American Journal of Physical Anthropology
Volume 137 Issue S47, Pages 70 - 99
Hunters of the Ice Age: The biology of Upper Paleolithic people
Brigitte M. Holt, Vincenzo Formicola
Abstract
The Upper Paleolithic represents both the phase during which anatomically modern humans appeared and the climax of hunter-gatherer cultures. Demographic expansion into new areas that took place during this period and the diffusion of burial practices resulted in an unprecedented number of well-preserved human remains. This skeletal record, dovetailed with archeological, environmental, and chronological contexts, allows testing of hypotheses regarding biological processes at the population level. In this article, we review key studies about the biology of Upper Paleolithic populations based primarily on European samples, but integrating information from other areas of the Old World whenever possible. Data about cranial morphology, skeletal robusticity, stature, body proportions, health status, diet, physical activity, and genetics are evaluated in Late Pleistocene climatic and cultural contexts. Various lines of evidence delineate the Last Glacial Maximum (LGM) as a critical phase in the biological and cultural evolution of Upper Paleolithic populations. The LGM, a long phase of climatic deterioration culminating around 20,000 BP, had a profound impact on the environment, lifestyle, and behavior of human groups. Some of these effects are recorded in aspects of skeletal biology of these populations. Groups living before and after the LGM, Early Upper Paleolithic (EUP) and Late Upper Paleolithic (LUP), respectively, differ significantly in craniofacial dimensions, stature, robusticity, and body proportions. While paleopathological and stable isotope data suggest good health status throughout the Upper Paleolithic, some stress indicators point to a slight decline in quality of life in LUP populations. The intriguing and unexpected incidence of individuals affected by congenital disorders probably indicates selective burial practices for these abnormal individuals. While some of the changes observed can be explained through models of biocultural or environmental adaptation (e.g., decreased lower limb robusticity following decreased mobility; changes in body proportions along with climatic change), others are more difficult to explain. For instance, craniodental and upper limb robusticity show complex evolutionary patterns that do not always correspond to expectations. In addition, the marked decline in stature and the mosaic nature of change in body proportions still await clarifications. These issues, as well as systematic analysis of specific pathologies and possible relationships between genetic lineages, population movements and cultural complexes, should be among the goals of future research.
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25 comments:
" The ... incidence of individuals affected by congenital disorders probably indicates selective burial practices for these abnormal individuals".
Possibly. But taken at face value it may also mean a level of inbreeding within isolated populations. The arrival of any new group would restore hybrid vigour. And this pattern would explain the bit, "others are more difficult to explain. For instance, craniodental and upper limb robusticity show complex evolutionary patterns that do not always correspond to expectations".
The arrival of any new group would restore hybrid vigour
This was not necesarily the result, even if you're right regarding the rest: the vigor could just be restored by internal natural selection, as happens with animal breeds (very much inbred but artificially selected for the best individuals only).
So yes, maybe there was a severe bottleneck in the LGM that restricted gene pool and led to genetic disorders (some of which may still linger, though surely not the most severe ones), but, after it, there was surely a restoration of genetic health by mere expansion and natural/sexual selection within the bottlenecked pool.
One thing is pretty clear: in Europe there was no external input between the Gravettian and the Neolithic (and the origins of Gravettian itself are still obscure anyhow). Magdalenian is a 100% European creation, and so are its Epipaleolithic derivates. Yet there was a Magdalenian expansion into Central Europe and a quite parallel Eastern Epigravettian one into the Caucasus and the Zagros. The expansive late UP and Epipaleolithic of all Europe cannot be explained with external origins.
"the vigor could just be restored by internal natural selection, as happens with animal breeds".
I very much doubt it. Can you give even a single example?
" maybe there was a severe bottleneck in the LGM that restricted gene pool".
You don't have to have a bottleneck. Genetic diversity will fall off in any population isolated in even just the smallest sense.
"there was surely a restoration of genetic health by mere expansion and natural/sexual selection within the bottlenecked pool".
Can't happen, unless the mutation rate is extreme giving an increasing proportion of heterozygotes.
"Yet there was a Magdalenian expansion into Central Europe and a quite parallel Eastern Epigravettian one into the Caucasus and the Zagros".
Increasing the heterozygosity of those regions, leading to expansions, in turn, from there. The Magdalenian expansion was probably the result of technological improvement rather than genetic anyway.
I very much doubt it. Can you give even a single example?
All animal breeds I can think of are resonably healthy. We would never breed unhealthy individuals, nor would Nature in the long run anyhow (yet species diverge, not converge).
You don't have to have a bottleneck. Genetic diversity will fall off in any population isolated in even just the smallest sense.
Size matters here. Isolation is relative... relative to the largest "isolated" population. If a people is isolated but large will show more diversity in the long run than another isolated but small.
