Australopithecus sediba Scan Provides Insights into Human Evolution

New findings reveal the highest resolution and most accurate X-ray scan ever produced of the brain case of an early human ancestor. The insight gleaned from these data is a significant clue into the obscure landscape of brain evolution across the transition from Australopithecus to Homo.

The publication is part of a series of five articles based on new evidence pertaining to various characteristics of the anatomy of the species Australopithecus sediba (reported in April 2010 by Berger et al) published in Science on September 9, 2011. Led by the University of the Witwatersrand (Johannesburg, South Africa), over 80 scientists from numerous institutes in Germany, the United States, United Kingdom, Australia, Germany, South Africa, and Switzerland worked on the project. The brain research includes a scientist from the European Synchrotron Radiation Facility (ESRF; Grenoble, France), where the X-ray microtomography scan was performed.

The exceptionally well-preserved cranium of MH1 (Australopithecus sediba) was scanned at the ESRF at a resolution (three-dimensional [3D] pixel size) of around 45 micrometers. Thanks to this high resolution, incredible details of the anatomy of sediba’s endocast could be revealed.

According to Prof. Lee Berger from the University of the Witwatersrand, who found the fossil in 2009, “the many very advanced features found in the brain and body make it possibly the best candidate ancestor for our genus, the genus Homo, more so than previous discoveries such as Homo habilis.”

Humans have a very large brain relative to their body size; approximately four times that of chimpanzees. Evolution from the brain of our shared ancestor with chimpanzees has seen this radical size increase. However, the reconstructed endocast (volume of the cranium) of MH1 is surprisingly small, with a volume of 420 cm³, on average only about 40 cm³ larger than chimpanzees.

The study of this brain demonstrates a remarkable conglomeration of characteristics. Its overall shape resembles humans more than chimpanzees and, given its small volume, this result is consistent with a model of gradual neural (brain) reorganization in the front part of the brain. “Indeed, one of our major discoveries is that the shape and form of sediba’s brain is not consistent with a model of gradual brain enlargement, which has been hypothesized previously for the transition from Australopithecus to Homo,” added Dr. Kristian Carlson from the University of the Witwatersrand, who is the main author of the paper.

Use of synchrotron X-rays was instrumental for this finding. The external shape of a brain is reflected, like in a mold, in the inner surface of a cranium. By mapping the contours of this internal surface, an image of the original brain located in the skull can therefore be generated.

However, the skull of MH-1 was not emptied from bedrock after its discovery, and only the powerful X-rays at the ESRF could penetrate deep into the fossil to reveal the cranium’s inside shape at the desired resolution. Leaving the rock inside the cranium also ensured that its delicate inner surface was not damaged or altered during its extraction.

“The ESRF is the most powerful installation worldwide for scanning fossils, setting the standard for what can be achieved during nondestructive studies of internal structures of fossils,” concluded Paul Tafforeau, staff scientist at the ESRF and a co-author of the article.

Related Links:
University of the Witwatersrand
European Synchrotron Radiation Facility