Okay. This is a bit of a roller coaster ride.
In this corner, we have Australopithecus. Australopithecus evolved up about 4 million years ago and became eventually extinct about 2 million years ago. Australopithecus is considered a hominid, in that he is part of the great apes: those apes that include us. Lucy was an Australopithecus.
Australopithecus is not normally considered a hominan, members of the human tribe and their extinct relatives. Pity these terms are so close but there it is.
In the other corner we have Homo habilis. H. habilis is of the genus Homo and lived from 2.3 to 1.4 million years ago. H. habilis has been associated with stone tools and may have had fire. He was about 1/2 the cranial capacity of a human. H. habilis is often considered the first on the direct line to humans.
This is the way we’ve considered it for some time. Now it’s time to throw in a couple of monkey wrenches.
Enter Australopithecus sediba.
Au. sediba was discovered in 2008 and two specimens have been dated to between 1.977 and 1.980 million years ago. The dates are particularly precise because of the sediment they were buried in contained Uranium salts from which accurate dates could be determined. (See here.)
The dates are important since it’s been determined that one of the drivers of human evolution have been climactic fluctuation (see here.) High variability started around 2.7 million years ago. By 1.5 million years the last hominan standing is Homo erectus.
Now, Au. sediba turns out to have many pre-adaptations to be human. They have hands that are closer to ours than Homo habilis. The pelvis more closely resemble ours and suggests that it used a form of bipedal walking. (See here, here and here.) The ankle looks human like but the heel looks like that of an ape.
The pelvis is particularly interesting since Au. sediba’s brain isn’t particularly big. However, the pelvis is upright and resembles a human pelvis, doing damage to the idea that the human pelvis got its shape because it had to cope with birthing a big human skull.
Most importantly, Au. sediba’s brain more closely resembles modern human brains than H. habilis.
How do we know this? you ask.
The brain closely fits in the skull cavity. By looking carefully (see here) at the skull cavity the shape of the brain can be deduced. Which is what these scientists did using precise X-ray micro-tomography.
Humans have a brain about 4 times the size of a chimp’s. H. habilis’ brain is about 1/2 ours and therefore about 2x on a chimp. Au. sediba’s brain is barely 40 cubic centimeters bigger than a chimps. Not much at all.
However– and this is a big however– Au. sediba’s brain is organized and shaped more like a human brain than either a chimp brain or one of H. habilis. (See here.) The frontal region and olfactory regions were similar to modern human.
First, these specimens have to be studied a lot more to figure out how they are to be placed in the heritage of Homo sapiens.
But there are a lot of interesting things going on here. I’ve talked before about evolutionary pre-disposition. Evolution takes advantage of the way things are rather than what things could be. Consequently, the hands and ankle of Au. sediba predispose him towards us. It gives him a leg to stand on, as it were.
I would guess that the brain of Au. sediba was not as good a cognitive engine as H. habilis. After all, it’s 1/2 the size. In brains, size matters but organization matters more. Even so, while Au. sediba had a better organized brain I doubt it was as smart as H. habilis. Think of Au. sediba as a finely tuned four cylinder engine compared to the hulking V8 of H. habilis. The little engine is built like a Swiss watch but still can’t crank out the raw torque of the inefficient V8.
The difference, though, is scalability. The brain of Au. sediba had the organization to expand. Quoting the abstract here regarding this organization: “… are consistent with gradual neural reorganization of the orbitofrontal region in the transition from Australopithecus to Homo, but given the small volume of the MH1 endocast, they are not consistent with gradual brain enlargement before the transition.”
That is, the Au. sediba brain organization was pre-disposed to grow better as it grew bigger.
That’s one idea. Faye Flam thinks the two species might have been friends with benefits, blending the best of both species. Which does damage to the idea they were separate species to begin with.
This is going to be fun.