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The Great Debate: Being Human

Notes from dayschool The Great Debate: Being Human
by Caspar Hewett

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Sex and the Choosy Girl

One theory which is seen by many as central to understanding what we are and how we became what we are is the theory of sexual selection. The idea was first suggested by Darwin in his book The Descent of Man and Selection in Relation to Sex. Twelve years earlier, in 1859, Darwin had published The Origin of Species. At this point his aim was to establish the principle of evolution by natural selection as the mechanism through which evolution takes place. Natural selection can be thought of as a filter which, over generations, makes the attributes of organisms better and better adapted to the environment in which they live. This filtering takes place simply through the fact that those individuals who are better suited to their environment have a higher likelihood of surviving for long enough to successfully reproduce. For example, an organism whose colours provide it with some level of camouflage is less likely to be seen by predators, and thus is more likely to survive than one whose colours stand out in its surroundings. What was so revolutionary about this theory is that it explained how there could be so much apparent design in the natural world without needing to invoke the existence of a designer. Given enough time, natural selection does the work.

Recognising the difficulties associated with getting the notion of natural selection accepted, Darwin held back from being too explicit about the implications for human origins. By 1871 he was more confident, and was willing to discuss in detail the idea that human beings are the products of evolution. In The Descent of Man he did just that. The start point for the theory of sexual selection then is his thoughts about female choice outlined in The Descent of Man.

First we note that it is quite common in sexually reproducing species for males to have bright colours while their female counterparts do not. For example peacocks have a very large, brightly coloured tail. This seems counter to what we have noted above about natural selection – what could be the survival advantage of this gaudy and cumbersome plumage? The answer lies in the observation that an individual organism must both survive for long enough to reach reproductive age and must attract a mate if it is to pass on its genes. Thus we would expect that the behaviour and appearance of an animal to be adapted not only to helping it survive, but also to helping it acquire the largest number or the highest quality mates.

Returning to the example of the peacock, let us suppose that some time in the past the peacock’s male ancestors were drab colours like peahens. What if, at some point in evolutionary history, there was a tendency for peahens to prefer slightly brighter coloured males? The gaudier males would then be more likely to find mates than their less gaudy rivals which would in turn lead to a tendency for brighter males in the next generation. This would provide a selective advantage for the brightest males which could lead to a runaway process where males would get brighter and brighter with each generation. Even females who bucked the trend would be at a disadvantage for, if they chose to mate with plainer males, they risk producing drab sons who would be less likely to be picked as mates in their turn. Thus selection would push both sexes into producing and choosing ever brighter males. Only when the disadvantages in terms of surviving for long enough to reproduce outweigh the advantages in terms of being selected as a mate would this process cease.

This idea of female preference driving the evolution of male gaudiness has been demonstrated to the satisfaction of most. However, it begs the question of why preferences begin in the first place. Echoing Darwin’s earlier argument Ronald Fisher argued in the 1930s that female preference for bright colours was completely arbitrary, and there are those that still hold to this idea (sometimes referred to as Fisherians). There are those who disagree with this theory and argue rather that the peacock’s plumage (or, equivalently, the length of a swallow’s tail, the sweetness of a blackbird’s song) is a signal that the male has high quality genes, and thus that the female’s selection is based on rational criteria (because such behaviour has been selected for – this does not mean that the females are rational or even know that they are choosing). Fisher rather favoured the idea that initially a preference may be based on some indication of health and vigour, but that once the selection process had begun it could run away and result in a preference for something which was unrelated to good genes. Another insight comes from Amotz Zahavi, who suggested that the fact that the tail is a handicap is itself a signal showing how strong and healthy a particular male is; he can survive in spite of his handicap. Regardless of these disagreements, the theory of sexual selection, based on the idea of female choice drives the evolution of particular traits, is accepted by most theorists.

Before we go on to look at the relevance of sexual selection theory to human evolution, we need to address the question; why female as opposed to male choice? In order to explore this, it is worth introducing the term parental investment. Parental investment (PI), an idea first introduced by Robert Trivers, is defined as any investment by a parent in one of her (his) offspring that increases the chance that the offspring will survive at the expense of that parent’s ability to invest in any other offspring (alive or yet to be born). PI then includes the provision of a wide range of resources such as food, energy and time expended obtaining food and maintaining the home or nest, time spent teaching children and risks taken to protect young. In terms of PI, there is a fundamental asymmetry between the sexes – females have an initial investment in their offspring far greater than that of males because female gametes (eggs) are much more costly to produce than those of males (sperm). This means that a female can have only a limited number of offspring, whereas a male can have a virtually unlimited number, provided that he can find females willing to mate with him. Thus females generally need to be much choosier about who they mate with. The criteria for what constitutes a good choice of male will vary considerably from species to species, but the basic point about female choice remains.

This brings us to the question of how sexual selection is relevant to human beings. Three million years ago our ancestor the upright ape Australopithecus afarensis had a brain size of about 400cc. Modern humans have a brain which is a remarkable 3½ times that size, at 1400cc. This inordinately large brain is very costly to run; 18% of our energy expenditure is consumed by the brain. From a Darwinian perspective this suggests that there must have been significant and immediate advantages to possessing a larger brain which outweighed the expense.

Thus we come to the question; why did we need to become so intelligent? The answers to this question are many and varied; was it that accumulated knowledge played a crucial rôle in enabling humans to develop a rich, varied diet, which in turn required the capacity for language and for a large memory? Or was it that a sexual preference for juvenile features drove us towards prolonged retention of such features (neoteny) which in turn allowed the development of a larger brain as a secondary effect of a longer period of growth?

