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2-million-year-old A. sediba cranium shows evidence of brain evolution, IU anthropologist finds

Originally published Sept. 12, 2011

It isn't necessarily a bigger brain, but it is a better brain. That's what Indiana University Department of Anthropology research associate Kristian Carlson reported Sept. 8 in Science in describing the internal surface of the braincase of Australopithecus sediba, a nearly 2-million-year-old hominin Carlson and six other scientists discovered 18 months ago in South Africa.


This skull reconstruction of A. sediba with the computer-generated cranial endocast area shown in green. The yellow section indicates missing elements that were mirror-imaged and the transparent blue highlight on the endocast indicates the anterior portion of the inferior frontal gyrus found in the frontal lobe.

Print-Quality Photo

As lead author on one of five reports in Science -- the others describing the skeleton, a pelvis, a hand, and a foot and ankle -- Carlson, now a researcher at the Institute for Human Evolution at the University of Witwatersrand in South Africa, and the six co-authors use a computer-generated internal cast of the early hominin's hollow cranium to piece together a critical period of human brain evolution also considered one of the most mysterious and confusing to scientists.

"The period of evolutionary history that these fossils represent (1.977 million years ago) is approximately the period during which our genus, Homo, may have emerged," he said. "And this is one of the most exceptionally well-preserved endocasts associated with early hominin cranial and post cranial remains."

The brain of MH1, as this juvenile male has been designated, was surprisingly small at 420 cm 3 , or about the size of a grapefruit yet only about a golf-ball size bigger than a chimpanzee brain. Both the cranium and teeth provide evidence of a developmental stage equivalent to a human child of 12 or 13 years old at the time of death.

But the endocast shows an expansion of an area of the brain, the prefrontal cortex, which potentially brings it closer to exhibiting human-like sifting of complex information and human-like planning -- multi-tasking, innovation, reasoning -- than other South African australopithecines.

"So these results are consistent with the gradual neural reorganization of the orbitofrontal region in the transition from Australopithecus to Homo," Carlson said. "But, given the small volume of the MH1 endocast, the results are not consistent with gradual brain enlargement before the transition to Homo."

While the other skeletal remains of MH1 showed some human-like features, such as a narrow pelvis, there were also australopith, or Lucy-like, features such as a tiny hip joint and tiny body size. In the accompanying article describing the A. sediba hand, members of the research team describe Homo-like features such as a long thumb and short fingers associated with gripping and possibly stone tool production.

Kevin Hunt, an anthropology professor in IU's College of Arts and Sciences who was Carlson's Ph.D. advisor, said the MH1 endocast deepens the mystery surrounding a period of human evolution right before the rise of Homo.

"This is a time when we have huge variation in morphology among the hominins. Some have huge teeth, long legs and larger brains. So when you think big brains, that must mean those are the human ancestors, right? But other specimens have more modern faces and smaller teeth, even though they have small brains," Hunt said. "So do we trust the faces or the brains? Or are they all our ancestors? It's almost like each fossil has its own separate human-like feature, and we'd have to break them all apart and put them together Frankenstein-like to get a modern human."

"So we can't say for certain A. sediba is a human ancestor, but it's a marvellous piece of the human jigsaw puzzle, whether it's the immediate precursor of humans or not," he noted.

The exceptionally well-preserved cranium of MH1 was scanned at the European Synchrotron Radiation Facility (ESRF), revealing internal anatomy with the highest achievable precision and contrast, Carlson said, adding that the ESRF is the most powerful installation worldwide for scanning fossils, setting the standard for what can be achieved during non-destructive studies of internal structures of fossils.

Co-authors with Carlson on the paper, titled "The Endocast of MH 1, Australopithecus sediba," included another Indiana University anthropology Ph.D., Dietrich Stout of Emory University, along with Tea Jashashvili, and Lee Berger of the Institute for Human Evolution, Paul Tafforeau of the ESRF, Darryl J. de Ruiter and Keely Carlson of Texas A&M University. Carlson is also a co-author on two other reports on A. sediba in the edition of Science, "A Partial Pelvis of Australopithecus sediba," and "The Foot and Ankle of A. Sediba."