It should also matter things like founder effects: an isolated population with five difefrent founder "tribes" would not be the same as an isolated population that has only one such lineage at their origin.
I don't see its should be something as lineal as you seem to believe. Also an expanding population should increase its diversity, even without hybridation, just out of increased mutation rates. It is only in a decreasing population (or in a very small and stable one, so drift can act effectively) when diversity can really decay. Otherwse it should increase out of mere recombination and mutation. Sex was invented to keep diversity high in fact.
Can't happen, unless the mutation rate is extreme giving an increasing proportion of heterozygotes.
A growing population gets paid in terms of growing effective mutation rates and derived new diversity.
But anyhow natural selction keeps acting among low diversity populations: unhealthy individuals will tend to die and not reproduce, so in the mid term you get vigor restored. Maybe not as effecively as with intense admixture (which is like a shot of new, presumably healthy, stuff) but you get it anyhow.
Increasing the heterozygosity of those regions...
Not necesarily. Caucasus and Zagros, as well as Central Europe, were surely deserted or nearly so in the Middle Paleolithic due to climatic rigors, so we get founder effects (with their corresponding initial low diversity levels) if anything with such (back-)migrations, not increase of the hetrozygosity of a non-existent population.
The Magdalenian expansion was probably the result of technological improvement rather than genetic anyway.
It's hard to say. I am more of the opinion of cultural or economical (rather than mere technological or genetic) advantage. For instance Magdalenians may have been very interested in exploiting sea mammals (Inuit-like harpoons everywhere) and the quality of their art appears to show a high "intellectual" orientation of sorts, but such things are more cultural than biological; and technologically I would not be able to argue that Magdalenian "neo-Aurignacian" (sometimes very hard to take apart from classical Aurignacian really, except for the harpoons and C-14 dates) is superior to, say, Solutrean super-refined bifacial retouch or even Gravettian (that had already displaced Aurignacian in the past).
One reason for Solutrean and later Magdalenian expansion may be that the country where these techno-cultures were developed (Franco-Cantabrian region) was with all likehood the most populated of UP Europe by large (and maybe of all West Eurasia). With a larger, reasonably well fed, population the chances of innovation and entrepreneurship increase just out of sheer statistical probability. This process has a positive feedback in it: one innovation bringing to another, one enterprise opening the gates of further challenges. Guess the creative dynamics can become exhausted or reach an unsurmontable limit at some point but there is some self-fed mechanism in such process: you have the people, the cultural background to allow them to innovate and explore, and you get developement and expansion just naturally from that. If additionally you have areas newly opened to colonization, as happened with Central Europe and later with Northern Europe as well, then it's even easier.
"All animal breeds I can think of are resonably healthy".
Only if they're not too inbred. Obviously any breed is inbred to some extent, otherwise it couldn't be classified as a breed. But inbreeding is a huge problem in many domestic animal and poultry breeds older than about 150 years.
"an isolated population with five different founder 'tribes' would not be the same as an isolated population that has only one such lineage at their origin".
It would still eventually become inbred if it consisted of less that about 500 individuals.
"Also an expanding population should increase its diversity, even without hybridation".
To some extent that's true, but in the present post we are almost certainly speaking of relatively stable population sizes in most regions as the early Upper Paleolithic gave way to the late Upper Paleolithic.
"But anyhow natural selction keeps acting among low diversity populations".
True.
"unhealthy individuals will tend to die and not reproduce, so in the mid term you get vigor restored".
Not so. Inbreeding often leads to total extinction of the population, or even of the species. I read recently that many bird species are on the verge of extinction even though there are many individuals left. Unfortunately the species have been divided into relatively small isolated groups. Inbreeding is carrying out its usual business.
Regarding Central Asia, the Caucasus and Zagros. We can guess that they have high heterozygosity because the regions have amoung the highest diversity of haplogroups in the world. Granted, possibly because the founder population was very diverse but I'd guess more likely because they have been subject to a whole series of immigrants.
"One reason for Solutrean and later Magdalenian expansion may be that the country where these techno-cultures were developed (Franco-Cantabrian region) was with all likehood the most populated of UP Europe by large (and maybe of all West Eurasia)".
Therefore suffering a minimum level of inbreeding?
It would still eventually become inbred if it consisted of less that about 500 individuals.
That's really tiniy, isn't it?
Anyhow, where there is intense inbreeding (like that remote Argentine "albino" town), some problems happen but you are really exaggerating their extent and dimensions. In classical Egypt, sybling marriage (the most extreme form of inbreeding) was common and it did not seem to affect the viability of the population. Cleopatra herself was the product of several generations of sybling marriages and she was still considered a sexy woman.
To some extent that's true, but in the present post we are almost certainly speaking of relatively stable population sizes in most regions as the early Upper Paleolithic gave way to the late Upper Paleolithic.
