lowing five passages encapsulate for us Darwin’s thinking about the place of humans in primate phylogeny and about the uniqueness of modern humans.
If the anthropomorphous apes be admitted to form a natural subgroup, then as man agrees with them, not only in all those characters which he possesses in common with the whole Catarhine group, but in other peculiar characters, such as the absence of a tail and of callosities, and in general appearance, we may infer that some ancient member of the anthropomorphous subgroup gave birth to man.
(Darwin, 1874, p. 160)
It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere.
(1874, p. 161)
In regard to bodily size or strength, we do not know whether man is descended from some small species, like the chimpanzee, or from one as powerful as the gorilla; and, therefore, we cannot say whether man has become larger and stronger, or smaller and weaker, than his ancestors. We should, however, bear in mind that an animal possessing great size, strength, and ferocity, and which, like the gorilla, could defend itself from all enemies, would not perhaps have become social: and this would most effectually have checked the acquirement of the higher mental qualities, such as sympathy and the love of his fellows. Hence it might have been an immense advantage to man to have sprung from some comparatively weak creature.
(1874, p. 65)
As far as differences in certain important points of structure are concerned, man may no doubt rightly claim the rank of a Suborder; and this rank is too low, if we look chiefly to his mental faculties. Nevertheless, from a genealogical point of view it appears that this rank is too high, and that man ought to form merely a Family, or possibly even only a Subfamily.
(1874, p. 158)
Nevertheless the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind.
(1874, p. 130)