Complex adaptations within a sexually reproducing species need all of their parts, or almost all of their parts, to be universal within the gene pool.
Let's say that you have a complex adaptation with six interdependent parts, and that each of the six genes is independently at ten percent frequency in the population. The chance of assembling a whole working adaptation is literally a million to one; and the average fitness of the genes is tiny, and they will not increase in frequency.
One bird may have slightly smoother feathers than another, but they will both have wings. A single mutation can be possessed by some lucky members of a species, and not by others - but single mutations don't correspond to the sort of complex, powerful machinery that underlies the potency of biology. By the time an adaptation gets to be really sophisticated with dozens of genes supporting its highly refined activity, every member of the species has some version of it - barring single mutations that knock out the whole complex.
Applying this logic to human brains in particular, we arrive at the explanation for what is called the psychic unity of mankind. (Though Eliezer Yudkowsky has used the phrase "psychological unity of humankind" instead.) In every known culture, humans seem to experience joy, sadness, fear, disgust, anger, and surprise. In every known culture, these emotions are indicated by the same facial expressions. (Citation needed to Paul Ekman.)
Donald E. Brown has compiled a list of over a hundred human universals - traits found in every culture ever studied, most of them so universal that anthropologists don't even bother to note them explicitly.
- The Psychological Unity of Humankind by Eliezer Yudkowsky
- Humans in Funny Suits by Eliezer Yudkowsky