Hmmm. I think here were periods of expansion and contraction. Cutural waves like Gravettian surely represent some population expansion (from Central Europe in this case), while the LGM would mean mostly a contraction. Magdalenian expansion specially after the LGM certainly means a large expansion in geography too and it was surely accompanied by a more dense colonization of already populated areas.
Not so. Inbreeding often leads to total extinction of the population, or even of the species. I read recently that many bird species are on the verge of extinction even though there are many individuals left. Unfortunately the species have been divided into relatively small isolated groups. Inbreeding is carrying out its usual business.
You are talking about extreme fractioning of the population. This was certainly not the case with UP Europeans, at least not to such extents. When we hear of inbreeding problems among mammals like Tsavo lions, we are talking of single reserve isolation: populations of maybe just a few dozens of individuals. Such extremes probably never happened in UP Europe.
To put it in another way: with your reasoning Andamanese (a much tinier, isolated and fragmented people) would have gone extinct long ago. They still thrive though.
Regarding Central Asia, the Caucasus and Zagros. We can guess that they have high heterozygosity because the regions have amoung the highest diversity of haplogroups in the world.
I said Central Europe, not Central Asia.
Anyhow, the increased genetic variability of those regions is probably a much more recent phenomenon, due to multiple waves of immigrants since Neolithic times.
"One reason for Solutrean and later Magdalenian expansion may be that the country where these techno-cultures were developed (Franco-Cantabrian region) was with all likehood the most populated of UP Europe by large (and maybe of all West Eurasia)".
Therefore suffering a minimum level of inbreeding?
Possibly. We tend to consider West Asia as the main source of West Eurasian population and dievrsity. But SW Europe was also a very densely populated region through all the UP. It's hard to evaluate because, unlike what happens in Europe, West Asia may not have been so throughtly explored in archaeological terms... but, in Europe at least, the Franco-Cantabrian province was all the time much more densely inhabited (as per the archaeological evidence) than any other European region.
I guess this can well mean comparable populations to those of the main West Asian provinces: the Levant and southern Anatolia, if not even larger. [The Iran-Iraq province seems to have gone deserted in the LGM].
"In classical Egypt, sybling marriage (the most extreme form of inbreeding) was common and it did not seem to affect the viability of the population".
In fact sibling marriage was common but it usually didn't last many generations before significant outbreeding. Besides the New Pharaoh was often not the product of these sibling marriages but of other wives.
"Such extremes probably never happened in UP Europe".
We don't know that and in fact there was probably significant isolation in many places.
"but you are really exaggerating their extent and dimensions".
I am convinced that the influence of inbreeding and hybrid vigour have been greatly underestimated as driving forces in evolution. And animal breeding.
Yes. Five hundred is a low number but human tribal groups have probably consisted of about that number for an extremely long proportion of our own evoltion.
Five hundred is a low number but human tribal groups have probably consisted of about that number for an extremely long proportion of our own evoltion.
I haven't done my maths but I'm pretty sure that 500 is not real even for the last hunter-gatherer survivals of our age, who live(d) in very marginal enviroments like the Kalahari or the Arctic.
I am convinced that the influence of inbreeding and hybrid vigour have been greatly underestimated as driving forces in evolution. And animal breeding.
While I can't argue that you don't have a point, I certainly cannot agree in your extremist vision of the issue.
I was chatting with Tim yesterday and he told me that you have a farming background. Then I realized that, when we talk of animal breeding, you are probably thinking in cows and I am thinking rather in stuff like cats (Im an urbanite, admittedly). The case is that while cows (or other cattle) are raised for their production, pets are bred for their "purebreedness" and "pedigree" (mostly for aesthetic reasons), so inbreeding is much more extreme among pets probably and it's "solved" by selection of only the best individuals wthin that narrow breeding pool. Such extreme cases of inbreeding are probably much more rare among agricultural cattle in fact because "purebreedness" itself is not the main goal but productivity.
In fact sibling marriage was common but it usually didn't last many generations before significant outbreeding. Besides the New Pharaoh was often not the product of these sibling marriages but of other wives.
I was thinking mostly on the common people. It seems that sybling marriage had become popular in Egypt at least by the Hellenistic period and did not cause particular troubles anyhow. At least that's what I've read.
"Such extremes probably never happened in UP Europe".
We don't know that and in fact there was probably significant isolation in many places.
You may be right for the smaller cultures, that were generally wiped off eventually. But at least since Gravettian and probably since the very Aurignacian the numbers involved in European demographics were relatvely high, as shown by the aboundance of archaeological sites. There was probably some bottleneck with the LGM (not necesarily a very massive one but enough to cause some major drift) but that's all.
