SA’s Taung Child’s skull and brain not human-like in expansion

The Taung Child, South Africa’s premier hominin discovered 90 years ago by Wits University Professor Raymond Dart, never seizes to transform and evolve the search for our collective origins.

By subjecting the skull of the first australopith discovered to the latest technologies in the Wits University Microfocus X-ray Computed Tomography (CT) facility, researchers are now casting doubt on theories that Australopithecus africanus shows the same cranial adaptations found in modern human infants and toddlers – in effect disproving current support for the idea that this early hominin shows infant brain development in the prefrontal region similar to that of modern humans.

The results have been published online in the prestigious journal Proceedings of the National Academy of Sciences (PNAS) on Monday, 25 August 2014 at 21:00 SAST (15:00 EST), in an article titled: New high resolution CT data of the Taung partial cranium and endocast and their bearing on metopism and hominin brain evolution.

The Taung Child has historical and scientific importance in the fossil record as the first and best example of early hominin brain evolution, and theories have been put forward that it exhibits key cranial adaptations found in modern human infants and toddlers.

To test the ancientness of this evolutionary adaptation, Dr Kristian J. Carlson, Senior Researcher from the Evolutionary Studies Institute at the University of the Witwatersrand, and colleagues, Professor Ralph L. Holloway from Columbia University and Douglas C. Broadfield from Florida Atlantic University, performed an in silico dissection of the Taung fossil using high-resolution computed tomography.

"A recent study has described the roughly 3 million-year-old fossil, thought to have belonged to a 3 to 4-year-old, as having a persistent metopic suture and open anterior fontanelle, two features that facilitate post-natal brain growth in human infants when their disappearance is delayed," said Carlson.

Comparisons with the existing hominin fossil record and chimpanzee variation do not support this evolutionary scenario.

Citing deficiencies in how the Taung fossil material has been recently assessed, the researchers suggest physical evidence does not incontrovertibly link features of the Taung skull, or its endocast, to early prefrontal lobe expansion, a brain region implicated in many human behaviors.

The authors also debate the previously offered theoretical basis for this adaptation in A. africanus. By refuting the presence of these features in the Taung Child, the researchers dispute whether these structures were selectively advantageous in hominin evolution, particularly in australopiths.

Thus, results of the new study show that there is still no evidence for this kind of skull adaptation that evolved before Homo, nor is there evidence for a link between such skull characteristics and the proposed accompanying early prefrontal lobe expansion, Carlson said.

New genetic data shows humans and great apes diverged earlier than thought

To calculate when a species diverged, researchers look at the average age of members of the species when they give birth and mutation rates. The older the average age, the more time it takes for mutations to cause changes. Insects that produce offspring in a matter of months, for example, can adapt much more quickly to environmental changes than large animals that produce offspring many years after they themselves are born. To find such data for both chimps and gorillas, the research team worked with many groups in Africa that included studies of the animals that totaled 105 gorillas and 226 chimps. They also looked at fossilized excrement that contained DNA data. In so doing they found that the average age of giving birth for female chimps was 25 years old. They then divided the number of mutations found by the average age of birth to get the mutation rate. In so doing, they found it to be slower than humans, which meant that estimates based on it to calculate divergence times were likely off by as much as a million years.

The end result of the team’s research indicates that humans and chimps likely diverged some seven to eight million years ago, while the divergence of gorillas (which led to both humans and chimps) came approximately eight to nineteen million years ago. To put the numbers in perspective, humans and Neanderthals split just a half to three quarters of a million years ago.