Thursday, 30 April 2009

Paternal investment and agriculture

In sub-Saharan societies, female-dominated agriculture is associated with low paternal investment and high polygyny rates. Why? The short answer is that year-round tropical agriculture enables women to meet their food needs and those of their children without a male provider. Paternal investment thus tends to fall to zero and men are free to maximize their reproductive fitness by mating with as many women as possible.

Long answer: this difference between tropical and non-tropical humans was already present before agriculture. Even during the hunter-gatherer stage, men provided more food for their mates and offspring with increasing distance from the equator, apparently because the longer winters made food gathering impossible for women during much of the year. Thus, when agriculture became the new mode of food production, men were much more predisposed to exploit its possibilities outside the tropics.

For instance, most livestock were domesticated in Europe and Asia, the guinea fowl being the only food-producing animal to have been domesticated in sub-Saharan Africa. Livestock domestication thus seems to have been limited not only by technical constraints (availability of wild animals with the right characteristics) but also by psychological constraints (relative predispositions of men and women to take part in food production).

Wherever men had previously provisioned their families with food from hunting, they were more inclined to do the same by domesticating game animals—often the same ones they used to hunt.

I discuss some of these points in my latest article: "Sexual Selection and Human Geographic Variation."

Reference

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.

http://www.jsecjournal.com/articles/volume2/issue4/NEEPSfrost.pdf


12 comments:

Tod said...

"Paternal investment thus tends to fall to zero and men are free to maximize their reproductive fitness by mating with as many women as possible".

Male genital mutilation: an adaptation to sexual conflict "This ‘sexual conflict’ hypothesis predicts that MGM should be associated with polygyny"


Wherever men had previously provisioned their families with food from hunting, they were more inclined to do the same by domesticating game animals—often the same ones they used to hunt.
10,000 Year Explosion site says that Africa was where "the big game had a chance to adapt as mankind gradually became formidable hunters and thus managed to survive until today". Presumably European animals had less fear of man and were easier to tame. An "ethnic group of Siberia (Altai/Tuva area).. uses large reindeer as mounts and cargo animals". Africans trying to tame wildebeest would have a much more difficult task

Anonymous said...

Nice paper and nice theory Peter, it makes a lot of sense.

I was reading a "paper" from Tacitus, the roman historian who described early german tribes.
He said that, in his own views, german men didn't do much most of the day, mostly drinking, eating and playing. I understand that it is first century CE, long after the period of sexual selection that you describe in your paper, but not so long after, maybe just 2 or 3 millenia.
If this leasure time is indicative of a relatively easy life and sufficient food supply, then by first century, the harsh conditions of sexual selection that you say are necessary for the fair skin genes to spread, seem reduced or not present anymore.
How can these genes be maintained and spread so quickly if the selection pressure is already gone ?

RG

Tacitus at
http://www.sacred-texts.com/cla/tac/g01000.htm
see paragraph4

Peter Frost said...

Tod,

Modern humans entered Eurasia 50,000-40,000 years ago, and agriculture began about 10,000 years ago. I presume that 30,000 years would have been long enough for Eurasian game animals to develop fear of humans.

Anon,

I argue that sexual selection of women was unusually intense in the continental Arctic of the last ice age. This ecozone was larger and more habitable in western Eurasia, specifically the Great European Plain of northern and eastern Europe. In this region, the costs of polygyny were prohibitive -- women were almost wholly dependent on men for food provisioning. Meanwhile, men had high death rates because of the long hunting distances they covered in search of highly mobile herds. Together, these two factors created a permanent surplus of mateable women who had to compete for a limited supply of mateable men -- hence, the intensity of the sexual selection.

Tacitus described northern Europeans about eight thousand years after the end of the last ice age. Sexual selection of women would have been much weaker, though still present to some degree. As Tacitus himself noted, polygyny was rare among the Germanic tribes. But, again, the show was already long over by then.

Tod said...

30,000 years would have been long enough for Eurasian game animals to develop fear of humans.

I was thinking it was the selection imposed by killing at a distance by throwing that produced an adaptation in animals. The distance at which they would bolt from hominids would have been increased.

I don't have a date for modern human's use of projectiles but in Europe that was less than 30,000 years Experimental evidence concerning spear use in Neandertals and early modern humans.
"data, in combination with fossil humeral cross-sectional data and the lack of evidence for throwing spears among Eurasian Neandertals, suggest that previously documented humeral strength asymmetries in Eurasian Neandertals and early Upper Paleolithic Modern human males can be plausibly linked to spear thrusting."

