possibility for reconciling the dental data with established ranges for Neanderthal-human split times,” she added.
Indeed, while the new study provides intriguing food for thought, it’s clear that more evidence will be needed to bolster the conclusion reached by Gómez-Robles. Until then, the last common ancestor of Neanderthals and modern humans will have to remain an enduring mystery.