Daily Archives: February 24, 2009

Chimpanzee Food Sharing

Is chimpanzee food sharing an example of food for sex?

One of the most important transitions in human evolution may have been the incorporation of regular food sharing into the day to day ecology of our species or our ancestors. Although this has been recognized as potentially significant for some time, it was probably the Africanist archaeologist Glynn Isaac who impressed on the academic community the importance of the origins of food sharing as a key evolutionary moment. At that time, food sharing among apes was thought to be very rare, outside of mother-infant dyads. Further research has shown that it is in fact rare … the vast majority of calories consumed by human foragers in certain societies and at certain times of the year comes from a sharing system, while the fast majority of calories consumed by chimpanzees is hand to mouth without sharing.
Continue reading Chimpanzee Food Sharing

Marta’s (good) questions, … fur

Why did humans evolve hairlessness? Hair (fur) protects mammals from heat and cold, what would be the benefit from losing this asset?

I think the most commonly held theory is that fur works on quadrupeds, but once you stand upright, it is less effective, and less fur works better. For later time periods, clothing works better than fur because it is more adaptable. Consider that whatever fur-based system human ancestors had was based on needs in the tropics where it does not get that cold, so it is not hard to imagine that clothing is much more effective.

Recent studies of body parasites suggest that body lice unique to humans differentiated genetically only fairly recently, in the range of several tens of thousands of years. This body lice requires clothing … human clothing on human bodies is the habitat for these lice. This suggests there may have been a reduction from a certain level of furriness only with modern humans living in a wide range of environments and using controlled fire, clothing, and some kind of shelter (hut/house) to deal with the elements. So it is possible that the immediate ancestors to modern humans (perhaps Homo erectus?) were actually fairly furry.

As for details of the body hair, this is also interesting. Why do humans have pubic hair but not a lot of other hair? Why to males have more body hair than females in many cases? Why to human males have facial hair? The African Apes have much less facial hair than most modern human males. It has been suggested that this has to do with sexual selection. It is important to distinguish between the idea that the starting condition is a lot of fur and that females may have lost more than males, vs. the starting condition was very little hair and males have added more. The amount of fur, it’s appearance, etc. may be related to testosterone (this is true in males and females but more obvious in males) so facial hair may be a signal of “quality” in males.

Marta’s (good) questions, … continued

Why did the evolution of a large brain happen only once (among mammals, and in particular, primates?)

Larger brains have evolved a number of times. It seems that there has been a trend over several tens of millions of years of evolution of larger brains in various clades, such as carnivores and primates. There is probably a kind of arms race going on among various species in which a larger brain is an asset.

However, as you imply, a really large brain (like the extraordinarily large human brain) seems to be very rare. One of the reasons for this is that there are at least two major kinds of costs of a large brain that outweigh the benefits. One kind of cost is the energetic expense of having this large brain. Over 10% of the day to day energy demands of an adult human go to the brain. The total energy requirement of an infant can be over 60% while the brain is both a relatively large proportion of the infant’s body, and is undergoing a great deal of growth. The brain tissue is very picky about things like the temperature it requires for normal function and the kind of nutrient it needs.

Continue reading Marta’s (good) questions, … continued