April 9, 2010 | 31
By now you’ve probably heard of Australopithecus sediba, the 1.95-million-year-old human species that made news on April 8. In a nutshell, researchers have found two beautifully preserved partial skeletons that they say represent a previously unknown member of the human family–one that may have given rise to our genus, Homo. You can read my story on the find here.
There’s a lot to talk about with this discovery, so I thought I’d supplement the story with some tidbits from the cutting room floor and material that came in after my deadline.
First, some tidbits:
With regard to that last point about A. sediba’s small cranium, this photo of Lee Berger holding the skull of the juvenile male specimen gives a sense of how tiny it is. Granted, it’s a juvenile (the adult female skeleton does not include a skull, although the discovery team expects to find that with further excavation), but the researchers say it had probably already attained at least 95 percent of adult brain size.The cranial capacity of this fellow is an estimated 420 to 450 cubic centimeters. In contrast, we modern humans have brains that typically range from about 1,200 to 1,600 cubic centimeters.
As I mentioned in the news story, what’s really striking about A. sediba is its mix of primitive and advanced traits. Here’s an image of the new fossils, superimposed over an illustration of an A. africanus skeleton (Berger and his colleagues argue that A. sediba is descended from A. africanus.) A list of representative primitive and advanced characteristics follows below.
Lastly, here’s another perspective on A. sediba, from paleoanthropologist Donald Johanson, discoverer of the Lucy skeleton (full disclosure: I co-wrote a book with Johanson called Lucy’s Legacy: The Quest for Human Origins). He e-mailed the following comments, which I received after my news story had already been published:
"The South African finds from Malapa are most interesting, but I find it curious that the authors point to so many anatomical features that indicate that the finds belong to our genus, Homo, yet they place it in Australopithecus, so I think they missed the boat here. Finding 1.8-million-year-old Homo in southern Africa is news-worthy since previous traces have been fragmentary and controversial.
Additionally, I do not see these fossils as evolving from Australopithecus africanus, which I believe gave rise to A. robustus in South Africa. The specimens from Malapa are not the ancestor to later all Homo as the authors believe, since we have evidence of Homo in eastern Africa at 2.33 million years. My team found an upper jaw of this age in the younger sediments at Hadar where Lucy was found, and this palate represents the oldest anatomical evidence, thus far, for our genus. It is probably best attributed to Homo habilis. Let us not forget that the Turkana Boy is about 1.8 million years old and is without doubt Homo and is attributed to Homo ergaster.
The Malapa [hominids], with so many Homo features but with relatively short limbs, resemble Olduvai Hominid 62 which we found in the mid-80s. Although fragmentary, OH 62 does have relatively shorter legs and longer arms, like earlier Australopithecus, and the appearance in the fossil record of a more modern body built, as in the Turkana Boy, comes later. However the Olduvai material, OH 62 and several other specimens, are attributed to Homo on the basis of diagnostic features in the teeth, jaws and cranium. Some scholars have suggested we place H. habilis into the genus Australopithecus and until there is a modern body build Homo should not be used as the genus for these fossils.
It is also rather possible that Homo, like Australopithecus underwent a diversification (adaptive radiation) resulting in several different species, this would not be unusual. However, within the greater framework of Australopithecus and Homo, I believe emphasis should be placed on the diagnostic anatomy of the teeth, jaws and cranium…so, I would continue to use Homo for habilis, and for these new specimens from Malapa. Until a more comprehensive comparative study is undertaken (I know other specimens have been recovered from the site), the relationship between the Malapa material and Homo in eastern Africa is not very clear. I would not be surprised if the Malapa material represents a newly recognized species of Homo.
The South African finds about a half a million years younger are probably descendents of the eastern Africa Homo. 500,000 years is a long time and Homo could easily have migrated from eastern to southern Africa in that time.
There are two partial skeletons, one a female and the other a male. The skull of the male is refreshingly complete and should be attributed to Homo. Just after Lee found the first hominids at Malapa, he invited me to see the material at Witswatersrand. The mandible is lightly built, not very deep or thick resembling Homo. The first and second permanent molars are erupted and there is little occlusal wear, suggesting a diet quite different from Australopithecus. In Australopithecus, by the time the second molar erupts the first shows rather heave wear. Also, the teeth are small in size, like in Homo, and unlike Australopithecus.
We have a very comprehensive understanding of the dating, diversity and relationships between the species of Australopithecus, but we know relatively little about the origins of our own genus, thus anything found that represents early Homo is potentially of some importance. I think these finds will re-focus attention on the South African fossil sites and strengthen the importance of these sites for a more complete understanding of the human family tree."
Images: Photo courtesy of Wits University; skeletons courtesy of Science/AAAS
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