Who Lies Sleeping?

Carl Sagan in Dragons of Eden gives four reasons, all to do with the brain, why intelligence in apes emerged only in the last few million years:

1. The brain had to grow bigger than a critical size
2. The ratio of brain to body mass had to exceed a certain value
3. The brain had to make more neural connections than before
4. The brain had to evolve particular functions (in the frontal and temporal lobes perhaps).

The first three imply only a quantitative change was needed but the fourth requires qualitative change instead—or as well.

It is plain from the fossil record that in the series of species that led up to man there has been a growth in brain capacity from about 350 cm³ in the apes to a maximum of about 2000 cm³ in recent and modern men. The human baby’s brain is now so large that mothers often experience difficult and painful childbirth. The English anatomists of Piltdown fame, Woodward and Keith, defined 750 cm³ as the critical size for intelligence. Later Le Gros Clark reduced it to 700 cm³. These definitions were purely arbitrary.

Brain size alone is not now regarded as sufficient to define a species—it is too variable. Cranial capacity in people today ranges from 1000 cm³ to 2000 cm³. Earlier hominids had brain capacities that overlapped considerably with preceding and succeeding ones. The H erectus range was 700 to 1250 cm³, overlapping with the H sapiens range. The H habilis range was 500 to 800 cm³, overlapping that of H erectus. And hominid brains only started to get bigger than apes’ about two million years ago, apparently concurrently with the making of tools. The range is continuous and offers no basis in itself for deciding when intelligence occurred. What is more, brain size does not correlate with intelligence, though there is a correlation between brain size and body size. Men have larger brains than women because they have larger bodies—but they are no more intelligent than women. When fossil craniums are found that appear the same save for their size, they are likely to differ only in the sex of their owner.

Big brained people are not generally more intelligent than smaller brained people. Our brains seem to have overdeveloped for some reason—only a part is used. The bulk varies in size from person to person but, being unused, does not affect intelligence. A creature with a much smaller brain using it more efficiently might be capable of behavior just as sophisticated as our own.

According to David Attenborough, macaques are one of the most successful and versatile of all primates. Although they have a small brain compared with ours, it is complex and large for their size and they are intelligent and adaptable. Some macaques have become media personalities in the last few decades. A group of Japanese macaques living in the cold mountains of northern Japan found some volcanic springs. Quickly the whole troop learnt how to shelter in the warmth of the hot spring water. In 1953, Japanese scientists were attempting to study a troop of macaques on the small island of Koshimu. To lure them into the open for ease of observation the scientist took to burying sweet potatoes in the sand. The monkeys were troubled by the dirt on the tubers until one female in a flash of gestalt took a tuber to a rock pool and rinsed away the particles of sand. Within a few months the whole troop had spotted and picked up the trick. The same monkey also discovered how to separate rice and sand by throwing the mixture into a pool and skimming off the rice. If the thinking animal’s niche were vacant, these monkeys could evolve into it over the next few million years—and they might still have the chance!

A bigger brained animal will be more intelligent than a smaller brained one, all else being equal. But since all else is not equal, we cannot be definite about a threshold brain size for intelligence.

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