Per Enflo ■ Web Publications
A simple reason, why Neandertal ancestry can be consistent with current DNA information
Reproduction strategy as a driving force for Human Evolution
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.
To refer to this article:
Enflo, P 2011, A simple reason, why Neandertal ancestry can be consistent with current DNA information. Retrieved [day month, year,] from http://www.perenflo.com/sida16.html
Summary: Humans form couples and are sexually active without long interruptions. This is an important difference between humans and the great apes. Thus, humans produce more children than they can easily handle. Yet, humans have a strong desire to take good care of their children. This creates a conflict. In this paper, we will show that quite a bit of human evolution can be understood as a consequence of this one conflict, and the attempts to resolve it.
One usually distinguishes between two strategies of reproduction among animals, the R-strategy and the K-strategy.
The R-strategy is to produce a very large number of offspring, but not take much care of them. Because of the large numbers, some offspring will survive, and the species survives. This strategy is represented i.e. by some shrimps.
The K-strategy is to produce a small number of offspring but to take well care of them, so that they survive. This strategy is represented i.e. by the great apes, our closest relatives among animals. After having a baby, the female chimpanzee will carry it around for several years. She will not be sexually active and have another baby until the first one can manage pretty well on its own.
Humans have a reproduction strategy which falls between R and K and which creates a conflict. In the way humans form couples and are sexually active without long interruptions, they will produce more children than they can easily handle. Yet, there is a strong desire among humans to take good care of their children. Quite a bit of human evolution can be understood as a consequence of this one conflict and the attempts to resolve it.
Some important and natural strategies to try to resolve the problem of having many children and still take good care of them are
(1) To colonize new regions to accomodate an increasing population.
(2) To develop new technology
(3) To develop new social structures
(2) and (3) will increase the capacity of existing habitats to accomodate larger populations.
There is abundant evidence, that humans over the last 2-3 million years have adopted the strategies (1)-(3). One can expect that this leads to a biological and social evolution according to (A),(B) and (C) below.
A/ One can expect natural selection to favor ability and willingness to adopt the strategies (1), (2) and (3).
If just colonization (1) is used, one can expect that a population in a region will increase to a larger population in a larger region, which will increase to an even larger population in an even larger region etc. This is "exponential growth".
Thus we have
B/ One can expect a fast colonization of all regions which are habitable and can be reached by existing technology.
If the development of new technology (2) or new social structures (3) is used in some region one can initially expect a local population expansion in that region.
But the capacity of that region will soon be reached and there will be pressure to either develop more new technology or new social structures, or, to move into adjacent regions, or both. The pressure to move into adjacent regions creates a pressure to use the new technology/social structures also in the adjacent regions to accomodate the population increase, and so the new technology/social structures quickly spread to the whole human population.
Thus we have
C/ One can expect the following feedback loop:
New technology/social structures give a local population increase and the one way to handle this population increase is to use more of the new technology/social structures. So, the new technology/social structures will spread fast through the entire human population. This feedback loop will be combined with A/ and B/.
To test this model, we will consider the evidence that A/,B/ and C/ have actually happened.
Brain size and intelligence, complicated social structures and willingness to colonize new regions now constitute differences between humans and the great apes. So, there is strong evidence that A/ has happened.
In the relatively recent past we have some examples feedback loops represented by (B) and (C). When farming was introduced there was a local population increase. The way to handle this increase was to introduce farming in more regions which led to more population increases in those regions. So, by this feedback loop, farming quickly spread to the whole world. And after having been hunters-gatherers for a few million years, humans became farmers in a few thousand years.
The introduction of farming led both to increased population densities and to colonization of new regions. But the introduction of farming did not resolve the conflict between having many children and having a desire to take well care of them.
In an even more recent past, a few hundred years ago, the industrial revolution started. Initially, it represented development of new technology. But soon it gave rise to new types of feedback loops in (2) and (3) like: With new technology, more people can be educated and develop more new technology. This feedback loop leads to an accelerated development of new technology.
Today we are in the midst of such a feedback loop. And from having been farmers for a few thousand years, humans have adopted a modern lifestyle in a few hundred years. And much technology, that is a few decades old, is now obsolete.
This development has also done more to increase population densities than to colonize new areas. And the development seems to also gradually resolve the millions of years old conflict of having more children than can comfortably be taken care of. In a growing number of countries today, the reproduction rate is below 2 children per individual.
It is natural to consider whether we can find feedback loops like (B) and (C) in a more distant past, and what they may have been like. One obvious case to consider is the early expansion out of Africa, that started approx. 2 million years ago. The scant archeological evidence seems to be consistent with the idea that there was a fairly fast colonization of large areas outside of Africa. And there seems to be growing evidence that some impotant technology, like Acheulean handaxes, spread fast through the human population. And there is evidence that there was also increased brain size.
Was this expansion connected to the ability to use fire systematically? Presently, there seems to be no consensus among researchers on this. Some researchers believe that the systematic use of fire is approx. 2 million years old, and there is some evidence for that. Others believe that it is much more recent, perhaps approx. 400000 years old, and there is evidence for that, too.
For an even more distant past one can consider how the introduction of stone tools affected regions of habitat for early humans, more than 2 million years ago.
In conclusion - we see that there is strong evidence that A/, B/ and C/ have happened in the past. Further research may decide whether it happened to a larger extent than is known today.
To refer to this article:
Enflo, P 2011, Reproduction strategy as a driving force for Human Evolution. Retrieved [day month, year,] from http://www.perenflo.com/webpubl.html