Of course not all regions were equal and the FC province shows along time a very apparent advantage, probably caused by a much better climate and resource aboundance. But even in less advantageous regions like Eastern Europe there doesn't seem to have been any major problems, as shown by cultural continuity from Gravettian to Chalcolithic and also some expansion into West Asia, the Balcans and other areas maybe.
"I'm pretty sure that 500 is not real even for the last hunter-gatherer survivals of our age".
Aborigine family groups, for example, usually considered themselves as belonging in a wider group of about 500 according to many scientists who studied them. I'll see if I can find references on the net.
"inbreeding is much more extreme among pets probably and it's "solved" by selection of only the best individuals wthin that narrow breeding pool".
I currently breed poultry as pets, not for production, and am aware that many pet breeders are extremely aware of the problem of inbreeding. Many breeds are very prone to genetic abnormalities if breeders are not careful. The problem is not confined just to 'productivity'.
"cases of inbreeding are probably much more rare among agricultural cattle in fact because 'purebreedness' itself is not the main goal but productivity".
And that productivity goes down with just a relatively small level of uncontrolled inbreeding. It's possible to calculate the drop mathematically when considering which bull to put with which cow during artificial insemination. Ancstry records go back many generations for several dairy cattle breeds.
"You may be right for the smaller cultures, that were generally wiped off eventually".
My argument would be that they were absorbed by the expansion of the larger cultures. Humans are not usually as extremely racially segregated as many at this blog seem to believe.
I agree that population was relatively high at times through parts of Europe but as resources diminished we would expact the popualtion fell, especially locally. This certainly seems to be the case as people moved through the Pacific Islands. In fact we seem to find periodically large populations in Europe followed by periods of relatively little evidence of inhabitants.
By the way have you checked out Mathilda's additions to her Cavalli-Sforza post? I'd be very interested to hear your comments.
Aborigine family groups, for example, usually considered themselves as belonging in a wider group of about 500 according to many scientists who studied them. I'll see if I can find references on the net.
But do they only and exclussively marry/reproduce within that group? Or is it rather an exogamous clan?
Many breeds are very prone to genetic abnormalities if breeders are not careful. The problem is not confined just to 'productivity'.
But most genetic abnormalities are of the kind of twisted tail and silly things that do not matter much for survival (though may matter for pet sales). Everybody has some sort of genentic abnormality... it's within the way genetic works.
My argument would be that they were absorbed by the expansion of the larger cultures. Humans are not usually as extremely racially segregated as many at this blog seem to believe.
I did not mean people was wiped off, just the cultures.
Anyhow, I'd like to know how would you explain that, after the spread of Gravettian and the Solutrean counter-wave... a small Aurignacian pervivence, Magdalenian by other name, (arguably more endogamous than the rest, at least culturally) managed to overcome the others? Wouldn't it be more logical that they would have become not viable by mere inbreeding?
Or think of early Indo-Europeans of Samara basin: they were practically isolated in their remote corner of Europe/Asia border but yet they had the vigor to invade many much better connected peoples around them. How if they were probably extremely inbred?
With your model, more extended cultures should always dominate, just because they had the numbers (much lower inbreeding rate). But facts show that, now and then, smaller groups (presumably much more inbred) became more successful.
In fact we seem to find periodically large populations in Europe followed by periods of relatively little evidence of inhabitants.
Not far from my great-grandfather's former household there is a cave that has continuous record since Gravettian to the Iron Age. Obviously it's not the only one. I don't think there's any evidence to sustain such claim in the Franco-Cantabrian region at least (nor probably other European UP provinces). Said that, it's possible that there were some contractions at times like the LGM but not massive contractions in any case.
For example compare the map series for the Basque Country at Wikipedia: Basque Prehistory. You can see there was always, in the UP, about the same apparent density (though the maps only report major sites, mostly caves).
By the way have you checked out Mathilda's additions to her Cavalli-Sforza post? I'd be very interested to hear your comments.
No. I took a look at her original post but the only comment that came to my mind was: "it's a divulgation book, not a paper". The study anyhow it's too old to be really relevant nowadays.
"But do they only and exclussively marry/reproduce within that group?"
Pretty much endogamous.
"But most genetic abnormalities are of the kind of twisted tail and silly things that do not matter much for survival".
One of the first effects is reduced fertility. Domestication can compensate for this difficulty to some extent, although it interferes drastically with productivity.
"Wouldn't it be more logical that they would have become not viable by mere inbreeding?"
Yes. But there would almost certainly have been contact even if the culture remained Aurignacian.
"they were practically isolated in their remote corner of Europe/Asia border".
But they had become a mix of a large number of groups on the steppes by the time they took off. Influences had arrived from all around, especially south of, the steppes. Hardly inbred.
"With your model, more extended cultures should always dominate".
