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Thursday, October 17, 2013

What a Small Brain Can Tell Us

New information about an early human skull sheds more light on the very first members of the human genus.  The skull, found in Dmanisi, Georgia in 2005, has now been freed from the stone casing that has preserved it for the past 1.8 million years. An international team led by David Lordkipanidze of the Georgian National Museum report its findings in the October 18 issue of the journal Science. 

Photo Caption: The Dmanisi D4500 early Homo cranium in situ. Photo courtesy of Georgian National Museum.

When the world first learned of early human remains in Georgia, the news came as a bit of a shock.  These early humans seemed quite similar to other remains found in Africa and dating to the same time.  That suggests they were able to travel and adapt to new settings. 

The latest analysis contains a new surprise.  The skull described in the new report has an unexpectedly small brain size, at or below the range usually seen as minimal for our genus.  At 546 cubic centimeters, its small brain widens our view of variability of humans at this time. 

Does this skull, identified as Skull 5 from Dmanisi, really measure up to being in the genus Homo at all? It is something else, like Australopithecus?  The researchers argue that it is clearly part of the genus Homo for the simple reason that Skull 5 is found with other, larger-brained skulls, all clearly part of the same community.  One Georgian brain was as large as 730 cc.  What this suggests is that Skull 5 is part of Homo but that our definition of Homo should be broadened. 

In fact, all this diversity at one site provides support for one side in an ongoing debate.  Are species defined broadly in terms of variability, or does small to moderate variation indicate separate species.  This finding supports the view that at least in terms of early humans, a species can be quite variable.      

Not too long ago, Lordkipanidze and his team took the opposite view.  They believed that these early humans from Georgia were a distinct species, what they called Homo georgicus.  The new paper retracts that claim, saying that the new evidence of variation in Georgia means that these fossils fit within the widened range variability of Homo erectus, a globally dispersed species.  More precisely, they see the Georgian samples as best classified as Homo erectus ergaster georgicus, part of the species Homo erectus but distinct because of modifications over time and because of location. 

Commenting on the variation in the skulls found almost literally on top of each other at Dmanisi, co-author Christoph Zollikofer notes that the skulls “look quite different from one another, so it's tempting to publish them as different species.  Yet we know that these individuals came from the same location and the same geological time, so they could, in principle, represent a single population of a single species,” Zollikofer said in a press release issued by the journal Science. 

The key claim advanced in the article, however, is that these samples from Georgia and Africa, together with other samples from Asia, are all part of one global species.  The report describes them as Homo erectus, seen as “a single but polymorphic lineage.” 

The diversity found in Georgia also suggests that the number of individuals in that region may have been larger than first thought, possibly numbering 10,000 or so.  And the small size of Skull 5’s brain suggests that they traveled all this way before brains began to expand.

The report, “A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo," is published in the 18 October 2013 issue of the journal Science, published by the American Association for the Advancement of Science.    

1 comment:

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