Two recent news items, coupled with earlier reports that Neanderthal children matured faster than human children, could be relevant to Neanderthal intelligence and perhaps to the species becoming extinct.
The paper New insights into differences in brain organization between Neanderthals and anatomically modern humans by Eiluned Pearce, Chris Stringer and R. I. M. Dunbar notes that while the Neanderthal brain size was similar to Homo sapiens, more of the brain was devoted to sight and controlling a larger body.
The paper Barium distributions in teeth reveal early-life dietary transitions in primates by Christine Austin et al examined a Neanderthal tooth as estimated that breast feeding in Neanderthals may have only continued for about 14 months compared with 30 months in human non-industrial societies.
The important thing to remember is that brains are expensive organs and no species is going to evolve a bigger brain than it needs to be successful in the evolutionary stakes. In addition there is no advantage in having a larger brain unless childhood is long enough to learn enough to exploit the extra capacity.
In human terms a bigger frontal lobe means that it is possible to learn more cultural information – and most of human intelligence depends on what were learn. By the time Homo species appeared it seems likely that group culture passed from generation to generation had become an important factor in leading to survival. The rules for cultural evolution are different to biological evolution and there are two different ways in which biological evolution could react.
By lengthening the period of childhood learning and the parental involvement in teaching, and increasing the size of the frontal lobes, a species would be able to acquire a more intelligent culture – but would pay by having less children. Homo sapiens followed this route and as a result early hunter-gather mothers probably only had an average of five live births – meaning that 40% of all infants needed to reach breeding age. In good times the population could only grow comparatively slowly, and might be at more risk of extinction in bad times, when the number infant deaths increase. (It is becoming increasingly clear that over the last 5 million years there were many early hominid species which became extinct.) As the brain got bigger, and childhood lengthened, the cultural load passed from generation to generation increased. As natural language is an important part of human culture a bigger brain makes it possible to develop a more sophisticated language. And a more powerful language means more effective teaching. The result is positive feedback that could have lead to the cultural explosion that archaeologists think started about a hundred thousand years ago.
The evidence suggests that the Neanderthal line took a different approach, which is some circumstances might have proved better. That would be to keep the brain and the childhood training as it was in the common ancestor. The result was that it could have more children than the human line – leading to faster population growth in good times. However the brain did not have the spare capacity to taken on a greatly expanded cultural load – which could also have limited their language skills.
So when modern man and Neanderthals met the difference in the frontal lobe capacity would mean that humans were, in effect, better educated and had a more efficient language. In effect we won out over the Neanderthal because our brain could support a bigger load of cultural information. In fact we were more intelligent!