The Penis and the Brain

Why have I paid so much attention to the evolution of the penis? One reason is its importance as a genetic conduit. Every gene in every human body has passed through thousands of penises over thousands of generations of human evolution. Equally, every gene has passed down through thousands of eggs inside female ancestors who chose to copulate with particular males. In sexually reproducing species, copulation is the genetic gateway from one generation to the next, which is what makes it so important evolutionarily, physically, and psychologically.

The penis is an easy trait to study because it is visible, measurable, and directly comparable to the corresponding organs of other species. Yet even for such a simple trait we have seen how the biases of male and female scientists may have influenced their views on penis evolution. We have considered both the sperm competition model and the "symbol of dominance" model for penis evolution. I could have mechanically run through the checklist of criteria for identifying sexually selected traits, but that would get rather tedious for every adaptation I shall be assessing in the rest of the book. The penis's fit to the criteria is rather obvious anyway: the penis shows distinct sex differences (it is much larger than the homologous female organ, the clitoris), grows mainly after puberty, is used during copulatory courtship, is considered sexually attractive by internal touch if not by sight, and differs markedly between species.

Physical organs shaped by sexual choice can also be seen as metaphors for mental organs shaped by sexual choice. Just as the human penis has been misunderstood as nothing more than plumbing for delivering sperm, the human mind has been misunderstood as wiring for processing information. In both cases, I argue that the organ evolved for the stimulation it can deliver, not to solve some straightforward physical problem of insemination or toolmaking. The sexual choice that mattered did not focus directly on the physical form of the organ, but on the shared experiences it could generate. Ancestral females did not apparently favor penises directly as visual ornaments, but favored them indirectly for the copulatory pleasure that they afforded, so they came back for more. Perhaps our ancestors did not favor intelligence and creativity directly, but indirectly: for how they contributed to having a great time with someone. If the penis really did evolve through female choice as a copulatory stimulator, then it should be considered not just a physical organ that reaches inside the body, but a psychological organ designed to reach inside the pleasure systems of another individual. It happens to have a physical form only because the other individual's pleasure systems happen to be connected to tactile sensors.

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