Why did we evolve to have so many nerve endings in our anuses?

That’s so we can tell when the ghost peppers have left our system, or when yesterday’s chili was hot enough. Otherwise we might be oblivious to this vital information.

One possible anal invader for our ancestors might have been blow flies and other fly species, attracted by the fecal odor there. The adult flies wouldn’t stay to occupy the anal canal or rectum, but would lay their eggs there. Their larvae (maggots) could then live there feeding on the abundance of bacteria to be found, or even chewing away at anorectal tissues as some maggots do. Sheep are very prone to perianal maggot infestations if nothing is done to prevent it; it’s a disease called sheep strike, and can destroy an entire flock. This is why ranchers bob the tails of lambs at birth, so a long natural tail doesn’t get contaminated with urine and feces and attract flies to the anus. For a prehistoric cave person or modern bush person, it would pay to feel a tickle or itch if a fly was crawling or laying eggs there, and (even in his/her semi-sleep) brush or scratch it away.

In some primitive cultures today, the floors of their huts are crawling with parasitic and scavenging arthropods—bedbugs, fleas, lice, and more. That’s a reason for sleeping on elevated platforms that make it a little harder for those organisms to invade the body. (The chigoe flea is a good example, if you want to read up on that). A bit of anal scratching when irritated would help further to repel such invaders (yet it also helps the spread of pinworms).

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To be a more serious, though, every body orifice is a potential route of invasion by other organisms. If we think of prehistory and our distant, naked ancestors sitting and sleeping on the ground—or even the various tribes of bush people today—it’s not far-fetched to conceive of other organisms attempting to invade the body by way of nose, ears, mouth, anus, urethra, or vagina. Not only do we have dense sensory nerve endings at these openings, but also high concentrations of immune cells (lymphocytes) and protective secretions (mucus, earwax) on guard against invasion, especially by microbes.