They usually do.
"now and then, smaller groups (presumably much more inbred) became more successful".
Not necessarily more inbred.
"Not far from my great-grandfather's former household there is a cave that has continuous record since Gravettian to the Iron Age".
When did mammoths become extinct in that region?
"The study anyhow it's too old to be really relevant nowadays".
On what grounds does age of a study automatically negate its conclusions?
But they had become a mix of a large number of groups on the steppes by the time they took off. Influences had arrived from all around, especially south of, the steppes. Hardly inbred.
That's an assumption. I don't imagine them totally isolated but they were very much on their own in their small niche before espansion began. They were agriculturalists though.
Yes. But there would almost certainly have been contact even if the culture remained Aurignacian.
Sure, the very process of spread from (apparently) Central Europe to Aquitaine is very obscure itself. What don't understand is why you claim that these cultural minorities were not isolated and yet the whole European genetic pool was. It's very contradictory really.
Also, if Aboriginal Australians can survive well with endogamous units of 500 people, I don't see why others can't.
When did mammoths become extinct in that region?
Hmmm... doubt there were ever any mammoths: too hilly. Would have to check but the typical hunt was more like bisons, horses, deer, goats, some bears...
On what grounds does age of a study automatically negate its conclusions?
Just meant that much other data has poured since that pioneering study (or rather set of studies) by Cavalli-Sforza. He also put forward a large ammount of ideas and hypothesis around the hard data that himself has had to review later on.
The very understanding of what statistical interpretation of autosomal DNA (or proteins in the case of CV's studies, if I'm not wrong) means has also widened. PCs are not automatically anymore meaningful without a good understanding of the sample locations and ammounts (something he did not provide in the book certainly)...
All that make the study limited in interest (for readers in 2008 - it was very interesting in 1996) on its own. At least in my opinion.
"What don't understand is why you claim that these cultural minorities were not isolated and yet the whole European genetic pool was. It's very contradictory really".
Agreed the European gene pool was largely isolated (although by no means completely). This inbreeding is why Europeans all look roughly alike. However within that larger gene pool there would have been even more isolated groups. Isolated by forest, rivers, mountains etc.
Evolution can only work through the spread of new genes, but it cannot work on the genes themselves. They must become fixed (double recessive) before selection can act on them. This can only happen through inbreeding. If you can think of some other way please let us all know.
This process is the most obvious way to account for the mysterious changes of the Early through to Late upper Paleolithic people covered in the article. Again, if you can think of another explanation please let us know.
They must become fixed (double recessive) before selection can act on them. This can only happen through inbreeding.
Not really: evolution acts all the time, just that not-expressed recessive genes are not directly affected. But they are indirectly affected by much worse odds of succesful reproducion in the long run. It's precisely only inbreeding what can fixate an unfavorable trait (yet one not fatal in early age); a trait like Rh- blood group (which was in the past probably very dominant, if not fixated, among Europeans).
This process is the most obvious way to account for the mysterious changes of the Early through to Late upper Paleolithic people covered in the article.
I understand that sometimes we are comparing pears and oranges, so to say, when doing these kind of comparisons. Ideally we should copare only skulls in the same micro-region (say Dordogne, for instance, surely the area with more remains in all Europe) and for many different "moments", not just the vast ages described as EUP and LUP (what happened to the two phases of the MUP, btw?), that include periods spanning many many milennia, but the remains are scarce and spread around so we only have so much to compare.
We can only see a few pictures, not the whole sequence. The craniometrical differences are anyhow not so radical, IMO: the skull tends to be more slender, more modern and there may have been a factor of sexual selection in that (hard to say but we do tend to follow some arbitrary patterns of beauty in which "archaisms" are often not favored - that may be the main reason why they have become archaisms). But they can be related to the Cro-Magnon prototypes and even to the older Aurignacian ones, just that we really don't know well the details of the evolution: it's like trying to understand a film with just a handful of photograms.
As for the bodies, smaller bodies can certainly be caused by poor nutrition and harsh conditions in general. For instance, just two decades ago Galicians were the shortest group within Spain, while now the young generations are the tallest ones (or so I have read from someone better informed than me). This obviously does not imply any genetic change, just that the living conditions have improved immensely and a hidden potential has arisen in consequence.
Anyhow most of the remains available are "Cro-Magnons" (a somewhat wide term anyhow) from the Gravettian period (also Gravetto-Solutrean in southern Iberia though here there are already some changes, specially in the jaw), AFAIK. The very sample is limited and unequal. I may even suspect that particularly robust skulls may have had much better chances of surviving the ages and reaching us than lighter built ones. In any case the sample is very limited: in all the Basque country, for instance, only one UP skull was ever found, was only poorly described and eventually got lost. Hopefully there are other richer regions where some localized comparison can be made maybe but in general the sample is very spread around, both in space and time.
