aurhor: Pierre Barthélémy
translation: doxa-louise
What was the ancestor to Neanderthal and Modern Man like
In conversation with Aurélien Mounier
A paleoanthropologist at Cambridge University (UK) with the Leverhulme Center for Evolutionary Studies, Aurélien Mounier wants to know how homo became sapiens, how individuals from this lineage came to possess the chatacteristics that are theirs today. In a collaborative effort with Marta Mirazon Lahr, director for the 'In Africa' project on the role of East Africa on the evolution of man, he has just published an article in the Journal of Human Evolution, wherein these two researchers describe how, with the aid of a digital model, they have recreated the skull of the ancestor common to Modern Man and his Neanderthal cousin. Aurélien Mounier kindly accepted to answer questions on this 'virtual fossil'.
What do we know about the common ancestor to Homo Sapiens and Neanderthal Man
We know that we a have a common ancestor with the Neanderthals. This ancestor, or rather this ancestral population generally known as Homo heidelbergensis lived during the Middle Pleistocene (a period covering roughly the years 800 000 to 200 000 before ou era), in Africa and probably in Eurasia. Unfortunately, for such a a long timespan and such a large geographic area, we possess but a few dozen (dizaines) fossils. In fact, there is quite a bit of debate about the precise definition of this population: what species? What geographical dispersion? and quite naturally one of the central questions is that of determining when these two lineages, that of modern men and the Neanderthals, separated.
The objective of our study was to build mathematically a virtual skull which would stand for a fossil of this ancestral population. After that, we could take this virtual skull to be a normal one, and compare it to actual fossils to see if it resembled certain known specimens. Finally, to take into account the debate about the moment of separation between the modern and Neanderthal lineages, we have complexified our model to create three virtual ancestors corresponding to three possible dates for separation: 1 million years, 700 000 years and 400 000 years. At the present time, the most probable date of separation, based on estimates form DNA and the molecular clock is closest to 400 000.
How did you achieve this virtual reconstruction
We constructed a simplified tree showing the evolution of the genus Homo. this tree contains three different species, Homo erectus, Homo neanderthalensis and Homo sapiens, represented by 15 specimens (2 erectus, 5 Neanderthals and 8 modern men) without including fossils from the population we are looking for. We used only 15 specimens because of the poor condition of such fossils which are often quite fragmentary. Because in this study we were looking to reconstruct the entire skull.
These specimens are defined through 797 'benchmark' points describing the whole skull. It is this information, the form created by these points, that will be used by the algorithm and constrained by an evolutionary model based on random chance as causative of evolution, in order to 'go back in time'. From there we are in a position to find a virtual ancestor for each of the nodes in our simplified genealogical tree which represents the history of the genus Homo.
What was this ancestor like and what do we still carry from him
He is like us mostly by the form the lower face, which offers a small slump which, in modern man is very marked and contributes to the formation of relatively prominent cheekbones. This ancestor resembles more Neanderthal man than he does us because the form of the skull in modern men is farther to that of other species of men who have today disappeared than that of the Neanderthals. Our uniqueness lies in having a face retracted under the cranial arch much higher and rounder than any other cranial arch of other human species from the past.
How to be sure that such a modelization approach can yield significant results. Is this in contradiction or complementary to results obtained during work on ancient DNA.
We have compared our virtual ancestors to over 50 skulls out of Africa or Eurasia and dated between 1.8 million years and today. The three ancestors have a morphology which resembles fossils dated in the middle Pleistocene, that is to say the period in which we know there existed a population ancestor to modern man and the Neanderthals. This tends to show that the estimation works well enough because the model to calculate the ancestors did not include any individuals from this period..Between these three models, the virtual ancestor which most resembles the fossils of the period is number 2 built on the assumption of a split between the Neanderthals and and Modern Man around 700 000 years ago. This is something of a surprise, I expected that the most recent ancestor, 400 000, best correspond to the morphology of fossils from the Middle Pleistocene. That would have matched results from analyses of ancient DNA.
What does this new dating tell us of the evolution of man
The new date is but a hypothesis from a model, it is not necessarily the truth. On the other hand, it is possible that certain genetic analysis, the speed at which genetic mutation occurs is a slight overestimation, which entails an underestimate of the date at which separation occured. I think the truth is somewhere between 700 000 and 400 000 years. We have here studied three dates only and do not know what our model would have shown for a different date, say 500 000 years. If that date were to be confirmed, it would indicate a slower evolution of the Modern and Neanderthal lineages and relative stability during the Middle Pleistocene: given that the eldest ancient Modern men and Neanderthals are at most 200 000 years old, this would give some 500 000 years to this population to diversify.
To what extent could we use such an approach for other species or other key moments in human evolution
This approach is perfectly suited to other species. It permits the reconstruction of a bone or a set of bones for any organism. Of course, the model used here is very simple. It can be made more complex by integrating other factors that play a role in evolution, such as selective pressures due to the environment, climate or geography. Building up virtually the last common ancestor to Tyrannosaurus and Diplodocus is thus a possibility...Of course this is not a priority for me and I am currently using this approach to find the common origins of man and chimpanzees, building the missing link that is most important to the history of the human family.
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