The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  It is more likely that the human brain evolved to be as large as it is by virtue of a number of different circumstances that interacted with one another—sometimes reinforcing, sometimes opposing—over the course of human evolution. The evolution of the human brain, like the evolution of anything else, must be thought about in terms of Darwin’s tangled bank, rather than the misreading of evolution as linear, progressive, and governed by purpose.

  But first, to the brain’s bothersome bigness. A clue to why the human brain is so big can be found in the timing of human brain expansion in evolution.

  It took off sometime after the evolution of Homo erectus but before the appearance of Neanderthals.8 Significantly, this expansion occurred long after the invention of tools and technology.9 When Louis Leakey and colleagues announced the discovery of Homo habilis in 1964, the whole idea of technology was linked with intelligence and brain size, leading to a long and fruitless discussion about the size of brain a fossil hominin ought to have before it could be considered either intelligent or technologically capable—as if, when the brain exceeded a certain size, a mental light would switch on and, like the apes in 2001: A Space Odyssey confronted by a monolith, they would be catapulted into a new realm of cognition.

  That period of prehistory—between 1.5 and 0.5 million years ago—is particularly murky. The world was inhabited by one or more species of hominin, collectively referred to as Homo heidelbergensis,10 which presumably evolved into Neanderthals in Eurasia, Homo sapiens in Africa, and possibly other species in China and elsewhere. We know that Homo heidelbergensis individuals were big and beefy, which alone would have contributed to their large brain size. But we also know that they were technologically fairly accomplished. Well-fashioned wooden spears dating back some 400,000 years ago, and miraculously preserved in peat in Schöningen, Germany, were arguably made by Homo heidelbergensis.11 They look like well-balanced hunting javelins, but would have required a person of some stature to use them effectively. Another habit possibly started by Homo erectus and continued by Homo heidelbergensis was the controlled and deliberate use of fire, which would have led, very quickly, to the invention of the barbecue. As I discussed in the previous chapter, some have suggested that the invention of cuisine contributed in no small measure to the evolution of the large brain in humans.12

  Whereas herbivores have relatively large guts and rather small brains, the opposite seems to be true in carnivores. The reason seems clear. Plants don’t take great skill to eat, if you are doing nothing more complicated than grazing or browsing, but they require formidable powers of digestion. Plant cells hoard their nutritious innards behind tough walls of cellulose, digestible only by bacteria living in the gut. Plants also contain many substances that are poisonous, and these need to be neutralized (plant poisons, a kind of defense against herbivory, form the basis of many drugs used today). Even then, the amount of nourishment one gains from a given mass of plant is rather small. Hence the large gut, and the enormous amounts of time that herbivores spend eating. