George W. Bush was no dummy, but he was stubborn. No matter what new facts developed, he claimed that they proved what he had said all along. He devoted so much effort to gaining intelligence and then when he got the facts he did not rethink..
The “Ardi” fossil (see: New Details on 4.4 Million Year Old Hominid) lived long before speech is suspected; even so it has overturned scenarios of human origins by removing chimpanzees, bonobos, and gorillas as reference points when considering the habits and appearance of the last common ancestor (LCA) of chimpanzees/bonobos/humans.
The discovery that knuckle-walking evolved separately in the chimpanzee and the gorilla is not a total shock—I have read anatomists who predicted as much—but it did contradict conventional opinion. It used to be that if chimpanzees had one trait (C) and humans had another trait (H), we tended to assume that C was a trait of the LCA and H was an evolved trait. This tendency was especially strong if the C trait was also found in gorillas. But what can we assume now about the LCA? Did it gesture like chimpanzees? Was it as smart as a chimpanzee? How far back toward square one must we go?
Fossil evidence establishes several surprising facts about Ardipithecus ramadus. Most unexpected was the information that adult males were only slightly bigger than the females and its upper canine teeth had already shrunk from the fang-like daggers of many primates to something more stubby. The male and female canines are similar in size and shape.
The long, sharp canines of the primate world have been used as the chief indicator of male on male fighting—typically over females and territory. Even primates, like gibbons, that have little size difference between the sexes, show males with larger, sharper canines. This early disappearance has come as a complete surprise and forces a serious reconsideration of the timing of the many changes along the path from ape to human. Is it possible that the canines shrank because of much reduced tensions between the males?
It has long been known that the canines of Australopithecus was small and stubby, but reduced tension between males was not generally accepted as the reason. For one thing, there was a notable difference in size between the sexes, and that too is a good indicator of male on male aggression. Alternate explanations offered was that the Australopithecus mouth had many more teeth than common among primates and the canines had to shrink to make room for them, or the use of cutting tools might have reduced the need for canines, or even better control of vocalization might have been the reason. But all those explanations now go by the wayside. Ardipithecus already had stubby canines long before the extra teeth or cutting tools or any hint of vocalization.
This blog has long held the position that speech origins are to be explained more by a radical social change than an intellectual one. Could it really be that the social change was already underway 4.4 million years ago?
In my original post on the new Ardipithecus work, Terrence Deacon pointed out the main argument against betting strongly on a very early change in social structure. Whatever their teeth, Australopithecus had a noticeable difference in male/female sizes, and that too is a strong indication of male vs male aggression.
The importance of these changes are considered in the series of papers that are available for free from Science magazine for those who register: here. In particular, of interest to this blog is C. Owen Lovejoy’s paper, “Reexamining Human Origins in Light of Ardipithecus ramidus,” and Lovejoy specifically considers the question of the difference between male and female Australopithecus [p 74e3].
- First, he casts doubt on the reliability of the claim of a difference, noting it is based on comparisons of fossils that are separated by space and time. The differences, therefore, might not have been so great in any local assemblage of individuals.
- Second, he says that even if the difference is real there is no “useful correlation” between sex-based skeleton size differences and social structure. Both chimpanzees and gorillas exhibit strong male against male aggression, but their skeletal differences are striking. I wrote to Lovejoy for clarification and he replied that "in gorillas male skeletons are very much larger than those of females, whereas in chimpanzees and bonobos, the skeleton is the same size in both sexes--chimps and bonobos are not skeletally dimorphic." (I wrote back to say, doesn't this mean that we cannot assume from the skeletal differences alone that Ardipithicus males and females were the same size. Lovejoy replied, "Yes, at least in body mass, but all studies of living primates show that CANINE dimorphism--not body size dimorphism--is the key to aggressive behavior in males.")
- Third, in many primates larger male body size is not associated with male-on-male aggression. The difference may arise from environmental pressures unrelated to combat for mates.
In other words, we cannot assume that because Australopithecus sexes had different skeletal sizes but similar canine sizes they had a more aggressive social organization, and we especially cannot assume there was a return to an earlier male-on-male combat zone. But this argument cuts both ways. Taking the second and third points, we cannot really be sure that male-on-male aggression shrank just because sex-based skeletal sizes became much more similar. The skeletal and teeth evidence contradict. We can pick one and say ahaa, or we can claim uncertainty.
