Trapped on Flores - Why did the Hobbit have such a small brain?

I have currently be doing a short online course Homo floresiensis uncovered, run by the University of Wollongong. This hominin was only about one metre high and had a disproportionally small brain. In terms of my own interest in the evolution of human intelligence  this raises the interesting question of whether a smaller brain means less intelligence.

Homo floresiensis

First some of the background. Flores is an island in the Indonesian chain east of Bali and the Wallace line which separates the Asian fauna and flora from the Australian fauna and flora. As such it lacked, (until Homo sapiens arrived about 50,000 years ago, on his was to Australia) any Asian carnivores, herbivores or primates - with the exception of a pigmy elephant, Stegodon. and the diminutive Homo floresiensis. The only native carnivore which could threaten the hominin was a Komodo dragon.

Two Flores sites are relevant - the cave of Liang Bua where there are signs of occupation, with stone tools and bones of Homo floriensis and evidence of fire between 190,000 and 50,000 bp, and then more recent Homo sapiens deposits. Recent discoveries of a small jawed hominin which might be the ancestor of Homo floresiensis, and stone tools have been found at Mata Menge and dated to about 700,000 bp.

Liang Bua

More finds, in further locations, will almost certainly be needed to draw an accurate picture of what happened but if further information appears it could say something about the evolution of human intelligence.

Because Flores is on an active plate movement boundary there will have been frequent tsunamis in the area, and a couple of early hominins could have been washed across the strait between Bali and Flores. The result could have been a colony of hominins which lived in isolation from other hominin groups for at least 600,000 years and perhaps over a million years. This could be a unique occurrence. In Africa and on the Eurasian land mass it is clear that different hominin groups met and regularly exchanged genes by interbreeding, and could also have exchanged cultural skills. With no competition from other hominins, and with low levels of threats from powerful carnivores, it could be very interesting to know how the tool-making skills (at least the stone-making skills and the use of fire) of Homo floresiensis changed over a period of over half a million years.

Coupled with this is the question of change in both body size and relative brain size. Brains are expensive organs - especially in species such as the great apes where success depends on being intelligent enough to ensure a significant proportion of infants reach breeding age. A move from an environment where there is a serious threat from fast moving carnivores to an island with much less danger means that infants have less survival skills to learn and can become mature earlier, while the parents have more time to bring up more infants. In addition parents don't have to be as strong to protect their infants, so don't need to be so big. If you look at the overall species economics spending less on being big and strong and less on learning/teaching frees up resources to have more infants for the same level of food supply. In addition because infants spend less time learning, and parents spend less time teaching, there is no need for such a big brain.

The following raises the question of whether there is a direct relationship between brain size and tool-making capacity. If the brain size of Homo floresiensis steadily shrunk over the years did its tool-making skills decrease, stay about the same, or actually increase.

The disappearance of Homo floriensis about 50,000 years ago coincides with the movement of Homo sapiens along the island chain towards Australia. At about the same time the pigmy elephant and a giant stork also disappeared. If the invading humans behaved as they did elsewhere they may have moved comparatively fast, as active hunters, killing off and eating the larger species as they went. While Homo floresiensis may have been treated as bush meat they may have also died from diseases brought by the newcomers. Such events could have happened on such a short timescale that fossil evidence could be very difficult to find,

However the possibility of interbreeding should not be ruled out, despite the difference in size. A recent report claims that the descendants of the modern humans who were probably involved not only carry Neanderthal and Denisovan genes but also genes of an as yet unidentified hominin which they may have encountered when they migrated along the island arc.

Clearly further detailed discoveries and analysis of hominin remains on Flores and elsewhere on the island chain could well provide evidence of the factors that affected the evolution of human intelligence in way which were not possible from areas where different hominns frequently encountered each other.