One idea which has become influential in the last few years is known as the Machiavellian hypothesis, which has its origins in the work of Richard Alexander in the 1970s. He suggested that the main evolutionary pressure for human beings to increase in intelligence was competition with other people, in particular, sexual competition between individuals of the same sex. The argument runs thus; the primary function of most animal communication is to manipulate others, not just to impart information; the ability to deceive and to detect deception in others is highly important for many social animals; thus it is reasonable to assume that this principle underlies the evolution of our highly developed communicative ability. What is more, in order to be really convincing to others in our deception it became necessary to develop the ability for individuals to deceive themselves; Robert Trivers has argued that this may be why we developed a subconscious. In a similar vein Nicholas Humphrey argues that the need to guess the likely actions of other individuals required us to develop the capability to imagine what is in others’ minds; a theory of mind, which was a key factor in the development of self-consciousness.

Evolutionary psychologists Matt Ridley and Geoffrey Miller favour the theory that only runaway sexual selection as described by Fisher is sufficient to explain the huge increase in brain size. Being intelligent, witty and entertaining was sexy to our ancestors! This hypothesis requires only that there was an initial preference for more intelligent mates which drove the process from then on. This helps to explain why only humans developed in this way. First, because the initial preference was quite arbitrary and only came about by chance and we can thus assume that an equivalent preference never arose in the other apes. Second, because the human mating system is unique among apes in that it is characterised primarily by monogamous pair bonding (with occasional polygamy) and shared parental effort in child rearing. This leads to a whole set of tendencies untypical of apes such as the preference of males for young or youthful-looking mates and the preference of females for high status, often older, men.

Intention and Intelligence

Let us now turn to a quite different take on what makes a human mind; that of the philosopher Daniel Dennett. His book, Kinds of Minds: Towards an Understanding of Consciousness provides a good discussion of what we mean by consciousness and mind. Dennett takes an evolutionary perspective, looking at the minds of animals and attempting to understand why natural selection favoured the development of minds like ours. He emphasises three key factors in the evolution of human consciousness: tool use, the ability to predict future and the ability to take the intentional stance.

For Dennett, the importance of tool use is that it is a two-way sign of intelligence. On the one hand it requires intelligence to recognise, use and maintain a tool. On the other hand tools themselves also confer intelligence on the organisms which possess them. Probably the most important tools humans use is a mind-tool - language.

On our ability to predict future Dennett says “the process of evolution by natural selection - has no foresight at all, but has gradually built beings with foresight . . . A mind is fundamentally an anticipator, an expectation-generator. It mines the present for clues, turning them into anticipations of the future. And then it acts rationally, on the basis of those hard-won anticipations."

In Dennett’s view the ability to take the intentional stance is a key attribute of the human mind. The intentional stance is the position we often intuitively take when we analyse animal behaviour. For example it is hard to resist describing the behaviour of a hare on spotting a fox in the following terms: If the fox is far enough away for the hare to be confident that it can escape if pursued it stands on its hind legs to allow the fox to see it and to see that it is aware of the fox's presence. The hare then returns to whatever it was doing without running away, confident that the fox will leave it alone, which it invariably does! While the rationale for its actions may be real, we must be aware that the hare itself has no inkling of that rationale. Dennett describes this as “free floating" rationale.

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Dennett suggests that our ability to take the intentional stance towards others (and towards the self) provides a key example of clever behaviour that can only occur in the presence of clever thoughts. He argues that a key point in the journey towards human consciousness was the step taken between first and second order intentional systems: A first order intentional system is one that has beliefs and desires about some things, but not about beliefs and desires themselves; a second order system is one that has both. He proposes that our ability to take the intentional stance towards others may have come before our ability to apply it to ourselves, and could thus be a prerequisite for self-consciousness.

Dennett proposes four stages in the evolution of conscious minds, focussing on his key factors in the evolution of consciousness: tool use, the ability to predict future and the ability to take the intentional stance.

1. Darwinian Creatures - Various simple organisms are blindly generated and natural selection insures that only the best designs survive.
2. Skinnerian Creatures - Individual organisms that are not wholly designed at birth: they have phenotypic plasticity. Such organisms would use simple trial and error to establish how best to deal with a new situation. (after B.F. Skinner)
3. Popperian Creatures - Such animals have the ability to preselect among a variety of possible behaviours or actions. This requires some sort of filter which amounts to an internal environment or model of the external environment. However, Popperian creatures allow the body to inform decision-making. (after Karl Popper)
4. Gregorian Creatures - Organisms whose inner environments are informed by the designed parts of the outer environment. (after Richard Gregory) For example scissors are an endower of intelligence.

Popperian creatures would be expected to make much smarter moves than their Skinnerian counterparts, partly because they are adaptively responsive to a wider range of high fidelity information. However only Gregorian creatures can benefit from others' experience.


C. R. Darwin., On The Origin of Species by Means of Natural Selection, John Murray, London, 1859
C. R. Darwin., The Descent of Man and Selection in Relation to Sex, John Murray, London, 1871
D. C. Dennett., Kinds of Minds: Towards an Understanding of Consciousness, Phoenix, London, 1997
R. A. Fisher, The Genetical Theory of Natural Selection, Clarendon Press, Oxford, 1930
G. Miller., The Mating Mind, BCA, 2000
N. Humphrey., The Inner Eye: Social Intelligence in Evolution, 2003
M. Ridley., The Red Queen: Sex and the Evolution of Human Nature, Penguin, 1993 (reissued 2000)
R. L. Trivers., ‘Parental Investment and Sexual Selection’, Sexual Selection and the Descent of Man, ed. B. Campbell, Aldine-Atherton, Chicago, 1972

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© C J M Hewett, 2005