At this point I lean toward uncertainty. About six million years ago an arboreal lineage of apes moved onto a woodland ground. (The difference between woodland and forest is in the canopy. Forest trees are tall and dense enough to hide most of the sky, while woodland trees are sparse enough for the sky to be visible and grass and brush to grow on the ground.) There they adapted a new form of bipedal locomotion and, apparently, a new social structure. (We would expect some social rearrangement, even without the teeth and skeletal evidence, just on the basis of habitat change.) Since we know nothing of the social system of pre-Ardipithecus (not even that it had a social system; the LCA could have been solitary, like orangutqans) we cannot say much about the change.
Lovejoy’s paper provides a great deal of speculation, but it is based on very little evidence about Ardipithecus beyond the skeleton and teeth. His principle point is that instead of looking at each of the different traits in humans as resulting from one or another environmental pressure, we should consider all or many of the traits as an “adaptive suite,” a range of traits all arising from some basic shift behavior or environment. He then proceeds to make a plausible, elaborate, case linking bipedal skeletons, shrunken canines, and loss of sexual seasons in females to monogamous social structures in a territory shared with many other couples and their children.
Maybe he is right, but lots of rival conspiracy theories sound plausible too. I think I could work out just as reasonable a story in which, instead of domesticating the males, the females became much more aggressive. Or maybe we could devise a third suite in which symbolic ritual appears much earlier in the human lineage than previously supposed.
It used to be, and in popular science sometimes still is, that the whole of human evolution was explained as a suite of responses to bipedal locomotion. Get bipedalism, the rest of humanity follows as night after day. That line only failed because it turned out that bipedalism is so early. It is hard to argue that things which appeared only millions of years later and only in some lineages were the inevitable result of the first cause. So I mistrust suites that are not backed by extensive empirical evidence.
I’m also suspicious because, for Lovejoy, this paper simply proves what he has been saying all along. It is very common for people to respond to unexpected news with (a) now there’s a surprise, but (b) it just proves what I have been saying all along. Of course somebody might have been right all along, but in writing about science history I have learned that surprises usually indicate that something critical was missing from the thinking. What everybody was saying all along was wrong.
And lets not kid ourselves. The break from the chimpanzee/bonobo model is a big surprise. Yeah, yeah, we all knew that chimpanzees and bonobos had been doing quite a bit of evolving themselves, but people took it for granted that despite whatever diddling around the margins took place, we could get a pretty good idea of what the LCA had been like, in the abstract, by looking at chimpanzee or bonobo behaviors. Michael Tomasello’s book Origins of Human Communications begins with a long look at chimpanzee voluntary communications (gesture). Why? Because he takes it for granted that by knowing the abstract basics of chimpanzee gesture we know the basics of LCA voluntary communications. But do we? The same assumption holds in Deacon and Pinker.
We struggle so hard to come up with information about human origins, the least we can do when we get some is pause, appreciate the surprise, and wonder what else we’ve been assuming a bit too casually.
What makes so sure Ardi is an ancestor of man? The small brain, less than a chimp, does not support this. It is well established that bipedalism occurred many times.
--------------------------
BLOGGER: I'm not sure what the bipedalism many times refers to, but it is not settled that Ardipitheicus ramidus is part of the human lineage. Because of the bipedalism it is considered a hominim, one of the descendants of the LCA. Our line might have been different, but the key thing for this post is that Ardipithecus was not at all like what was expected.
Posted by: bl | October 19, 2009 at 04:16 PM
Unless we have D.N.A.'s of fossils, we will not be able to describe correctly the lineage of man. All we have right now are synthetic lineages, proposed descriptions that count on likeness that can be the result of convergevce rather than direct evolution. One must not exclude the possibility of two and more lineages of apes that developed bipedals to live some very similar fossils. The 99% sameness of human and chimp D.N.A. and 4-6 m. years of presumed separation still suggest strongly that there could have been a chimplike - human line with Ardi a neglected spectator.
-----------------------
BLOGGER: Although I agree with the general principles of this comment, I'm troubled by the general 'pay no attention' to the news thesis. Everything we know is tentative. Einstein once said over coffee in Berlin 1924 that everything science says is wrong. But I think we do better to embrace the errors of the moment, with the understanding that divorce is possible at any time, than we do by cautiously waiting for all the evidence to be in. The latter will never happen.
Posted by: Yair shimron | October 20, 2009 at 08:59 AM