William H. Calvin has specifically studied the evolution of the human mind. In ``The Evolution of Intelligence'' [CGC+98], he outlines major stages our ancestors went through (see table 2.1). Items marked with an asterisk (*) are added to highlight the exponential speed-up which is often observed in evolutionary processes.
|3,500,000,000||years ago: First single celled animals*|
|700,000,000||years ago: First neurons*|
|ca. 250,000,000||years ago: First brain*|
|1,800,000||years ago: Homo Erectus|
|200,000||years ago: Big brained Homo Sapiens|
|50,000||years ago: (Creative) Homo Sapiens Sapiens|
The most recent radical transition only took place at /th of Homo Sapiens' existence. Archaeological skull research showed a (roughly) exponential trend in terms of brain volume, for which several explanations have been proposed.
Archaeological records show relatively sudden jumps in toolmaking sophistication, each followed by long periods without significant improvement. During these periods of so called conservatism, brain size had been gradually increasing.
There are several things that we do much better than apes that are demanding for the brain, that may provide insights into our intellectual growth. First, it may be very important that we have developed language (first a proto-language, a rudimentary way of communicating through sounds). ``Without the flexibility of language that grammar allows, there can also be no `higher-level' thought. Without syntactic language, thought is stuck in the world of concrete, here-and-now perceptions.'' [KW01, p. 533]. Hadamard's study shows otherwise, that thinking is non-verbal [Pen90, p. 414].
Secondly, Calvin argues that throwing has been crucial for the development of the brain. Early hominids differ from most animals in their tool usage. There has been a big premium in handling those tools. For throwing, the trajectory has to be imagined and planned in advance. It can hardly be corrected in the short interval that is typical for ballistic movements. The target distances may vary greatly, and there's big reward for getting better at this (less likely to be noticed from longer distances, more chances of food), but it also becomes disproportionally harder2.8. [Cal04, Ch. 8]
Sharing is also crucial, since the `catch' is often more than the hunter could eat. It was tolerated to take from the catch of others, but to prevent freeloading, remembering `who had what' became important. This developed in confluence with language.
Of course our evolution did not only take shape because of the action of man. Many cycles of rough and followed by mild climate conditions created challenges and opportunities. Static or invariant conditions create pressures for the evolution of modularized systems2.9whereas dynamic conditions create pressures for modular plasticity and the evolution of less modularized, domain-general systems [Gea05, p. 125]. Based on recent datasets (contemporary subjects), larger brain volumes were associated with higher general fluid intelligence2.10 (r = 0.49), larger short-term memory capacity (r = 0.45), and faster speed of processing (rs about -0.4) but were unrelated to general crystallized intelligence2.11 (r = 0.06) [WVL00,Gea05, p. 114]. Fluid intelligence related traits seems to have arisen together with the increase in brain size. There is a moderate correlation between IQ and kinship2.12 (r = 0.86) for identical twins. This influence increases with age [Sha02, pp. 89-92].
Macroscopically, humans were quite flexible and often filled biological niches that became available. Purely (evolutionary) biological changes suggestions have also been made. The increase of brain size is constrained by a problem of heat dissipation. The so called `radiator hypothesis' states that more adequate cooling that can be found in humans may have made this increase possible [Fal05].
A recent meta-study by McDaniel confirms a significantly positive relation (0.33) between brain volume and (general) intelligence [McD05]. Azam nuances this by saying that ``[i]t can be deduced that an increase in brain size does not necessarily increase the sophistication or behavioral diversity, unless accompanied by a corresponding increase in specialized brain modules'' [Aza00, p. 15]. Others suggest that the correlation of brain volume to intelligence is poor and that factors such as glia-to-neuron ratios2.13 can serve as a possible determinants [KW01, p. 572].
Erik de Bruijn 2007-10-19