Humanity: a Sexual History or Make Love, Not War

Apart from the obvious “pass the gene” role sex had a crucial role in the modern humans history. At some point, the old chimpanzee-like strategy  (an alpha male had sex with any female he wanted and a new leader killed the old leader’s babies) was replaced by more or less monogamous society structure. Thus allowing for the long childhood under the protection of both parents.

A scheme  of modern humans movements according to the single origin hypothesis. The resident species different from H. sapiens shown in different colours. Note that the denisovan a not on the map, as the scheme predates their discovery.  Looking at human evolution is like guessing the picture on 1000-pieces jigsaw puzzle when you have parts of 10 random pieces. Image by NordNordWest via Wikimedia Commons.

There are  two competing theories about the modern humans (Homo sapiens) origin:  the single origin vs multiregional theory. The mainstream, single origin theory postulates that the modern humans originated in Africa and spread to the other continents. It means that  despite our morphological differences such as skin colour we are a single species. This theory is confirmed by successful cross-breeding of humans everywhere – from Australia to North America.

The multiple origin theory postulates that  “H. sapiens” is not just one species, but  rather a composite, which originated in several places, so different human races have independent origins. While there wasn’t much evidence to support this hypothesis,  recent  human genomes sequencing shows that, as it often happens in natural sciences, the marginal, multiple origin theory had some merit.

Human evolutionary tree according to multiregional theory. By Fred the Oyster via Wikimedia Commons.

It’s been known for a while that migration of H. sapiens brought it in contact with other, resident Homo species, such as H. heidelbergensis.  As we are the only modern Homo species, you know who won. So the question is not who but how.

Given our propensity to kill everything in sight, including our closest surviving relatives, chimpanzees, for a while the prevailing theory was that we killed them all, including “the Northern cousin”, the neanderthal (H. neanderthalis). But the neanderthal genome sequencing brought an interesting discovery: modern European humans have a couple of percent of the same DNA as the extinct species. Neandertals did not vanish without a genetic trace, they  crossbred with us.

Discovering new humanoid species, such as Denisovans, allowed us to realise, that  “sapiens x  neanderthalis” wasn’t the only hybrid”. Apparently, people in South Asia who don’t have neanderthal DNA in their genetic makeup, have  DNA of yet another hominid species, denisovan (H.denisovan), instead.  The resident African humans, such as pygmies, which stayed put, do not have an admixture of  either neanderthal or denisovan, but some other archaic hominid species. So different races are bit different, although nowhere as much as much as the multiple origin theory postulated.

A current version of human evolutionary tree. Note links between sapiens, denisovans, helderbengensian, and neanderthals in the upper right corner. The tree looks more fluid than the very straight “no alternatives allowed” multiorigin hypothesis tree. By Homo-Stammbaum, Version Stringer.jpg: Chris Stringer derivative work: Conquistador [CC BY-SA 3.0] via Wikimedia Commons

The other question of course how did hybridisation happen? In chimpanzees,  to prevent excessive crossbreeding young males leave the family group and travel,  eventually settling with an unrelated female. This has a parallel in pre-industrial human societies, where males had more freedom to travel. Also, unlike males, “foreign” females were considered non-threatening and even desirable.

 Bible: Ezra 9:1-2

…The people of Israel and the priests and the Levites have not separated themselves from the peoples of the lands with their abominations, from the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians, and the Amorites. For they have taken some of their daughters to be wives for themselves and for their sons, so that the holy race has mixed itself with the peoples of the lands.

I’d like to speculate that prehistoric human males probably didn’t mind shaking up with some neanderthalian or denisovan maidens.


*Modern human genomes reveal our inner Neanderthal

*New DNA analysis shows ancient humans interbred with Denisovans

*Evolutionary History and Adaptation from  High-Coverage Whole-Genome Sequences of Diverse African Hunter-Gatherers

Penis size interacts with body shape and height to influence male attractiveness

Women on Top: How Real Life Has Changed Women'...An interesting topic, isn’t it? Unfortunately, the authors,  probably  not wanting to attract attention of the IgNobel Prize Committee, obscured their findings in an impenetrable academic jargon  (See below).

I provide a translation to  (hopefully ) human-readable language:

In many animals male genitala (MG) is a trait, which is selected post-copulation, e.g. if the male genitalia work better during copulation, fertilisation is more probable. However,  in humans  MG can be selected before copulation through the female’s choice.  This was easy before the clothes and there is hypothesis that human penis is larger than  in the other apes and lacks a penile bone  as a result of sexual selection: women like bigger penises.

The authors have shown (105) females videos of men with different bodies and penises. They discovered that women preferred bigger penises, but after MG reached a certain size, their attractiveness started to decline.  Size mattered more for selection of shorter  men  and men with more masculine bodies – wide shoulders, narrow hips (professional swimmers must be very popular with the ladies):  a short man with a larger penis is more attractive than the same height man with a smaller penis, but less attractive than a taller man.

The most interesting finding was that larger penis and greater height have almost equal attractiveness.  I think this may explain gradual evolution of average human height from petite to inching above 2 meters. Which in turn increases chances of cancer. Also it would be interesting to see a symmetrical study: boobs vs female height.

Compelling evidence from many animal taxa indicates that male genitalia are often under postcopulatory sexual selection for characteristics that increase a male’s relative fertilization success. There could, however, also be direct precopulatory female mate choice based on male genital traits. Before clothing, the nonretractable human penis would have been conspicuous to potential mates. This observation has generated suggestions that human penis size partly evolved because of female choice. Here we show, based upon female assessment of digitally projected life-size, computer-generated images, that penis size interacts with body shape and height to determine male sexual attractiveness. Positive linear selection was detected for penis size, but the marginal increase in attractiveness eventually declined with greater penis size (i.e., quadratic selection). Penis size had a stronger effect on attractiveness in taller men than in shorter men. There was a similar increase in the positive effect of penis size on attractiveness with a more masculine body shape (i.e., greater shoulder-to-hip ratio). Surprisingly, larger penis size and greater height had almost equivalent positive effects on male attractiveness. Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans. More broadly, our results show that precopulatory sexual selection can play a role in the evolution of genital traits.

Proc Natl Acad Sci U S A. 2013 Apr 23;110(17):6925-30. doi: 10.1073/pnas.1219361110.