Early Neanderthal teeth shed light on the identity of our own ancient ancestors
By looking at the teeth of ancient humans, researchers have been able to hone in on when modern humans and Neanderthals may have split.
They estimate that it most likely occurred by at least by 800,000 years ago, but potentially as far back as 1.2 million years.
The origin of our species, Homo sapiens, has long intrigued scientists.
We know that our closest relatives were the heavier-set Neanderthals (Homo neanderthalensis), and the fossil record for our own group is more complete than that of many other animals. But when exactly the two species went their separate ways is still debated.
Thanks to the ability to extract and analyse ancient DNA, these last few decades have seen a sea change in the field of human evolution. Scientists can now start to ask more complex questions about the history of our species and how we fit into the larger hominin family tree.
Even then, depending on what assumptions are made and which part of the genome is being looked at, researchers have come up with differing dates as to when the two branches on the evolutionary tree split.
To resolve this, Dr Aida Gomez-Robles, researcher at University College London and scientific associate at the Museum, has been looking at the teeth of hominins.
By analysing the rate of change in tooth shape between the species, she has been able to say when this split most likely happened, then compare it to what the genetics say. Her findings are published in in the journal Science Advances.
Aida found that the best match for when our last common ancestor lived was sometime between 1.2 million and 800,000 years ago.
'Genetic analysis tends to indicate a younger divergence time between modern humans and Neanderthals,' explains Aida. 'Even the most recent estimates that are based on nuclear DNA indicate a divergence time of around 600,000 years ago.
'This means there is at least a 200,000-year gap between what the DNA analysis indicates and what these new analysis shows.'
A new look at old fossils
This latest piece of research did not involve the discovery of new fossils to fill in gaps in the record, but rather the analysis of existing data using a quantitative approach. This means taking detailed measurements of the shape of hominin teeth and calculating the rate or speed at which they changed over time.
'I am not estimating the date of divergence,' explains Aida. 'Rather, I am just calculating the rate of change in the teeth assuming different divergence times.'
This is an important, although slightly complicated, point. Aida logged the changes in the tooth shape of Neanderthals over time. Then she mapped this onto the evolutionary tree of hominins and modelled what would happen to this evolutionary rate of change if the split between modern humans and Neanderthals took place at different points in history.
'If you have a very young divergence time what we see is a very high evolutionary rate for the change in Neanderthal teeth shape, and if we have a very old divergence time then we have a very low evolutionary rate,' says Aida.
'In all the other branches on the hominin tree we can see that there are much lower evolutionary rates, so what I was looking for was the point where the rates that I see for both Neanderthals and other hominins match.'
From this, Aida arrived at a date of divergence between 1.2 million and 800,000 years ago. 'This result is not based on general similarities and descriptions, but on a quantitative analysis,' she says.
'Of course, the analysis is based on some assumptions - and it can be argued that those inferences could be different - but this is a numerical analysis that anybody can repeat using the same or different assumptions.'
The wrong ancestor
In the grand scheme of the fossil record, to most people a difference of a few hundred thousand years might not sound that significant. But when it comes to dealing with ancient human remains, this can have important consequences.
There have been a number of contenders for the last common ancestor of modern humans and Neanderthals. For a long time, one of these species was Homo heidelbergensis.
Remains attributed to this hominin species have been found in various parts of Africa and Europe. This distribution led many to think that it could have been the ancestor to both our own species that originated on the African continent and the Neanderthals who dominated Europe.
This new work, however, adds to the evidence that this was unlikely the case.
'The difference is how we interpret the fossil record,' says Aida. 'The H. heidelbergensis remains would be compatible with a divergence time of 600,000 years ago that the genetic analyses indicate, but those fossils are younger than the 800,000 years that my analysis indicates.
'This is important because it suggests that H. heidelbergensis would not be a good ancestral species for Neanderthals and modern humans.'
This therefore indicates that researchers should perhaps be looking elsewhere for our ancestral species, for example Homo antecessor.
Known from fossil remains from Europe that date to around 1.2 million and 800,000 years old, H. antecessor could be a better contender.
As is often the case, more fossil remains from all ancient human species could help solve this mystery. But this study also shows that novel ways to interpret already-existing data can be used to add new and exciting insights into our species' own origin.