Runaway brain
A hypothesis that explains the evolution of the human brain through runaway sexual selection. Geoffrey Miller favors the brain-as-fitness-indicator hypothesis instead.
Plausibility of the runaway hypothesis
If creative intelligence varies among male hominids, and that intelligence is genetically inherited, two of the three prerequisites for sexual selection are met. The third is that female hominids show a sexual preference for creative intelligence for some reason. Once this condition is met, highly creatively intelligent males attract more sexual partners and have more offspring. This assumes, of course, that our ancestors were not strictly monogamous. …
The crucial point is that creative intelligence need not provide hominids with any survival advantage. Through the runaway process, it can evolve purely as a sexual ornament. Why did our ancestors evolve brains so rapidly and to such extreme sizes during a period when they apparently did not help with toolmaking or competition with other African hominid species? In the early 1990s, I regarded the runaway process as the most promising explanation. …
Other theorists proposed several other candidates for positive feedback. In 1981, Edward Wilson proposed that large brains enabled complex culture, which fed back to produce even larger brains. … Richard Dawkins supported this view, describing the human brain as a storehouse of learned cultural units called memes. Larger brains could accommodate more memes, which in turn favored larger brains. … In 1976, Nicholas Humphrey argued that pressure for social intelligence turned into a positive feedback loop that drove brain evolution. In 1988, Andy Whiten (AnswerMe) and Richard Byrne (AnswerMe) extended Humphrey’s view by emphasizing the survival benefits of social deception and scheming. … Another possible source of positive feedback was competition between groups rather than social competition within groups. In 1989, Richard Alexander (AnswerMe) proposed that warfare between tribes triggered an arms race in which each tribe competed for superior technical and strategic intelligence. This arms race drove brain size and intelligence upward together.
All these theories have some plausibility. Cultural, social, and military selection pressures were probably important. But these positive feedback loops seemed too speculative. Biologists of the time did not admit them to the hall of fame of evolutionary forces. These models were also hard to apply to interesting traits in species other than primates. They were special models limited to primate and human evolution. Ronald Fisher’s runaway process was different. It was already part of mainstream evolutionary theory, and a leading candidate for explaining complex, costly, ornamental traits in species other than primates. —p116-118, The mating mind
Problems
The mind’s evolution was too slow to explain as a single runaway event:
Human brain size tripled over two million years. From a macroevolutionary perspective, this is tremendously fast—much faster than any known increase in brain size in another lineage. Music, art, language, humor, and intelligence all evolved during this explosive brain growth. … Yet … the mind’s evolution was actually too slow to explain as a single runaway event. Two million years spans 100,000 generations even in an ape as slow to reproduce as humans. Over that enormous time, brain mass increased by only 1kg (0.01g per generation). A sustained runaway process should have been much faster. Assuming ordinary heritability and ordinary variation in brain size, I estimate that runaway selection could increase brain size by at least 1g per generation. This rough estimate uses the lowest sexual selection pressures measured in other wild species. If correct, a single sustained runaway event would be at least 100 times faster than the actual evolution of the human brain. …
The speed problem might be solved by assuming that human brain evolution, like most evolution, proceeded in stops and starts. … Fossil evidence suggests several dramatic bursts in human brain size. One was the transition from the 450g brain of Australopithecus to the 600g brain of Homo habilis (though the current consensus is that Homo habilis was not a direct human ancestor). Another burst produced the 800g brain of early Homo erectus 1.7 million years ago. There were probably further bursts during the following million years of Homo erectus evolution. Another produced the 1,200g brain of early Homo sapiens. The final burst produced the 1,300g brain of modern humans 100,000 years ago. Each burst was brief on a geological timescale, but still lasted hundreds or thousands of generations—enough time for ordinary selection pressures to shape a trait. Fossil evidence remains insufficient, however, to tell whether these bursts were driven by a very fast process such as runaway sexual selection, or a slow one such as ordinary survival selection. …
The problem is why every runaway process worked only to increase brain size, rather than decrease it. In principle, runaway selection could be equally likely to produce a species that behaves as stupidly as possible with the smallest possible brain. Even if stupidity harmed survival, a blundering, incompetent species could evolve if that stupidity were sexually attractive. …
In the next chapter, I will examine another sexual selection process that explains this one-way development much better. (see Brain as a fitness indicator —p125-129, The mating mind
Traits adapted through runaway sexual selection tend to show large sex differences, but human male and female brains differ little:
Another problem with the runaway brain theory is that all traits governed by this kind of sexual selection produce large sex differences. A peacock’s tail is far larger than a peahen’s. If human brains tripled in size because of runaway sexual selection, the increase should have been limited to males. … But it was not. Male brains average 1,440g, female brains 1,250g. Adjusting for body size reduces the difference to 100g. … The same applies to creative intelligence. If creative intelligence evolved through runaway sexual selection, male IQ should be much higher than female IQ. There are sex differences in specific cognitive abilities, of course, but they are very small. Moreover, men excel in some areas and women in others. —p129-130
I examined several factors that might have minimized sex differences: first, genetic correlations between males and females; second, overlap between courtship and mate-choice abilities; third, mutual mate choice. But the most powerful of these, mutual mate choice (See Mutual mate choice), is incompatible with a pure runaway process.
I think mutual mate choice is very important in humans. I must therefore abandon the runaway brain theory. Having opened this chapter by praising runaway theory, I now close by burying it again. … In the next chapter, I will examine a sexual selection model compatible with mutual mate choice. It shows how males and females advertise their fitness to the opposite sex through sexual ornaments. This mate-choice function connects with the origins of sexual reproduction itself. —p153-154