Where there was a shorter period of selection by projectile predation animals may have been relatively phlegmatic about the propinquity of humans. A related possibility is that African animals may be faster and more powerful, (eg the 11 foot leap of the Springbok). which would make the initial stages of domestication more difficult.

While I think your post gives the primary cause of the lack of domesticated African animals there are some differences between them and the ones that were tamed elsewhere.

Tod said...

On closer reading the lack of people out food gathering would mean Eurasian animals would be, if anything, more likely to have evolved to fear humans as the presence of humans would almost always mean hunters with spear-throwers like the 27,000 year-old atlatl made of reindeer antler found in France. (An interesting fact is that the Gray Wolf is the most effective natural predator of adult reindeer. Reindeer travel the furthest of any terrestrial mammals." " Wikipedia Reindeer
When hunting wolves have an actual travel rate of about 50km (30mi) a day. (Wolves: Behavior, Ecology, and Conservation 2003) Hunting reindeer on foot would entail some very long distances to be covered.

Anonymous said...

Peter, I understand you well, and I think your theory makes lots of sense.

My point was more about the timing. If by the first century, the form of sexual selection that you described is mostly over, you are left with 8 thousands years, at most, to generate modern european features.

But modern europeans were present already. In his book "Saxons, Vikings and Celts" Bryan Sykes showed how one 12000 years old and one 9000 years old individuals found in the Cheddar cave were related to bristish people today. Although it doesn't say what they looked like, we can assume that they didn't show all modern characteristics such as fair skin, hair and eyes, since the mutations for these characters are all younger than 12000 years. And recently 9500 years old skelettons have been found near Paris.

The mechanism you describe might be true, but how do you see these genes spreading geographically so fast in populations so scattered. You said once "a girl with blond hair is snatched from her tribe by the neighbors". How long will it take for the genes to spread from the Caucasus to Gaul and Ireland and become 100% predominant by the time of Tacitus or Strabon ?

Wouldn't it more simple to assume that these mutations popped up independantly in geographically distant european clans and were then selected by the mechanisms that you indicate?
How do you see that ?

thanks,
RG

Peter Frost said...

Tod,

You have a point. The entry of modern humans into Europe only goes back c. 35,000 years. But even in Africa, modern humans are not all that old. There is good evidence that as recently as 100,000 years ago humans were still behaviorally 'archaic' despite being anatomically modern.

Anon,

We don't have firm dates for the advent of white skin and diverse hair and eye colors among Europeans. The data I've seen point to the late ice age or perhaps the early Holocene. I agree that we're looking at a relatively narrow time frame, perhaps only five thousand years or so. If I had to bet, I would place it in the second half of the last ice age, about 10,000 to 15,000 years ago.

If this is the correct time frame, any pigmentary mutations, once selected, could have circulated rapidly from one end of Europe to the other. The Great European Plain was inhabited by highly mobile hunting bands who had wide-ranging trade networks for precious stones and the like. I imagine these networks could have also been used for mating. There are certainly precedents for this sort of thing in other hunter-gatherer societies.

Tod said...

For instance, most livestock were domesticated in Europe and Asia, the guinea fowl being the only food-producing animal to have been domesticated in sub-Saharan Africa.A critical recasting of that might be that the farmed animals are absent from the indigenous fauna of sub Saharan Africa.
" Among the striking negative characteristics of the Ethiopean zoogeographical region of Wallace -that is to say the whole of Africa from the southern part of the Sahara to the extreme south of the contenent -is the absence from the indigenous fana of certain animals that have long been domesticated in other parts of the world. Among these are the subfamilies Ovinea (sheep) and Caprinae (goats and their allies and the generalBos /i>(cattle), Sus (boar)and Gallusfowl.(Baker 1974)

Sub Saharan Africans were capable of domesticating (eg Guinea fowl), so why did they never tame the one animal that might be expected; dogs are not in the category of farmed animals. 100,000 years ago dogs were tamed in Asia, presumably to help with hunting. As hunting is a pleasurable pastime for Africans they did not lack incentive to domesticate dogs.
I think it has to be taken into account what was available to them, the reason they never tamed dogs might partially to do with the "dogs" that were available. True dogs - tribe Canini are not found in sub-Saharan Africa.

The " African Wild Dog (Lycaon pictus) is a carnivorous mammal of the Canidae family, found only in Africa", it has a completely different dominance/ mating /social structure to that of the gray wolf "Males typically do not leave the pack they were born to.[7] This is the opposite situation to that in most other social mammals, where a group of related females forms the core of the pack or similar group. The females compete for access to males that will help to rear their offspring. In a typical pack, males outnumber females by a factor of two to one, and only the dominant female is usually able to rear pups."

