Biological organisms in general, and human brains particularly, contain complex adaptations; adaptations which involve many genes working in concert. Complex adaptations must evolve incrementally, gene by gene. If gene B depends on gene A to produce its effect, then gene A has to become nearly universal in the gene pool before there's a substantial selection pressure in favor of gene B.
A fur coat isn't an evolutionary advantage unless the environment reliably throws cold weather at you. And other genes are also part of the environment; they are the genetic environment. If gene B depends on gene A, then gene B isn't a significant advantage unless gene A is reliably part of the genetic environment.
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.
In a sexually reproducing species, complex adaptations are necessarily universal.
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.
So you can't have the X-Men. You can't have "mutants" running around with highly developed machinery that most of the human species doesn't have. And no, extra-powerful radiation does not produce extra-potent mutations, that's not how it works.
Again by the nature of sexual recombination, you're very unlikely to see two complexly different adaptations competing in the gene pool. Two individual alleles may compete. But if you somehow had two different complex adaptations built out of many non-universal alleles, they would usually assemble in scrambled form.
So you can't have New Humans and Old Humans either, contrary to certain science fiction books that I always found rather disturbing.
This is likewise the core truth of biology that justifies my claim that Einstein must have had very nearly the same brain design as a village idiot (presuming the village idiot does not have any actual knockouts). There is simply no room in reality for Einstein to be a Homo novis.
Maybe Einstein got really lucky and had a dozen not-too-uncommon kinds of smoother feathers on his wings, and they happened to work well together. And then only half the parts, on average, got passed on to each of his kids. So it goes.
"Natural selection, while feeding on variation, uses it up," the saying goes. Natural selection takes place when you've got different alleles in the gene pool competing, but in a few hundred generations one allele wins, and you don't have competition at that allele any more, unless a new mutation happens to come along.
And if new genes come along that depend on the now-universal gene, that will tend to lock it in place. If A rises to universality, and then B, C, and D come along that depend on A, any A' mutation that would be an improvement on A in isolation, may break B, C, or D and lose the benefit of those genes. Genes on which other genes depend, tend to get frozen in place. Some human developmental genes, that control the action of many other genes during embryonic development, have identifiable analogues in fruit flies.
You might think of natural selection at any given time, as a thin froth of variation frantically churning above a deep, still pool of universality.
And all this which I have said, is also true of the complex adaptations making up the human brain.
This gives rise to a rule in evolutionary psychology called "the psychological unity of humankind".
Donald E. Brown's list of human universals is a list of psychological properties which are found so commonly that anthropologists don't report them. If a newly discovered tribe turns out to have a sense of humor, tell stories, perform marriage rituals, make promises, keep secrets, and become sexually jealous... well, it doesn't really seem worth reporting any more. You might record the specific tales they tell. But that they tell stories doesn't seem any more surprising than their breathing oxygen.
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.
This may seem too natural to be worth mentioning, but try to take a step back and see it as a startling confirmation of evolutionary biology. You've got complex neural wiring that controls the facial muscles, and even more complex neural wiring that implements the emotions themselves. The facial expressions, at least, would seem to be somewhat arbitrary - not forced to be what they are by any obvious selection pressure. But no known human tribe has been reproductively isolated long enough to stop smiling.
When something is universal enough in our everyday lives, we take it for granted; we assume it without thought, without deliberation. We don't ask whether it will be there - we just act as if it will be. When you enter a new room, do you check it for oxygen? When you meet another intelligent mind, do you ask whether it might not have an emotion of joy?
Let's go back to biology for a moment. What if, somehow, you had two different adaptations which both only assembled on the presence, or alternatively the absence, of some particular developmental gene? Then the question becomes: Why would the developmental gene itself persist in a polymorphic state? Why wouldn't the better adaptation win - rather than both adaptations persisting long enough to become complex?
So a species can have different males and females, but that's only because neither the males or the females ever "win" and drive the alternative to extinction.
This creates the single allowed exception to the general rule about the psychological unity of humankind: you can postulate different emotional makeups for men and women in cases where there exist opposed selection pressures for the two sexes. Note, however, that in the absence of actually opposed selection pressures, the species as a whole will get dragged along even by selection pressure on a single sex. This is why males have nipples; it's not a selective disadvantage.
I believe it was Larry Niven who suggested that the chief experience human beings have with alien intelligence is their encounters with the opposite sex.
This doesn't seem to be nearly enough experience, judging by Hollywood scriptwriters who depict AIs that are ordinarily cool and collected and repressed, until they are put under sufficient stress that they get angry and show the corresponding standard facial expression.
No, the only really alien intelligence on this planet is natural selection, of which I have already spoken... for exactly this reason, that it gives you true experience of the Alien. Evolution knows no joy and no anger, and it has no facial expressions; yet it is nonetheless capable of creating complex machinery and complex strategies. It does not work like you do.
If you want a real alien to gawk at, look at the other Powerful Optimization Process.
This vision of the alien, conveys how alike humans truly are - what it means that everyone has a prefrontal cortex, everyone has a cerebellum, everyone has an amygdala, everyone has neurons that run at O(20Hz), everyone plans using abstractions.
Having been born of sexuality, we must all be very nearly clones.