Enflo P, Hawks J, Wolpoff MH. 2001. A simple reason why Neanderthal ancestry can be consistent with current DNA information. Am J Phys Anthropol 114:S62.
A simple reason, why Neandertal ancestry can be consistent with current DNA information
Summary: Are the Neandertals part of the ancestry of today´s humans? Observations in fossils and mtDNA are not easy to interpret, and seem to contradict each other. Thus, over the years, we have seen several FINAL PROOFS that the Neandertals are part of our ancestry, and several FINAL PROOFS that they are not. The purpose of this talk is to show that there is not necessarily any contradiction in these observations. All of them and several other observations can be understood as consequences of the following simple and well-known phenomenon: REGIONAL INSTABILITIES IN LIVING CONDITIONS, which leads to REGIONAL DIFFERENCES IN REPRODUCTIVITY which leads to MIGRATION OF PEOPLE.
Are the Neandertals part of the ancestry of today´s humans? Observations in fossils and mtDNA are not easy to interpret, and seem to contradict each other. Thus, over the years, we have seen several FINAL PROOFS that the Neandertals are part of our ancestry, and several FINAL PROOFS that they are not.
FINAL PROOF 1. The Neandertals are part of our ancestry, since several of their traits are found in the people, that followed them.
FINAL PROOF 2. The Neandertals are not part of our ancestry, since mtDNA-variation today leads us to a young Eve in Africa.
FINAL PROOF 3. The Neandertals are not part of our ancestry, since their mtDNA is very different from that of today´s humans.
FINAL PROOF 4. The Neandertals are part of our ancestry, since there is a Portugal child, which is a hybrid. (There are more FINAL PROOFS).
Then we have BREAKING NEWS FROM AUSTRALIA: A MODERN HUMAN WITH MITOCHONDRIAL DNA VERY DIFFERENT FROM THAT OF TODAY´S HUMANS.
There seems to be a contradiction between compelling evidence pointing in one direction and compelling evidence pointing in the opposite direction, and some confusion caused by the Australian fossil. The purpose of this talk is to show that there is not necessarily any contradiction in these observations. All of them and several other observations can be understood as consequences of the following simple and well-known phenomenon: REGINAL INSTABILITIES IN LIVING CONDITIONS, which leads to REGINAL DIFFERENCES IN REPRODUCTIVITY which leads to MIGRATION OF PEOPLE.
Assume that in a REGION, every couple has on average, LESS THAN TWO CHILDREN, and that the population size is kept up by immigration. We say that the REGION is at a REPRODUCTIVE DISADVANTAGE. Then the following will happen:
(1) mtDNA from the original population in the region will DISAPPEAR at an expotential rate (i.e. VERY FAST).
(2) Genes, which go by male lineage, will DISAPPEAR at the same rate.
(3) Genes, neural to selection, will DISAPPEAR at the same rate.
For (1), (2) and (3) IT DOES NOT MATTER, whether there is MUCH, LITTLE or NO INTERBREEDING BETWEEN NEWCOMERS AND LOCAL PEOPLE. THE RATE OF DISAPPEARANCE IS THE SAME.
If there is interbreeding between newcomers and local people, genes for which there is favorable selection may survive in the region.
Assume that, in a region with 100000 males and 100000 females, in every generation of 20 years, 1% of the population is replaced by immigrants. Then, after 5000 years, less than 10% of the mtDNA of the original population remains in the region. After 35000 years, one can, with 99% probability, not find one single individual in that region with mtDNA from the original population. The same computation applies to genes neutral to selection.
Assume that, in a region, once every 100 years, there is a bad event (like an unusual long winter), which kills 10% of the population. Assume that after that event, population is restored, 5% by increased reproductivity and 5% by immigration. Then, this INSTABILITY OF LIVING CONDITIONS leads to the same REPRODUCTIVE DISADVANTAGE as in EXAMPLE 1.
REMARK. The DISAPPEARENCE of genes by REPRODUCTIVE DISADVANTAGE is, over time, MUCH FASTER THEN THE DISAPPEARENCE BY GENETIC DRIFT. The mathematics is the same as for a leaking bottle of wine which is replenished with water. After some time there is only water in the bottle, regardless of whether the wine and water mix or not.
Let us ASSUME, that in the last 60000 years, LIVING CONDITIONS WERE LEAST STABLE IN EUROPE, MORE STABLE IN ASIA AND MOST STABLE IN AFRICA. Assume, that this put Europe into a reproductive disadvantage, Asia into a lesser reproductive disadvantage and made Africa most reproductively successful. Assume that this led to migration. For now, make no assumption whether there was much, little or no interbreeding between newcomers and local people.
WHAT DO WE EXPECT TO OBSERVE?
IN MITOCHONDRIAL DNA
(1.) We can expect Neandertal mtDNA to be quite different from mtDNA of today´s people (since, by the model, it is not around today or it is extremally rare)
(2.) We can expect mtDNA from a 60000 years old Australian fossil to be quite different from that of today´s people (same reason as 1.)
(3.) We can expect, that, by looking at mtDNA variation in today´s population we will be led to a fairly young Eve in Africa (since, by the model, Africa has been most reproductively successful)
(4.) We can expect mtDNA variation i today´s population to be greatest in Africa, smaller than in Asia and smallest in Europe (since that will follow the reproductive success of the region).
IN NUCLEAR DNA
(5.) we can expect, that, by looking at variation in the Y chromosome in today´s population, we will be led to a fairly young adam in Africa (same reason as 3.)
(6.) We can expect most of genetic variation in Europe to be from Neolithic time (since, by the model, even a modest immigration rate will make most of the local genes disappear)
(7.) Overall, we can expect genetic variation to be greatest in Africa, smaller in Asia and smallest in Europe (same reason as 4.)
For all these 7 observations it does not matter whether or not, nor how much, the immigrants mix with the local population - these observations are to be expected under all different mixing scenarios. Now we assume that there is substational interbreeding between newcomers and local population. What can we expect to observe?
(8.) We can expect to see local continuity, some of the traits of an older population can be expected to carried by a younger population, possibly at lower frequency. The Portugal (hybrid) child and Frayer´s data on suprainiac fossa, H-O mandibular foramen etc. are to be expected.
(9.) In Europe, we can expect Neandertal traits to become increasingly rare ( like Frayer's data, since the larger reproductive disadvantage has made it hard for them to survive)
(10.) In Asia we can expect some local ancient traits to still be around in today's population at high frequencies (since a small selective advantage for such traits could compensate for a small reproductive disadvantage, which made mtDNA disappear).
Finally we should mention
(11.) The surprisingly low genetic variation in humans - compared to some other species - can also be understood as a consequence of regional differences in reproductivity and the migrations it has led to. It is not necessary to assume - as is often done -that there was a bottleneck in the human population some 50000-70000 years ago. This modeling suggests, that this low variation may be less pronounced in genes under selection.
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