Princeton University anthropologist Alan Mann, PhD, has an evolution-based explanation for why third molars don't erupt properly: As the size of the human brain increased, there wasn't enough space left for them, according to an article in the Daily Mail.
Up to 25% of individuals are born missing one or more third molars, which could be attributed to natural selection. The chronic pain of the tooth being unable to erupt normally perhaps led to less reproductive fitness, the article noted. Over generations, this could have given humans without third molars a slight advantage.
