A paper in Nature about the DNA in the above bone fragment, found in the Denisova Cave, Russia, has been widely reported in the scientific press over the last few days. The fragment is about 50,000 years old and comes from a girl about 13 years old. The girl's mother was a Neanderthal while the father was a Denisovan with some Neanderthal ancestry. It has been known for some years that there was some interbreeding between Homo sapiens, Neanderthals and Denisovans - and that possibly other species of hominins have been involved - but the finding of a bone from a first generation child is a surprise because early hominin remains are so rare.
The discovery emphasises the importance of sex in the development of human intelligence. Let me explain:
In evolutionary terms the existence of the more advanced lifeforms depends on the DNA in the genes, which are passed from generation to generation in a process which sometimes involves transcription errors. Some errors are beneficial and some are harmful. If an organism has only one parent (for instance breeding by a budding process) both beneficial and harmful changes will be passed from one generation to another. If two parents pool their DNA each of the offspring will get a different selection of genes - and those with more of the good changed gene changes and less of the bad changed genes are more likely to survive. For this reason sex is a mechanism which favours the selection of beneficial changes and discards harmful changes,
While sex driven evolution using DNA has led to the development of animal brains there is a limit to what it can do. The brain is an expensive organ to run, and has to compete for other resources. No animal will evolve a brain bigger than it needs because all the information learnt in a lifetime is lost when the animal dies. For most species having many offspring, most of which die, is better than cutting back on the number of offspring and investing resources in a bigger and better brain.
In fact the mammal species with the largest brains proportionate to body size tend to be social animals where adults spend much time caring for a small number of offspring, who learn by copying what their parents do. The most obvious non-primate example is the orca(killer whale) which is widely distributed around the world. They form matriarchal social groups and different groups use very different hunting techniques to exploit the parts of the ocean they inhabit. There is evidence that that these groups are leading to the formation of different sub-species depending on their different lifestyles. For instance some form large pods catching fish in coastal waters, while others form smaller family groups catching marine mammals, while others live in the deep oceans hunting for large fish shoals.
The orca provides a good model for the situation the early hominins were in about 5 million years ago, when the Ice Ages started to disrupt the climate worldwide. Not only did the ice cover varying amounts of the northern continents, but areas like the Sahara oscillated from being green to being an arid dessert. The result was not only that Africa contained many different habitats, but that these were often changing over a period of a hundred thousand years or so. Our early ancestors probably had a level of intelligence comparable with modern chimpanzees or bonobos and could also have tools (including food gathering techniques) at the same primitive level. As the climate changed there would be different environments to exploit and the species would split into different groups eating different kinds of food and using different tools. Each successful group (and as we are talking evolution many groups would have failed) carved an evolutionary niche for itself, becoming a subspecies adapted for fishing on the banks of a river, or making the most of forest fruits, or hunting on more open survanahs. Such differences would develop until disturbed by the next climate change.
This is where sex again becomes important - but not just sex between closely related individuals but sex between hominins of different subspecies. Thus some of the offspring might inherit the more versatile hands of its mother's subspecies and the ability to run faster from its father's subspecies. In addition one of the parents might teach the fishing skills they had learnt as a child, while the other might teach how to make a better throwing spear,
However, as no animal has a brain bigger than it needs it is likely that on average the new mixed species generation will lack a brain with the capacity to learn all the tool-making skills of both the parent species. In such a situation natural selection will chose offspring with the genetic advantages of both parents and with a more intelligent brain than average. If we assume that at any one time there are perhaps half a dozen successful potential subspecies, each developing different traits such as a better vocal tract for communicating, bigger hips to allow infants to be born with bigger heads, etc., and each with its own special tool set, it is clear that sex between sub-species which have been separated for a few hundred thousand years could greatly speed up evolution of a far more intelligent hominin with a bigger brain.
Fossil remains, and the earliest stone tools, fit with the brain growing slowly in size over the last five million years, and similar very slow increases in the evidence for tools. This is what one would expect of normal genetic evolution, with the pressure for a bigger brain coming from the impact of sex between subspecies every few hundred thousand years or so. This increase of brain size with increasing tool sophistication seems to break down at some stage in the last quarter million years - which includes the Middle Stone Age. By about a hundred thousand years ago the rate of invention of new tool types increased, as did tool complexity so one might expect a rapid increase in brain size to accommodate all the new knowledge.
If anything our brain seems to be getting smaller.
The reason is almost certainly because one group of humans (probably Homo sapiens) develaoped a tool for describing how to make tools - in effect a reasonably sophisticated language. This allowed information about tools to be transferred far more quickly, and for reasons related to how the brain's neural net works (to be described elsewhere on this blog) far more efficiently in terms of the amount of memory needed. The result was a tipping point where, as language developed, more information could be transferred to between generations without requiring the brain to get any bigger.
Language also allows individuals to specialise - one gathering the food, while another makes stone tools - while writing allows information to be stored effectively over generations. In fact the more the human race learns collectively the less each individual needs to learn in order to survive. We end up with brains that have spare capacity to be creative - paint pictures, compose music, carry out scientific research, etc. - because we no longer have to know how to gather food, build a shelter, etc.. Because we no longer need to learn a full set of survival tools (especially as most predators apart form other humans have been eliminated) the evolutionary pressure for bigger brains has been removed - and evolution tells us that is an organ is bigger than needed it will tend to get smaller.
In a sense we have invented a new kind of sex where, instead of sharing DNA by combing gametes, we share information with other members of society by using powerful language tools. The evolutionary product of doing this over thousands of generations is what we call human intelligence.
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