As wolves were tamed to help in the hunt the difference in their method of operation when hunting in the wild might be most significant.

Wolves are capable of coordinating deceptive tactics and the staging of ambushes, they have special calls to anounce their arrival at an ambush site. The wolf would easily adapt to driving game towards a hunter. The African Wild Dog "is a cursorial hunter, meaning that it pursues its prey in a long, open chase". This would be very likely to result in the prey being driven away from the human hunter.

This is an example of a sub Saharan African equivalent of a Eurasian animal that was unsuitable for domestication. There may be others, I still think paternal investment is the main reason for the dearth of tamed animals in Sub Saharan Africa though, just trying to anticipate the line sceptics would take.

Tod said...

Wolves outperform dogs in following human social cues "Our findings indicate that domestication is not a prerequisite for human-like social cognition

RG said...

" any pigmentary mutations, once selected, could have circulated rapidly from one end of Europe to the other. The Great European Plain was inhabited by highly mobile hunting bands who had wide-ranging trade networks for precious stones and the like"

I have trouble with that. Is it possible that in every corner of Europe, every island, every isolated valley, there has been so much genetic input that by the time of Tacitus there is no polymorphism left for the old genes, at least for the skin.
Maybe it's posible, but the more these remote places are densely populated with people with "old genes", the more of this mating trade is needed to replace them.
It's not like the new genes carriers/traders enter into unpopulated territories. Gaul population alone was estimated around 10 millions by the time of Tacitus. The quantitative aspect seems very dependant of the time imparted and the size of the initial genetic stock.

RG

Peter Frost said...

Tod,

Dogs are used in West African societies, but I don't know the time depth for their introduction. I'm still skeptical about the argument that African game animals are not suitable for domestication. I've heard it before and it strikes me as being circular. The same hurdle existed with the ancestors of Eurasian farm animals -- the unsuitable characteristics were bred out.


RG,

Actually, swarthy skin is still present among indigenous Europeans, even in northern Europe. Fleure mentioned this point in his article on skin color variation:

"In a few places in Sweden, Britain, and France people have been noticed who show characteristics of the skull and face that remind one of late-Paleolithic man: these people are usually darker, in hair and eyes, than their neighbors; sometimes they even have swarthy skins. Although this fact may not have great weight in argument, it does hint that there has been depigmentation in this region. The many stories of golden hair and blue eyes suggest that sexual selection may have helped the change."

Fleure, H.J. (1945). The distribution of types of skin color, Geographical REview, 35, 580-595.

If we go back to the Upper Paleolithic, or even the early Holocene, we're looking at a relatively small breeding population in Europe. If memory serves me right, the estimates are in the range of twenty thousand or so (I'll have to track down that reference).

Tod said...

Dogs are a special case of 'domestication', in a sense they came ready made. What must explain their astoundingly early domestication 100,000 years before any other animal is that they had behaviors that made them useful to hunters (who gave them the entrails and bones) from the beginning, and social adaptations that let them live with humans as one of the 'pack'.

Breeding over all this time should have meant dogs have devoloped abilties that wolves lack . Until recently I was under the impression that due to their modification by man dogs, but not wolves, are able to realize where a food token is hidden if a human points or just looks ( chimpanzees usually can't do this ).

Seems like that's wrong, wolves being extemely social animals already had such ability proof of this is that they can do this better than dogs. Wolves outperform dogs in following human social cues I think they just fitted in to a great extent due to having useful behaviors while needing very little modification. Of course dogs were not farmed animals so there no parallel to sheep, cattle, and pigs or their African equivalents.

(Africanis "There is ample evidence that no canine domestication took place in Africa and that the traditional African dog is a descendant of dogs that had been domesticated in the East and came to Africa. Their earliest presence has been established in Egypt and dated at 4700 BC. Archaeological records show that, from then on, the dog spread rapidly along the Nile into Sudan and even beyond. At the same time, migrations, trade, and transhumance took it deep into the Sahara. By 2000 BC, this moving frontier stopped for a long period. Meanwhile, throughout the Egyptian dynasties, the breeding of swift and slender hounds together with a variety of common dogs became very popular.

For thousands of years, the aboriginal Stone Age San (Bushman) populations in Southern Africa hunted without the help of dogs. Although the Khoikhoi brought domestic sheep along a western migratory route to the Cape of Good Hope just before the Christian era, there is no conclusive evidence that dogs were part of their party")