Darwin, "Ardi", and the African Apes

When I wrote about the public unveiling of Ardipithecus ramidus (or "Ardi" to the public) last week I contrasted the description of the hominin with the bombastic rollout given to the lemur-like fossil primate "Ida" (Darwinius masillae) this past May. In the latter case it was clear that a media production company rushed the scientific process and overhyped the conclusions presented to the public, but now there are questions about the relationship between the scientists who described "Ardi" and the Discovery Channel.

This coming Sunday the Discovery Channel will air a special called "Discovering Ardi" which obviously was in production long before the collection of Science papers were released. Could the release of the papers in Science and the documentary have been timed for maximum impact? During a journal club meeting I attended about this time last year one of the co-authors of one of the "Ardi" papers said that the team was trying to get a journal like Science or Nature to publish all the Ardipithecus ramidus papers at once, so obviously the high-impact-release strategy had been in place for some time. But was this strategy something that the scientists came up with, or did the Discovery Channel ask the scientists to do so (thereby delaying the release of research in order to "go broad" with findings that would benefit the media network)? If it is the latter, should we be concerned with the possible control media companies may exert over the release of scientific information?

Uncomfortable as it may be to consider, these are valid questions. There have been several examples of scientists working closely with media companies to give their discoveries more attention ("Ida", "Predator X", "Dakota", "Lyuba", &c.) and, from my perspective, it often seems that scientists can often find that their hands are tied by the interests of those media companies. Coordinated release efforts have become a common thing, and perhaps we should start asking about how to foster greater transparency about the relationships between media companies and scientists with a "hot" discovery. Whether such relationships hinder or help scientists and the public is open for debate.

John Hawks has also pointed out that the Discovery Channel is really playing up the "Darwin predicted this/would have loved this" aspect of the "Ardi" story. This is annoying, especially since Darwin never covered paleoanthropology in any detail. It was a new science when he was working, only really kicking off in 1858 with the Brixham Cave discoveries, so there was not much for him to write about. Still, there is that old "African apes" quote from the Descent of Man, and what follows in a repost of an older essay explaining how our present reading of Darwin's thought on the common ancestor of humans and living apes is quite different from what he actually meant.

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 the discovery of the oldest hominin (i.e. human) fossils in Africa. 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 remember 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. 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 "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, 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. 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 other living ape not included among their ranks, Homo sapiens. 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.

This arrangement 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, but discarded, ideas about human evolution, including the notion that we may be most closely related to more than one kind of living ape.

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? These questions are not easily answered, if they can be at all. There appears to be a logical contradiction here, but perhaps there was no such problem for Darwin.

During the late 19th century some anthropologists separated our species from other apes but suggested that we shared close ties with them on the basis of our resemblances to them. Our closeness to this or that ape, then, would be superficial rather than genetic. (Perhaps the public popularity of the recently-discovered gorilla even played a role in this matter.) Whatever the reason for the seeming disjunction between his words and phylogeny, however, Darwin's views on primate phylogeny were certainly more complex than most modern accounts do justice to.