At first glance, runaway's speed and creative power sound like just what we need to explain the human mind's evolution. Brain size in our lineage tripled in just two million years. From a macro-evolutionary viewpoint, that is very fast—much faster than any brain size increase in any other known lineage. Music, art, language, humor, and intelligence all evolved at some time during that explosive growth. On the geological timescale, the human mind's evolution looks faster than the flash from a nuclear strike does on the human timescale.
But evolutionary speed is relative. The human mind's evolution was actually much too slow to be explained by a single runaway event. Two million years is still a pretty long time— about a hundred thousand generations even for a slow-breeding ape like us. During that time, we added two pounds of brain matter—about a hundredth of a gram of brain per generation. A sustained runaway process would have been much more potent. Assuming a modest heritability and a modest amount of variation in brain size, I estimate that runaway could increase brain size by at least one gram per generation. That rough estimate assumes a sexual selection pressure on the low end of pressures that have been measured in other species in the wild. If this estimate is right, a single sustained runaway event would have been at least a hundred times too fast to explain human brain evolution. Brain size would have tripled in 20,000 years, not 2 million years.
Like a ramjet, runaway sexual selection has more of a minimum speed than a maximum speed. It just can't go slow. This is one reason why the simple runaway story makes a poor explanation for human brain evolution. Compared with runaway's hypersonic speed, human brain evolution was like a stroll through the park on a Sunday afternoon. Yet, if this speed objection seems to undermine the runaway brain theory, it undermines every other positive-feedback theory as well. The other processes proposed by E. O. Wilson, Richard Dawkins, Nicholas Humphrey, Andy Whiten, and Richard Alexander would also have run too fast.
This speed problem might be solved by supposing that human brain evolution, like the evolution of almost everything, happened in fits and starts. There were short periods of relatively fast evolution when selection pressures were pushing in some direction, and long periods of stasis when selection just maintained the status quo against mutation. Fossil evidence suggests that brain size increased quickly in a few dramatic bursts. The transition from 450-gram Australopithecine brains to 600-gram Homo habilis brains was one such burst (though Homo habilis is no longer thought to be our direct ancestor). Another burst produced the early 800-gram Homo erectus brain 1.7 million years ago. There were probably several more bursts during the evolution of Homo erectus over the next million years. Another burst produced the 1,200-gram archaic Homo sapiens brain. A final burst produced the 1,300-gram modern human brain about 100,000 years ago. Each burst looks short in terms of geological time, but lasted for hundreds or thousands of generations, plenty of time for standard selection pressures to mold traits. We do not yet have sufficient fossil evidence to tell whether each burst was driven by a very fast process like runaway or a slower process like ordinary survival selection.
So, where does this leave us? A single runaway event cannot explain two million years of human brain evolution because it would have been too fast and too transient. Instead, we could propose a multi-step runaway process, where each burst in brain size was driven by a separate runaway event. But that would beg the question of why all the runaway events increased rather than decreased brain size. In principle, a species could stumble into runaway sexual selection for the dumbest possible behavior produced by the smallest possible brains. A species of bumbling incompetents could evolve, despite the survival costs of their stupidity, as long as stupidity remained sexually attractive. Runaway is not supposed to be biased in any evolutionary direction, so it should be as likely to decrease a trait's size as to increase it. This makes it a poor candidate for explaining multi-step progressive trends.
Another possible answer to the speed quandary is to forget about fossil brains, and focus on human mental abilities. We do not know when language, art, and creativity evolved. Perhaps they all evolved together when modern Homo sapiens emerged about 100,000 years ago. Some archeologists even think that these capacities all evolved in a single burst 35,000 years ago, in an event they call the "Upper Paleolithic revolution." Such rapid evolution might reflect a single runaway process operating over a few thousand generations in a single population, transforming a large-brained but unintelligent hominid into an intelligent, talkative human. The earlier brain-size bursts may have occurred for some other reason. Perhaps the key transition to the human mind was a brain reorganization rather man a simple brain size increase. The reorganization may not be evident in the record of fossil skulls, but may be more psychologically significant than earlier size increases. It may have been driven by a burst of runaway sexual selection relatively late in human evolution.
However, this theory fails to explain why brain size increased in all those bursts before our species evolved. It seems to me that the multi-burst trend toward larger brains should be explained rather than ignored. Pure runaway cannot explain it, because runaway does not have any intrinsic bias toward larger ornament size, higher ornament cost, or greater ornament complexity. The problem with runaway is not just its rocket-like speed. Its more fundamental problem is its neutrality, which makes it weak at explaining multi-step trends that last millions of years. The next chapter examines another sexual selection process that is much better at driving sustained progress in one direction.
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