I'm doing some reading and ran across a 2009 post by Brian Switek ("Darwin, Ardi and the African apes"), who touched on a little-appreciated aspect of Darwin's conception of human relationships:
Yet there is something else that has long gone overlooked about Darwins oft-quoted African apes passage. Today we take it to mean that out of all living apes our species shared a recent common ancestry with chimpanzees and gorillas, thus suggesting that humans evolved in Africa. Darwin did not have the details but the consensus is that he turned out to be right in a general sense. In truth, however, Darwins conception of human evolution may not have been as modern as we have presumed.
As Switek describes, anthropologists often credit Darwin with a very modern conception of primate phylogeny. This credit comes because of a passage in the Descent of Man in which Darwin argues that chimpanzees and gorillas, the African apes, are the closest to people. In the course of my reading of the Descent this year, I will come to that passage and consider it in some detail. The important reason for anthropologists to note that passage is that it directly contradicted Haeckel, whose work on human evolution began earlier than Darwin, and who had claimed that humans are closer to the orangutans than the African apes.
The drawing above, which Darwin produced in 1868, does not follow the scheme described in the Descent. "Man", at the extreme left in the phylogeny, is a sister group to a three-way trichotomy of chimpanzee-gorilla, orangutan, and Hylobates branches. What I find even more interesting is that Darwin clearly changed the arrangement by reversing the branches with Hylobates and chimpanzee-gorilla written on them. If we take his tree strictly as a phylogeny, in which the topology is determined by the arrangement of the branches, then the left-right positions of these two branches do not matter since the branches form a trichotomy. But it is not obvious that Darwin had in the back of his mind what most undergraduates today think about these trees -- that putting species near each other on the page is a sign that they are more closely related.
This was a manuscript page in Darwin's notes, and from its context it is clear that Darwin himself was not ready to commit on the subject. The drawing was accompanied by a short description on the reverse side, which reads in part: "Arrangement as far as I can make out by comparing the view of various naturalists ... For myself I have no claim whatever to form an opinion."
Who were the naturalists on whom Darwin depended? One of them was Haeckel, whose 1868 Natrliche Schpfungsgeschichte included a phylogeny that connected humans with Hylobates and orangutans as Asian "man-like apes" in opposition to African chimpanzees and gorillas.
But probably Darwin depended on Thomas Huxley for most of his knowledge of these apes, who had published an extensive description of what was known about orangutans, gibbons, chimpanzees and gorillas up to that time in his 1863 Evidence as to Man's Place in Nature. Huxley did not provide any phylogeny or textual assessment of which of the "man-like apes" may be the closest to humans. Huxley observed that many of the descriptions of these primates were unreliable, and he took the attitude that comparing descriptions across different apes was a way to test their veracity. Reading his text with this in mind, it is easy to take away the feeling that these apes mostly share characteristics with each other that make them different from humans. The exception in Huxley's text is his evocative description of upright posture and gait in gibbons. If one were to take Huxley's description and plot a phylogeny from it, I think it would look like the first version of Darwin's drawing above, with Hylobates placed closer to humans, but not necessarily more closely related to humans.
This scenario is also consistent with Wallace's 1864 argument about human evolution; that we are a long, independent lineage from other primates that originated as early as the Eocene period.
As I get to this section of the Descent I'll be looking very carefully at why Darwin shifted his view from this 1868 note.