Island life - a probable reason for hobbit's small brain

The hobbit, Homo floresiensis, may have had a tiny brain because it lived on an island, according to a new study published in the recent (7 May 2009) issue of the scientific journal Nature. Scientists at London's Natural History Museum examining the skulls of extinct dwarf Madagascan hippos have discovered they evolved significantly smaller brains in relation to their body size compared with their counterparts on the mainland, an issue at the heart of the debate of the hobbit's origins.

Natural History Museum palaeontologist Dr Eleanor Weston, who led the research, explained, 'The recent discovery of a small fossil human, Homo floresiensis, discovered on the island of Flores (in Indonesia) with normal facial proportions but a brain the size of chimpanzee's has baffled scientists. It could be that H. floresiensis' skull is that of a Homo erectus that has become dwarfed from living on an island, rather than being an abnormal individual or separately-evolved species, as has been suggested. Looking at pygmy hippos in Madagascar, which possess exceptionally small brains for their size, suggests that the same could be true for H. floresiensis, and the result of being isolated on the island.'

Madagascar is a large island with diverse habitats. In the past, it has supported up to three species of hippo. Scientists are unsure when the hippos arrived, but their remains persist to within the last 6,000 years. Brain-body scaling trends in mammals such as the Madagascan hippos and their mainland ancestor, the large common hippopotamus, can be calculated from the relationship of brain to skull size. Although the phenomenon of dwarfism on islands is well recognised in large mammals, an accompanying reduction in brain size, as Weston and fellow Museum palaeontologist Professor Adrian Lister found, has never been clearly demonstrated before.

Dr Weston continued, 'It may be advantageous to the survival of animals that become isolated on islands to become dwarfs. Also, the brain is a costly organ, because it uses a lot of energy. We found that the brain sizes of extinct dwarf hippos were up to 30 per cent smaller than you would expect by scaling down their mainland African ancestor. If the hippo model is applied to a typical H. erectus ancestor, the resulting brain capacity is comparable to that of H. floresiensis.'

'Whatever the explanation for the tiny brain of H. floresiensis relative to its body size, it's likely the fact it lived on an island played a significant part in its evolution,' Dr Weston concluded.

Homo floresiensis has an estimated mass of 35 kilos and an estimated height of one metre. The brain matches the smallest yet known for any hominid, at roughly 400 mL. It is only 20,000 years old.

It is interesting to mention that some animals experience island dwarfism, such as the Holocene mammoths of Wrangel Island off Siberia or the Channel Islands off California. Many kinds of animals experience island gigantism, such as the Komodo dragon on Flores or the dodo on Mauritius, which was a giant relative of the pigeon. The strongest previously documented case of a small brain resulting from living on an island is that of fossil bovid Myotragus, which was isolated on the Mediterranean island of Mallorca for more than five million years. The brain mass of Myotragus was reduced by up to 50 per cent relative to values of living bovids of equivalent body mass.

The study was supported by the Biotechnology and Biological Sciences Research Council.