Also Gravettian is clearly intrusive in SW Europe. It rooted well in southern Iberia (from where Cro-Magnons may have spread to North Africa) but in the Franco-Cantabrian region it was soon replaced by Solutrean and this soon after by Magdalenian ("soon" meaning here a few thousand years, though in some specific cases the transition between newly arrived Gravettian and rapidly adopted Solutrean is really fast anyhow). I suspect that these transitsions in the most important (demographically speaking) archaeological province of all Europe may have been accompanied by some flows and counter-flows of peoples or just beauty standards. As we know, a whole big lot of things can happen in a few thousand years.
I'm beginning to see your problem. You don understand practical genetics. "evolution acts all the time, just that not-expressed recessive genes are not directly affected".
Of course they're not. That's the nub of the matter. Evolution cannot act on recessive genes until inbreeding leads to them being expressed. I would have though that was obvious.
"It's precisely only inbreeding what can fixate an unfavorable trait". And a favourable one too, obviously.
Earlier you wrote: "Not far from my great-grandfather's former household there is a cave that has continuous record since Gravettian to the Iron Age. Obviously it's not the only one". But now you say: "We can only see a few pictures, not the whole sequence". And, although you say, "The craniometrical differences are anyhow not so radical" the article claims, "Groups living before and after the LGM, Early Upper Paleolithic (EUP) and Late Upper Paleolithic (LUP), respectively, differ significantly in craniofacial dimensions, stature, robusticity, and body proportions".
I've just understood your problem. Not having a farming background you don't understand practical genetics. You may find it useful to refer to this essay:
http://remotecentral.blogspot.com/search/label/Human%20Evolution%20on%20Trial%20-%20Hybrid%20Vigour%20And%20Inbreeding
Especially the bit called "Wave Theory of Evolution".
And a favourable one too, obviously.
No. A favorable one can get fixated even in situations when inbreeding is low. It is actiavely selected every generation and in the mid run it should become dominant and eventually fixated (this last may vary depending on the level of selective advantage and the size and variability of the population).
Let's see, inbreeding does favor fixation in all cases certainly but, independently of this factor:
1. Disadvantageous genes will tend to become less common and disappear
2. Neutral genes will show no particular trend (just random fluctuations)
3. Advantageous gense will tend to become more common, dominant and eventually even fixated (it's much more than just mere drift: it's positive selection)
In small inbreeding groups, drift becomes more intense, incresing the chances of fixation of all kind of genes, even the unfavorable ones. But this doesn't mean that the general patterns mentioned above cease to exist, nor that the extreme drift in such populatons is the only possible factor favoring the dominance/fixation of favorable genes.
But it's the only possible factor favoring fixation of disadvantageous or even neutral genes. It can also speed up the fixation of advantageous genes but it's not the only nor the main factor in this trend.
Earlier you wrote: "Not far from my great-grandfather's former household there is a cave that has continuous record since Gravettian to the Iron Age. Obviously it's not the only one". But now you say: "We can only see a few pictures, not the whole sequence"
But one thing is seeing the whole sequence of technology and culture and another thing that of human anthropometry. There are lots of stone tools, even a lot of art pieces... but there are only a handful of skeletons or skulls. AFAIK in that cave I mentioned the only skull ever found is that of cave bear used by Neanderthals in their rituals apparently.
And, although you say, "The craniometrical differences are anyhow not so radical" the article claims, "Groups living before and after the LGM, Early Upper Paleolithic (EUP) and Late Upper Paleolithic (LUP), respectively, differ significantly in craniofacial dimensions, stature, robusticity, and body proportions".
"Significantly" (wouldn't it be "significativley"?) is not the same as "radically". AFAIK the overall consensus among prehistorians is that of human continuity at least since Gravettian times and probably since Aurignacian ones. Said that, the process is not fully understood precisely because we lack of really complete info.
...you don't understand practical genetics.
I think that it's you who may not understand that. I've been very patient with you but please do not judge me, judge what I say. Does it make sense or not? Why?
I don't know if breeding has too much to do with real genetics in the wild. Maybe or maybe not, the exceptionally high degree of control by humans on who survives and reproduces and who does not may resemble what works in nature but it's not quite the same. I'm under the impression that in artificial breeding both admixture and inbreeding are brought to unnatural proportions. Besides, the resulting animals often would not survive in reallife situations (cows without horns, c'mon!).
"Does it make sense or not?"
Not at all.
"Why?" You wrote: "I don't know if breeding has too much to do with real genetics in the wild. Maybe or maybe not".
Why on earth would genetics in wild species be different from that of domestic ones?
