Posted by jlwile on April 17, 2010
On April 8th, Scientific American’s website had a story entitled “Spectacular South African Skeletons Reveal New Species from Murky Period of Human Evolution.” In that post, the author said:
Scientists working in South Africa have unveiled fossils of a human species new to science that they say could be the direct ancestor of our genus, Homo.
So those who discovered the fossil think that it could be a direct ancestor of genus that contains modern humans. Do the data support this bold claim? Not at all. In fact, based on the actual fossils that have been published,1 A. sediba looks like nothing but a specialized form of extinct ape.
First, you need to understand the nature of these fossils. They seem to come from two individuals. One is assumed to be an adult female (and is called MH2), and the other is assumed to be a juvenile male (and is called MH1). To get an idea of what few remains we have from these two individuals, you should click on the link below, where the actual fossil remains are laid over sketches of what complete skeletons would look like.
So from the assumed juvenile male, we have a fairly complete skull, one fairly complete humerus, one fairly complete tibia, one fairly complete clavicle, and assorted bits from the spine, pelvic girdle, ribcage, femur, and feet. From the assumed adult female, we have a fairly complete humerus, a fairly complete radius, a fairly complete ulna, a fairly complete scapula, and assorted bits from the jaw, clavicle, spine, ribcage, pelvic girdle, femur, knee, fibula, and feet. That’s not much to go on.
However, you’ve got to go with what you have, so now that we know what fossils we’ve got, what do they tell us? Well, let’s start with cranial capacity. We can only get that from the juvenile, so automatically we must be worried about how well-developed the juvenile is. However, the authors think the juvenile’s cranium is probably 95% developed, and the estimated cranial capacity is 420-450 cc, which fits nicely into the australopithecine range of 350-600 cc.2
Next, let’s look at the arms. The arms are very long, exactly what you would expect of an australopithecine. One of the defining characteristics of primates is the size of the radius or ulna relative to the humerus, which is called the brachial index. Since the adult female has one set of mostly complete arm bones, the brachial index is easy to calculate. It ends up being just over 100, which is typical of an orangutan, very close to the 78-91 range of australopithecines, and very far from the 65-83 range of humans.3
So based on these three indicators, A. sediba seems like an australopithecine that might be slightly more orangutan-like than most. Why all the fuss, then? Well, because the authors claim that some of the features of the fossil specimens seem very much like those found in genus Homo, which is the genus to which human beings belong. For example, the authors think the coxal bones of the pelvis and the length of the lower limbs indicate that these creatures walked upright. Thus, even though their upper limbs are clearly adapted to life in the trees, the authors think their lower limbs were adapted to walking upright more than those of the australopithecines.
Of course, the problem with this is that in the adult specimen, no coxal bones are preserved, and in the juvenile specimen, only portions of the coxal bones are preserved. In addition, the only complete leg bone is the tibia in the juvenile. The rest of the remains of the lower limbs are fragmentary, and they mostly come from the juvenile. To make the bold claim that these creatures could walk upright based on such fragmentary evidence is risky, to say the least. I think the best you can say from these bones is that these creatures might have been able to walk on two legs, but it is not clear how comfortable this would be for them.
The other features that supposedly indicate these creatures are something other than australopithecines is that compared to typical australopithecines, the juvenile’s skull has a flatter face, a more projecting nose, and smaller teeth.
So notice what we have here. Using the adult, we have an ape that is a bit more orangutan-like than most australopithecines. Using the juvenile, we have many australopithecine features, but we also have some features that are closer to those found in genus Homo. That, as I see it, is the biggest problem with trying to say these creatures are anything other than specialized apes. It is very difficult to make decisions based on a juvenile skeleton. A juvenile skeleton is in the process of growth, and that can significantly distort the features paleontologists use to compare fossils.
Of course, I am not alone in this assessment. This study was published in the journal Science, and as is typical for that journal, a “commentary” article was published along with the scientific reports. In that commentary, we read:
But others are unconvinced by the Homo argument. The characteristics shared by A. sediba and Homo are few and could be due to normal variation among australopithecines or because of the boy’s juvenile status5
The article discusses two well-known paleoanthropologists (Dr. Tim White and Dr. Ron Clarke) who both think that this is either A. africanus or a “sister species.”
It is not surprising that Dr. White thinks these new fossils are australopithecines. In the chapter he wrote for The primate fossil record, he lays out seven “defining characteristics” of the australopithecines. 6 They are:
1. Leg/pelvis anatomy suggesting at least some bipedal locomotion
2. Large brachial index relative to other hominids
3. Sexual dimorphism (differences between the genders) between that of chimps and orangutans
4. Height between 1.2 m and 1.5 m
5. Cranial capacity between 350 cc and 600 cc
6. Teeth posterior to the canines relatively large with thicker enamel than extant apes and humans
7. Small incisors and canines compared to extant apes, and less sexual dimorphism in the canines
The fossils meet criteria 2, 4, and 5, and they probably meet 1 as well, at least from the standpoint that bipedal locomotion is possible. For 3, we know very little about sexual dimorphism, since we have only a juvenile male to compare to an adult female. For 6 and 7, we have mostly juvenile information, which is very hard to judge, especially when it’s the size that matters.
So the most scientific conclusion is that these fossils come from the genus Australopithecus, which is an extinct genus of specialized apes. That, at least, is what the data given by the fossils tell us, and it is probably why the authors put them in that genus. The authors’ idea that this is some ancestor of human beings is based on the hope that juvenile features can tell us a lot about adult features, and that is based on wishful thinking, not scientific reasoning.
1. Berger, L.R., de Ruiter, D.J., Churchill, S.E., Schmid, P., Carlson, K.J., Dirks, P.H.G.M. and Kibii, J.M., “Austalopithecus sediba: A new species of Homo-Like Australopith from South Africa,” Science, 328:195–204, 2010.
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2. White, T.D., “Earliest hominids,” Chapter 24 in The primate fossil record, ed. W.C. Hartwig, Cambridge University Press, p. 413, 2002
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3. Brian G. Richmond, et al.., “Early hominin limb proportions,” Journal of Human Evolution, 43: 529-548 (fig. 7), 2002
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4. White, T.D., Ibid
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5. Balter, M., “Candidate human ancestor from South Africa sparks praise and debate,” Science 328:155, 2010
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6. White, T.D, Ibid, pp. 407-418
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