Darwin and the African apes

In any book about evolutionary anthropology it is almost obligatory to cite Charles Darwin as the person who suspected that our species was most closely related to chimpanzees and gorillas, thus anticipating our modern understanding. In his famous 1871 book The Descent of Man Darwin wrote;

In each great region of the world the living mammals are closely related to the extinct species of the same region. 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.

Later anthropologists, like Raymond Dart, would connect discoveries made in Africa with this passage, but we should recall that Darwin was tentative about this hypothesis. The following portion of the paragraph reads;

But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates [gibbons], existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

Darwin was right that "our early progenitors lived on the African continent", but we should not lose sight of the fact that he did not have very much to go on. When he wrote the Descent of Man paleoanthropology was a brand new branch of science. More than that, most of the ancient human bones that had been recovered were so close to modern humans, or were interpreted as being so, that there was still a huge gap between our kind and living apes. Indeed, though he turned out to be wrong, the hypothesis of Eugene Dubois and others that humans first appeared in Asia was no more unreasonable than Darwin's view at the time.

Yet there is something else that has long gone overlooked about Darwin's oft-quoted 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, Darwin's conception of human evolution may not have been as modern as we have presumed.

On April 21, 1868 Darwin drew of hypothetical evolutionary tree of primates, including Homo sapiens. It is a bit messy, with lots of bits scratched out, but it provides an important insight into how Darwin thought we were connected to other primates. Let's start from the bottom up.

Near the base of the tree Darwin drew a split between lemurs on one side and other primates (generally anthropoids, being monkeys and apes) on the other. The anthropoid side of the split gave rise to three branches; the Old World monkeys, the New World monkeys, and a motley assemblage of apes and a few cercopithecoids. This latter branch, in turn, produced three new branches.

The first branch on the right consists of langurs, macaques, and baboons, or cercopithecid primates that are considered "Old World" monkeys today. It should be remarked, however, that langurs, baboons, and macaques have traditionally been used as behavior or functional models for certain aspects of human evolution, from infanticide (Hrdy's work with langurs) to Jolly's infamous "seed-eaters" hypothesis. They have often been considered the most human-like of the monkeys, so much so that at times some have preferred them over apes for models of human evolution.

The middle group consists of almost all the living apes. From right to left they are gibbons, orangutans, and chimpanzees + gorillas. Looking at this we see that all these apes are more closely related to each other than any are to the only ape not included among their ranks, Homo sapiens. Indeed, Darwin places humans all the way on the left on a branch that meets with that containing the apes, just above where the common ancestor between apes and the baboons, &c, would lie.

As I just noted this makes gorillas and chimpanzees more closely related to other apes than to humans. Darwin's conception at the time was thus not a precursor to our present understanding that chimpanzees are our closest living relatives, with gorillas, orangs, and gibbons being more distant relatives that bracket the group. (As I have stated before our present understanding makes it perfectly acceptable, if not necessary, to count our own species as an ape.) What, then, are we to do about the famous passage from the Descent of Man? Did Darwin really change his views on human phylogeny since he drew his sketch or are we taking one particular part of the Descent of Man out of context? To me it seems to be the latter.

Another passage in the same book provides a crucial clue. It reads;

If the anthropomorphous apes be admitted to form a natural sub-group, 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 sub-group gave birth to man. It is not probable that a member of one of the other lower sub-groups should, through the law of analogous variation, have given rise to a man-like creature, resembling the higher anthropomorphous apes in so many respects. No doubt man, in comparison with most of his allies, has undergone an extraordinary amount of modification, chiefly in consequence of his greatly developed brain and erect position; nevertheless we should bear in mind that he "is but one of several exceptional forms of Primates."

This is consistent with the figure that Darwin drew. We did not evolve from any living species of ape, of course, but our species arose from an ancient member of the lineage that also led to modern apes. It was a splitting of the two lineages that led away from each other, and so Homo sapiens remained distinct from other apes. (The classic Hominidae and Pongidae split that has now been abandoned.)

I must admit that this conclusion is not entirely the result of my own research. The conflict between how Darwin's "African ape" passage is presently interpreted and the primate phylogeny he actually had in mind was brought to my attention by Richard Delisle's Debating Humankind's Place in Nature: 1860-2000. Although I found it to be disappointing in some aspects, Delisle's book documents a number of interesting ideas about human evolution, including the notion that we may be most closely related to more than one kind of living ape.

Indeed, if Darwin drew the "anthropomorphous apes" as being more closely related to each other then how could chimpanzees and gorillas be "man's nearest allies"? If our species had arisen from some ancient ape stock that gave rise to the rest of living apes then wouldn't we be more or less equidistant from other living apes? This is not a question easily answered, if it can be at all. There appears to be a logical contradiction here, but perhaps there was no such problem for Darwin at the time. In the past some anthropologists have separated our species from other apes but suggested that we shared close ties with them. Perhaps our ancient ancestors even interbred with more than one kind of ape in the past. I am not ascribing such a view to Darwin, I have no evidence to do so, but his views on primate phylogeny were certainly more complex than most modern accounts do justice to.