Your comment: "A favorable one can get fixated even in situations when inbreeding is low. It is actively selected every generation and in the mid run it should become dominant and eventually fixated (this last may vary depending on the level of selective advantage and the size and variability of the population)".
Complete rubbish. You seem to believe favourable mutations are always dominant, therefore expressed and selected for, and unfavourable ones always recessive. That's not how it works.
There is no way for even favourable genes to be selected for until they are expressed. This can only happen through the formation of double recessives. Double recessives will not happen, or only very infrequently indeed, in populations without some level of inbreeding. Certainly once you have inbreeding your points 1, 2 and 3 will hold.
"AFAIK the overall consensus among prehistorians is that of human continuity at least since Gravettian times and probably since Aurignacian ones".
And probably for even longer than that. But humans are not a completely interbreeding species. Otherwise we would all look exactly the same as each other, or near enough to it. Observation will tell you we vary regionally, exactly as do all other widespread species.
These differences show up at extremely local levels, as many posts at Dienekes have shown. Isolation has been a factor in our evolution, as it has been in all species. Where to draw the boundaries is also a problem common to all species. And it may pay me to remind you that in the past we didn't travel around in aeroplanes or even motor cars. But populations isolated for some time eventually mix with incoming ones. And we get evolution.
You seem to believe favourable mutations are always dominant, therefore expressed and selected for, and unfavourable ones always recessive. That's not how it works.
Not at all. I was basically thinking in generalistic terms of co-dominance if anything.
What is clear is that favorable gense will always be selected for in the mid run, because that's what "favorable" actually means: it gives an advantage to its carriers at least statiscally speaking. In situations of very small populations though, Chaos becomes active in form of drift, founder effects and other random accidents and the favorable mutation may die off though. But generally speaking, they will very strongly tend to perpetuate and become more and more dominant, precisely because they are advantageous.
Instead, within the same geenral statistical parameters, neutral genes will tend to remain roughly stable and disadvantageous genes will tend to drop in frequency or even disappear totally. Only when a population is really small can truly disadvantageous genes "succeed", due only to random factors, that become much more important in low population scenaries.
And probably for even longer than that. But humans are not a completely interbreeding species. Otherwise we would all look exactly the same as each other, or near enough to it.
Even among syblings there are very noticeable differences. After all they only share roughly some 50% of the DNA. Most people get surprised that my sister and I are syblings at all. She would go unnoticed in Stockholm, while I can pass by a local in Istanbul instead (after some tanning).
And it may pay me to remind you that in the past we didn't travel around in aeroplanes or even motor cars.
But we did travel: on foot certainly but mostly by ship or on horse. Distances of 50-100 km were not rare for marriages, for example - and to that you must add the varied influence of mid/long distance colonists, fishermen, traders, migrating invaders... You see the first type of long distance colonization in Neolithic spread (even if much was surely just absroption of locals), traders were surely active in the Megalithic and Bell Beaker networks and later too, migrating invaders are certainly best related to Indoeuropean and Semitic waves. All them had impact across long distances, sometimes very noticeable.
Additionally, in the Paleolthic, tribes were at least seminomadic and often probably truly nomadic. As they migrated they surely entered in contact with other groups, made alliances (including marriages) and so on, only to part ways some years or generations later maybe.
So there was gene flow across mid/long distances, even if not dominant in most cases. Enough in any case to relieve inbreeding from becoming excessive. Of course there would always be more "cosmopolitan" and more isolated groups, but what I still fail to understand from your reasoning is how the "cosmopolitan" (much less inbred) groups were not always dominant thanks to that magic "vigor" of yours. My logic is that admixture, while generally improving the genetic pool by increasing its diversity, is not enough to give a decisive advantage in most cases. The real advantages are in the socio-cultural plane instead. Otherwise you just can't explain things like Germanic invasions (I can hardly think of any more isolated place than Scandinavia, at least in Europe - maybe Scotland?). These were not obiously driven by any sort of genetic advantage, but by socio-economic-cultural factors like the developement of stirrup and heavy cavalry, and the new weakness of their southern Celtic neighbours as they became semi-civilized an too dependent on fortified towns and their trade networks, whose destruction disorganized the very socio-economical system of the Celts.
Had the Celts lower genetic diversity and supposed "hybrid vigor" than Germanics? Not at all: they lived in middle Europe, at the very crossroads of all genetic flows and had recently absorbed many other peoples. But their supposed genetic advantage was not enough to prevent their fall to Germanics and (as reaction) to Romans. They just had evolved socio-culturally in a direction that made them vulnerable. Also they were surely never interested in invading the far cold ruthless North. This is a generic advantage of peripheric barbarians against "civilizations": the latter don't want to, or even cannot at all, get involved in the badlands, where ther's just no business to make at all. This badlands may be populated by relatively isolated and inbred people but at some point the civilizations fall in crisis, show their weaknesses and the hardy "cossacks" (or "bedouins") from the periphery can take over, plunder and even replace the ruling elite. Not sure how this may apply to Plaeolithic contexts though.
