An ape by any other name

As usual, I was looking for something else -- this time in the writing of Henry Fairfield Osborn -- and came across an interesting paper that he delivered as a lecture in 1927 [1]. He was addressing general evidence for human evolution, in particular as reflected in the anatomy of anthropoid apes. In the course of this, he rose to the defense of his own theory of human origins, which involved the evolution of our lineage from a Central Asian ancestor that had isolated from the other apes for many millions of years:

About three years ago I was a firm believer in the anthropoid ape theory of ancestry. I listened to a series of most able papers given by a number of investigators--Doctors Tilney, Morton, McGregor, all members of the Galton society--and felt then that their investigations of the anthropoid ape theory was quite established. A year later, however, I went into the central desert of Asia, in Mongolia; there I came under the influence of a new environment, a desert or semi-arid environment, and it flashed across my mind that this must have been the primitive home of man, that anthropoid apes could not have existed here. From that time to this the idea has been growing upon me, and last April, at the bicentenary meeting of the American Philosophical Society, I stated that I personally had abandoned the anthropoid ape theory and I advanced the opinion that man has a long line of Dawn Man ancestors and that the other theory rests upon a large amount of evidence which proves the kinship of anthropoid apes to man but does not prove the ancestry of man through an anthropoid ape type (Osborn 1927:221, emphasis in original).

Many people today make a point of saying that humans did not descend from apes, but that we share an ancestor with apes.

If we confine ourselves to living apes, that is of course true. Our common ancestors with chimpanzees and bonobos (the chumans) were not identical to either of these species, and may have been very different from both. That was one of the key issues raised in the interpretation of Ardipithecus. Lovejoy and colleagues [2] made the case that Ardipithecus is a better representative of many of the traits of our last common ancestor with chimpanzees. Chimpanzees have changed substantially since that ancestor lived, in some ways paralleling the evolution of gorillas and orangutans. If this interpretation is correct, then looking to living apes as models for our ancestors will mislead us on many aspects of their biology -- a point made at length by Lovejoy with Ken Sayers in a 2008 paper [3].

Still, we shouldn't misunderstand this line of argument. Saying that stem hominines were anatomically distinct from chimpanzees doesn't really change the plain English meaning of the word "ape." If we seek a high degree of phylogenetic precision, we shouldn't use the word "ape" anyway -- it's not a taxonomic term. But to introduce the concept of evolution, it's equally misleading to avoid plain language. We shouldn't shroud Miocene hominoids in mystery, as if phylogenetic branching could magically transform them into new organisms. They evolved. Where once there were only apes, now there are some different apes. And us.

Osborn's hypothesis marks the dark side of ape denial: If humans didn't evolve from apes, they may instead have evolved independently from some non-ape ancestor instead:

There is all the difference in the world between kinship and ancestry. When we come down to what we all believe in -- to an anthropoid stem stock, a group from which both the anthropoid apes and man were derived -- we get a neutral form which cannot be defined as either an anthropoid ape or man, but with that type, which has the potentiality of the human stock on the one hand and of the anthropoid ape stock on the other, we come to a parting of our ways, somewhere back in Oligocene time, millions of years ago (Osborn 1927:221, emphasis in original).

Were the chumans a "neutral form", definable neither as ape nor human?

In the 1920's, this was a serious scientific question. Living apes seemed to belong to a single family, humans to another. If orangutans and gibbons could be lumped with the chimpanzees and gorillas, then an independent lineage of apes might indeed go back to the Oligocene.

This idea stood against Darwin's view, and that of most of Osborn's contemporaries (Osborn mentions William Gregory and Arthur Keith explicitly). But it was more or less aligned with Alfred Russel Wallace's view of human evolution, which had been contemporary with Darwin's. Wallace had an independent line of human ancestry going back as far as the Eocene.

Today a heavy weight of genetic and fossil evidence supports a human-gorilla-chimpanzee clade. The ancestor of that clade, whether taxonomy calls it a hominid, hominine or something else, was in ordinary parlance an ape. In many characteristics it was "neutral" -- not assignable to either human or chimpanzee clades. Neither humans nor chimpanzees yet existed. But apes of many flavors did exist. Our ancestors were among them.

Here's Osborn's ending paragraph:

Science works by trial hypotheses. I have one hypothesis, my opponents another. To my mind there is a very strong evidence of the prolonged independent ancestry of man, an ancestry not of anthropoid ape type, but of a neutral, common type. I agree to many arboreal traces in human descent, but I dissent as to the geologic length of arboreal life which my opponents claim resulted in resemblance between apes and man; I dissent as to our ancestry from a type which had specialized as far in arboreal life as the anthropoid ape. My theoretic ancestor belongs to a pro-ape stage, which I call the Dawn Man line. But we are all keeping our minds open; only in that way can we get at the truth (ibid., 230).

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