Sex in Prehistory

Humans, especially males, tend to fret quite a bit about the size of their penises. Scientists do as well, but for different reasons. A quick look at theories behind this can be found in Mystery Dance: On The Evolution of Human Sexuality, by Lynn Margulis and Dorion Sagan. A lot of the book is theory and speculation, but that’s often the fun of books like those.

First, a few concepts have to be understood. From a geneticist’s point of view, the sexual characteristics and behaviour of all animals, including humans, can be explained through the chromosomes wanting to replicate themselves as much as they can.

For the female, there’s the matter of getting pregnant. The next generation carries her genes and can then pass them on. For the male, it’s a matter of being the one lucky enough to have access to a fertile female. There are two main strategies at opposite ends of a spectrum.

One strategy is typified by the gorilla. Males are larger than females, a phenomenon called sexual dimorphism. One male becomes dominant over all the others and wins exclusive access to all females in the group. The other strategy is total promiscuity. In chimpanzees, the dimorphism is less pronounced. Chimpanzee females are described as the sexual athletes of the animal world, mating several times a day with as many males as they can when they are in heat.

The first strategy, typified by the gorilla, is called “sperm competition avoidance.” One dominant male has exclusive access to the females, so only his sperm will be used to impregnate them. The second, typified by the chimpanzee, is called “sperm competition.” In this case, there is no sexual exclusivity, and one male will try to displace sperm already present in the female while leaving his own to impregnate her instead.

Sperm competition is essentially sperm competing to get to the ovum first. If a woman has sex with two different men only a few hours – or a few minutes – apart, even the sperm of the second man will try to get there first, and may even succeed. Indeed, with chimps, there will often be brothers and cousins getting together to mate successively with a same female. A geneticist would say they would be trying their best to have their shared genes passed on.

It so happens that species with “sperm competition avoidance” strategies will usually not have large genitals, nor large quantities of sperm. Since there is no competing sperm to worry about, there will be none to be displaced by large genitals. But for sperm competitors, the penis must be large enough to displace sperm left previously by others, and the testicles must produce enough sperm each time to compete with the sperm left behind by others.

What does all this have to do with human sexuality? Well, we’re generally a monogamous species (there are exceptions to every rule). Since monogamy implies sexual exclusivity, we are considered to be, by and large, sperm-competition avoiders. Our males shouldn’t need their genitalia to be as large as they are. So, how did we come to have such large organs?

Chimpanzees and humans share a common ancestor from which they diverged about six million years ago. There are many similarities between chimps and humans. Male chimps have fairly large genitals, larger testicles than those of humans, and a penis at least half the size of that of a human.

According to experts, large testicles and penises are usually found in promiscuous animals. The male gorilla, whose genitals are quite small, has a harem which it guards jealously. Like gorillas, the australopithecines showed a bigger difference in body size between sexes than men and women do today. This suggests they were sperm-competition avoiders, and the males would have had small genitals.

This would have started to change when subordinate male members of the earliest Homo species started upsetting the applecart by offering food to females in exchange for sex. Time went on, and by the time an intermediate species of Homo came on the scene, males weren’t all that much larger than females. Homo “intermediate” (either erectus or ergaster) started cooperating more and more, and this would set the stage for sperm competition. Eating and sleeping together in groups would have promoted more talking and socialising in general. A fair amount of promiscuity may have been the norm until pair bonding became a way to ensure peace among all the males and strengthen group cohesion.

(Mental note to self: What about the females? They get jealous, too.)

A trend toward monogamy and romance may have begun as Homo sapiens males took fertile females away from the group for some “alone time.” Chimps sometimes do this. Theoretically, it’s a way of making sure that male is the one with an opportunity to impregnate the female. However, the chimp that dares do this could be beaten by the other members of the group for breaking group conventions.

It’s thought early Homo species did the same thing and may have risked similar mistreatment by other group members. Only later when pair bonding became the norm did this become not only acceptable but almost mandatory.

The theory is that the human species is moving back to a state of sperm competition avoidance. Since this has only started relatively recently, we still have large genitals left over from humanity’s more promiscuous days.

What does the future hold? Given that we can now consciously decide to avoid pregnancy completely and have sex for pleasure only, and perhaps even throw off our jealousy and possessiveness in some cases, it would be interesting to see things play out, if only we could live long enough.

As I said, there are many similarities between chimps and humans, but one main difference is female chimps continue to experience oestrus cycles, while human females do not. How important a factor could that be in determining whether or not humans will be sperm-competition avoiders?