"Not sure how this may apply to Plaeolithic contexts though".
I wondered that from soon after I began reading your reply! Paleolithic people certainly didn't travel "mostly by ship or on horse". On the other hand I certainly accept that Paleolithic people moved huge distances, but their total numbers were much less than our numbers are today.
"I was basically thinking in generalistic terms of co-dominance if anything".
It's true that if a characteristic is a product of co-dominance selection for that gene can begin straight away. However study of domestic animals, admittedly perhaps obeying a separate set of genetic laws (Maju 2008), has revealed that most genes, favourable and unfavourable, do not show up until male and female with common ancestry are mated. This suggests they originate as recessive genes.
To clarify the situation regarding inbreeding I'll quote from my university text book ("Quantitative Genetics" by D. S. Falconer). Again I realise that you claim that any beliefs concerning genetics older than about last Thursday have been completely overthrown. But this from a section headed "Inbreeding":
"On account of the limitation in the number of independent ancestors in any population not infinitely large, all genes now present at a locus in the population would be found to be identical by descent if traced far enough back into the remote past".
How many populations are "infinitely large"?
Finally this recent Dienekes post totally supports my position on the current subject:
http://dienekes.blogspot.com/2008/12/isolation-with-migration-model.html
This all will help you understand why siblings can be so different (limited inbreeding). The fact that inbreeding can bring out favourable genes will also help you understand why some apparently inbred populations have been so successful.
So you think, based in commercial animal breeding research, that most favorable genes are recessive. Ok, I take note but I can't judge that assumption.
This would nevertheless contradict your theory of "hybrid vigor". Whatever the case, I rather tend to think that genes are not that important, that culture is instead. There's an excessive emphasis today in "differential genetics", as if genes would have the answer to everything - and obviously this is not the case: genes may help or hinder a bit but most of the variance is merely cultural. For example, Muslims and Westerners have about the same genes (at least for the bulk of their populations), yet culturally they have evolved in very different ways. And these differences were not real before Illustration I'd say. Instead more distant populations in the genetic coponent, say Singaporeans or South Africans, are very much culturally similar to Westerners (I'm excluding from this "western" category all those fundamentalist Christians, naturally, as they are anti-Illustration and can hardly be considered the same culture at all).
So, well, it's a cultural matter first and foremost. And I would even say it's a matter of freedom and rationalism, elements that are obviously not in the genes.
"I rather tend to think that genes are not that important, that culture is instead".
To a very large extent I agree with that. However humans tend to be born into a culture. It would be a rare person who was born Roman Catholic and grew up a Muslim for example. And culture can either unite or separate groups. So genes remain important.
"And I would even say it's a matter of freedom and rationalism, elements that are obviously not in the genes".
From my observation "freedom and rationalism" are not too common in the world, unfortunately.
And you may find elements of this interesting. From John Hawks. Some of it covers inbreeding:
http://johnhawks.net/weblog/reviews/genetics/mtdna_migrations/sub-saharan-africa-population-size-behar-2008.html
To a very large extent I agree with that. However humans tend to be born into a culture. It would be a rare person who was born Roman Catholic and grew up a Muslim for example. And culture can either unite or separate groups. So genes remain important.
I'm glad that you agree to some extent with my "culturalist" perspective.
But in other times people did change religious/cultural affliation more easily than you seem to think: Christians became Muslims and vice versa, depending on who was in power locally. Today maybe it's rare that Christians convert to Islam but to Atheism or Agnosticism is very very common. And that implies certain cultural paradigm switch (more emphasis in science and also vital goals in "this" life, for instance). This switch make the raditional rituals, like Christmas or religious funerals/wedings, for example, meaningless as unifying elements and sort of an intergenerational cultural barrier.
In any case I can't agree with your ultimate caveat of "genes remain important". Genes may be important but hardly in the context of modern (it has less than 1500 years of age) cultural divisions like the one between West and Islam. There's no actual genetic "barrier" between these two cultural groups nor genetic unity inside them. All that belongs to the imaginary realm of mythic genealogies, not real ones. It's cultural, ideological, not truly genetic.
And you may find elements of this interesting. From John Hawks.
I had already read it but I fail to see how it relates to our discussion, sorry.
I think we've pretty much exhausted the subject. However I meant to add before that culture itself can lead to inbreeding. We have a local, long established Christian sect, the Exclusive Brethren, who have been advised not to marry within their own congregation. Guess why.
And finally, I agree that many of our modern cultural divisions are "less than 1500 years of age" but we were originally talking about the Upper Paleolithic. Cultural divisions probably existed in those times too.
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