So far as cerebral structure goes... it is clear that Man differs less from the Chimpanzee or the Orang, than these do even from the Monkeys, and that the difference between the brains of the Chimpanzee and of Man is almost insignificant, when compared with that between the Chimpanzee brain and that of a Lemur.

We have to face the fact that we are the descendants of apelike ancestors. The truth, at first sight, is often ugly and repulsive to our personal feelings, but when it is the truth, its ultimate effects on us are always salutary. ...
... Man's brain does not stand as a thing apart; it is the culmination of an ascending series. There is no part of it and no function manifested by it that cannot be traced to humble beginnings lower in the animal scale. And what we postulate for man's brain we must in all justice apply to that of the ape, the dog, and all other beasts.

No one has any right to be angry with me, if I think fit to enumerate man among the quadrapeds. Man is neither a stone nor a plant, but an animal, for such is his way of living and moving; nor is he a worm, for then he would have only one foot; nor an insect, for then he would have antennae; nor a fish, for he has no fins; nor a bird, for he has no wings. Therefore, he is a quadraped, had a mouth like that of other quadrapeds, and finally four feet, on two of which he goes, and uses the other two for prehensive purposes.

Men have had the same origin as other animals; they have the same general architecture of body and mind; and they have the same destiny. And they stand on the same general level of ethical claim and obligation.

[Fred] Spoor's observations are truly startling, In all species of the genus Homo, the inner ear structure is indistinguishable from that of modern humans. Similarly, in all species of Australopithecus, the semicircular canals look like those of apes. Does this mean that the australopithecines moved about as apes do—that is, quadrupedally? The structure of the pelvis and lower limbs speaks against this conclusion. So does a remarkable discovery my mother made in 1976: a trail of very humanlike footprints made in a layer of volcanic ash some 3.75 million years ago.

All animals and people have the same core emotion systems in the brain. (p5)

In the past decade, there has been some fascinating research on the brains of transgender people. What is most remarkable about this work is not that trans women’s brains have been found to resemble those of cisgender women, or that trans men’s brains resemble those of cis men. What the research has found is that the brains of trans people are unique: neither female nor male, exactly, but something distinct.

We hold these truths to be self-evident that all P-brains are created equal.

What does it matter if two brains are isomorphic, or quasi-isomorphic, or not isomorphic at all? The answer is that we have an intuitive sense that, although other people differ from us in important ways, they are still 'the same' as we are in some deep and important ways. It would be instructive to be able to pinpoint what this invariant core of human intelligence is, and then to be able to describe the kinds of 'embellishments' which can be added to it, making each one of us a unique embodiment of this abstract and mysterious quality called 'intelligence.

That however the brains and abilities of men may differ, their stomachs are essentially the same.

As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between man and the apes, Nature has provided us, in the latter animals, with an almost complete series of gradations from brains little higher than that of a Rodent, to brains little lower than that of Man.

So far from the posterior lobe, the posterior cornu, and the hippocampus minor, being structures peculiar to and characteristic of man, as they have been over and over again asserted to be, even after the publication of the clearest demonstration of the reverse, it is precisely these structures which are the most marked cerebral characters common to man with the apes. They are among the most distinctly Simian peculiarities which the human organism exhibits.

From a comparative perspective, we are a relatively monomorphic mammal, with relatively monomorphic minds.

The human brain has left and right brain symmetry with its own nature and can process information which initially appears to have no pattern or order. However, the brain has the ability to process visual information much more efficiently.