New Delhi: Big brains weren’t necessarily the killer apps that allowed modern humans to prevail over their primitive and now-extinct cousins. Rather, the key was a gradual reorganization of nerves in specific brain regions that began in pre-humans and reached its peak in our species, say researchers after an analysis of the most complete and well-preserved pre-human fossils ever found.
A team of nearly 80 scientists has described in Friday’s issue of Science a new species of possible human ancestors called Australopithecus sediba (A. sediba); fossils of this species, found in a South African cave in 2008, stand out from existing fossils of archaic humans. Their hands and feet are very similar to that of modern humans, but their brain sizes are closer to chimpanzees.
While the findings aren’t enough to force paleoanthropologists to redraw man’s family tree, other researchers suggest that the quantity and state of preservation of these bones seriously challenge current interpretations of the evolution of human bone structure and brain anatomy, both largely based on a study of a limited number of fossils remains.
Intriguing revelations: The cranium of a juvenile A. sediba skeleton. Photo: Brett Eloff, courtesy Lee Berger & the University of Witwatersrand
A. sediba is a sub-species in the larger Australopithecus genus, the most famous of which is a 3.2 million-year-old fossil called Lucy, believed to be the mother of all humans. These were a class of ape-like pre-humans that are believed to have walked upright and foraged on land rather than treetops.
The transition from Lucy to Homo erectus (H. erectus)—a species that lived between 1.8 million and 1.3 million years ago, and unequivocally considered to be on the direct evolutionary line of modern humans—is extremely murky.
It isn’t still irrefutably clear whether A. sediba, which, at 1.97 million years, is a direct human ancestor, though the researchers involved with the study are quite convinced. “The fossil record for early Homo is a mess,” said study co-author Steven Churchill of Duke University in Durham, North Carolina, in an audio conference. “A. sediba has a number of derived characteristics (of Homo and Australopithecus), and this suggests it’s a good ancestor of the first species that everyone recognizes in the Homo genus—H. erectus.”
Among these characteristics is A sediba’s pelvis. It is widely believed that the modern pelvis grew bigger to accommodate for the increasing brain sizes (in babies), a hypothesis challenged by the latest finds. In the recent fossil finds, the pelvic blades are closer to humans, whereas other traits, such as the small birth canal are like Lucy’s, scientists say. This suggests that the redesign of the pelvis seen in Homo may have been driven by the need to walk upright more efficiently rather than to birth babies with bigger brains.
“Our brains have to pass through the pelvis, so accommodations must be made,” said Churchill, “What’s cool about sediba is their pelvises are already different from other Australopiths, and yet they’re still small-brained. It would be surprising if it was not for locomotion.”
To get an idea of how the fossil’s brain must have looked, Kristian Carlson of the University of the Witwatersrand, Johannesburg, and colleagues used the partial skull of the fossil and made a detailed scan of the space where the fossil’s brain would have been.
The scan showed that the frontal brain, or the region that lies directly behind the eyes, shows signs of neural reorganization, which, according to Carlson, perhaps indicates a rewiring towards a more human-like frontal lobe. “So probably there was first reorganization and then an increase in brain size. We could be seeing the beginning of multi-tasking.”
Other signs of multi-tasking come from studying the fossil’s hands, which are more human than ape-like, with shortened fingers and a relatively long thumb. This is a sign of precision gripping, which involves just the thumb and fingers but not the palm. “It’s even possible that A. sediba had already started dabbling with tool-making,” said Tracy Kivell, a researcher who studied the fossil’s hands.
Independent researchers say the work is groundbreaking.
“It gives us a fresh new tsunami wave of evidence at this critical time when Australopithecus went extinct and Homo originated in Africa,” paleoanthropologist Brian Richmond of George Washington University in Washington, DC, who was not connected with the study, said in an interview with Science.