Incest prevelant in early homo-sapiens?

[Undesired Walrus]

Would incest have been a common practice in early homo sapien history, devoid of the revulsion so many feel toward the prospect these days? If most groups were very small and family centered, surely incest would be natural?

[Mark6]

Wolves live in small, family centered groups. Yet when cubs reach maturity, they often leave to join other groups. They seem to have a natural aversion to incest.

AFAIK, solitary animals like tigers are far more willing to mate with their siblings or parents than social ones. Probably because in the wild their chances of running into a relative are fairly low.

[HansMustermann]

Human groups weren't probably _that_ small.

Look at the primitive tribes out there, because early humans probably lived like that too. And you can look at chimps too, since they're our closest non-extinct relative.

Virtually nobody splits off in family-sized groups. A tribe can number anywhere between tens and hundreds. According to Dunbar, the natural "mean group size for humans" is around 150 (the so called "Dunbar number"), which apparently matches the census data for a lot of tribes and low-tech villages.

But the Dunbar number is based on brain complexity and size, and basically means how many relationships you can keep in your head. It's based on plotting that against group sizes for a lot of primates, and when you punch in the values for a human brain you get 147.8.

It's basically what group size an ape with a human sized brain would tend to have. I.e., without inventing any further concepts or forms of organizing itself.

So it's likely that early humans, whose brains weren't that much different from ours, tended to be in groups of 150 or so. As the group grew larger than this, just the fact that nobody really had a relationship with everyone else in the tribe, would cause subgroups to form and the group would eventually split into two. Then it would be two groups of less than 150 for a while, but then population grew back. So anyway, some groups were probably larger temporarily, some smaller, but 150 seems like a good estimate for the average size.

And groups of 150 are plenty to not _need_ incest.

If we guess an average family would mean, say, two adults and three children (more would be born but die early), there would be some 30 such families in an average tribe. That's plenty to get a son- or daughter-in-law without incest.

But even the most primitive tribes have invented _some_ forms of relationships between groups. A primitive form of diplomacy if you will. No idea if the very early humans had that yet, but it's not that great a stretch of imagination that they at least talked to each other now and then.

Marriages across group lines would be very possible too in that setup.

[Soapy Sam]

All humans are related.
In smaller groups they will be more closely related.
Adam and Eve had two sons.

Not only was there incest; there was gay incest.
And it worked!