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Tiny brained Homo floresiensis shared common ancestor with humans

17 April 2013

Title: Brain size of Homo floresiensis and its evolutionary implications

Authors: Daisuke Kubo, Reiko T. Kono and Yousuke Kaifu

Journal: Proceedings of the Royal Society B

The small brain size of Homo floresiensis has been precisely measured for the first time. The surprising results imply that Homo floresiensis may have been dwarfed Homo erectus, according to research published in Proceedings of the Royal Society B today.

The first CT-scans of the only remaining Homo floresiensis skull, taken by scientists from The University of Tokyo, reveal that the brain size was 426 cc. This is larger than the previously cited figure of 400 cc, although still much smaller than that of modern humans (1300 cc on average).

The precise measurements allowed the researchers to construct new models of the brain size reduction in the evolution of H. floresiensis. Author Daisuko Kubo and his colleagues suggest that ‘the new model shows that, contrary to expectations by some researchers, it is possible that large-bodied Homo erectus migrated to a solitary island and evolved into Homo floresiensis by marked island dwarfism.’

Previous studies have differed as to whether Homo floresiensis descended from small-bodied and small-brained early Homo habilis (from circa 2 millions years ago) or from large-bodied and large-brained Homo erectus (circa 1.7-0.05 million years ago). However, there is a lack of fossil evidence for the presence of H.habilis in Asia, where the H.floresiensis resided, suggesting they instead evolved from H.erectus.
The theory that H.floresiensis evolved from H.erectus, as modern humans did, was also previously thought untenable, due to the mean brain size of H.erectus being very large (~991cc), and thus requiring an extreme evolutionary brain size reduction to that of the H.floresiensis. However, according to the researchers, not only was the H.floresiensis brain size larger than previously thought, but the earliest Homo erectus’ brain size was ~860 cc rather than the previously cited 991cc, and thus they concluded ‘H.erectus is the most appropriate ancestor for the H.floresiensis in this model’.

The Japanese researchers analysed 20 different modern human populations worldwide and showed that the relationship between body size and brain size is actually stronger than previously suggested. This lead to the authors’ suggestion that large-bodied, large-brained Homo erectus evolved into small-bodied, small-brained Homo floresiensis, possibly due to dwarfism after migration to a solitary island.

These results have important implications for our understanding of human evolution. Evolution of the genus ‘Homo’ is characterized by increases in brain and body sizes, but Homo floresiensis suggests that these ‘human’ characteristics may be substantially flexible.