Giant brains

The many convergences documented in the last chapter, from the agriculture of ants to the vocalization of birds, are strong evidence that the evolutionary emergence of many complex systems is highly probable, if not inevitable. Yet the sceptic will still pause in thought.

Pieter Plessis Jack The Signalman
figure 9.1 Jack standing against the trolley with Mr Wide; note the lever frame in the background. (Photograph courtesy of Euan Nisbet, Royal Holloway, University of London.)
Coefficient Encephalisation

figure 9.2 Relative size, or encephalization quotient (EQ), of mammalian brains plotted against a coefficient that relates total volume of the brain against the surface area and volume of the cerebral cortex. This comparison is less familiar than the EQ calculated on the basis of brain and body volumes, with an expected exponent of 0.67. (Redrawn from fig. 2 of M.A. Hofman (1982), Encephalization in mammals in relation to the size of the cerebral cortex. Brain, Behavior and Evolution, vol. 20, pp. 84-96, with the permission of the author and S. Karger AG.)

figure 9.2 Relative size, or encephalization quotient (EQ), of mammalian brains plotted against a coefficient that relates total volume of the brain against the surface area and volume of the cerebral cortex. This comparison is less familiar than the EQ calculated on the basis of brain and body volumes, with an expected exponent of 0.67. (Redrawn from fig. 2 of M.A. Hofman (1982), Encephalization in mammals in relation to the size of the cerebral cortex. Brain, Behavior and Evolution, vol. 20, pp. 84-96, with the permission of the author and S. Karger AG.)

Such similarities are indeed intriguing but do they really address the central question as to whether or not humans as a biological property are inevitable? This is because as well as having a complex social system, agriculture, placentas and live birth, warm-bloodedness, and vocalization, we have something else. Even if some reptiles give birth to live young, and others chewed the equivalent of celery, their mental powers neither were nor are conducive to rumination. Whatever may be said in favour of reptiles, their brain size is distinctly disappointing. Bigger brains are largely the prerogative of the birds and mammals, although as we saw earlier the electrosensory mormyrid fish also weigh in with a hefty brain. To a first approximation the size of the brain scales to the body mass (Fig. 9.2).86 Most mammals, the group upon which I shall now concentrate, have a brain whose size matches the body, but some have a brain smaller than would otherwise be predicted. The tenrec, an insectivore, is one such example.87 Conversely, other mammals have disproportionately large brains. Elephants are big, but their brains are even more massive than would be predicted. Humans, of course, are the exception of exceptions, with a brain approximately seven times bigger than it 'should' be. This strange condition was arrived at by an astonishing neural trajectory that began about four million years ago, with the later australopithecine apes. It is often thought that this must have been a biologically unique event, unrepeatable and dependent on a series of peculiar historical factors without parallel elsewhere. William Calvin,88 for example, regards attempts to conjure up particular explanations for the evolution of a large brain as little more than caricatures. In particular, and referring to an idea that has wide currency, he regards such a feature as intelligence as being an unforeseen consequence of a neural machinery that has been selected for some other reason. Intelligence, in this scenario, is not primarily adaptational and as such might be reduced to an evolutionary fluke, unique to humans and their nearest relatives and, to echo G. G. Simpson (see note 2), unlikely to be found elsewhere in the Galaxy.

In fact the evidence suggests otherwise, at least on this planet. To start with, there is strong evidence that among the primates those with bigger brains show more innovatory behaviours, social learning, and tool use, while among the birds those with greater behavioural flexibilities and adventurousness (or fewer neophobias) again have larger brains.89 This accords with the fact, returned to at various points below, that some monkeys, parrots, and crows are all markedly intelligent. So, too, of course, are the great apes, but at least in this case the fact that chimps, for example, have important parallels to human mentality is hardly surprising. To find striking similarities to human intelligence that might persuade the disinterested reader that such represents a general biological property likely to emerge on any suitable planet, we need to turn to the toothed whales (the odontocetes) and especially the dolphins.

It has long been recognized that for their size some of the toothed whales, of which the dolphins are one group, have large brains.90 A straightforward comparison with other groups of mammals, of which the humans and related anthropoid primates are the most relevant, nevertheless runs into some difficulties. Rather self-evidently the size

Pacific white-sided dolphin Tucuxi dolphin

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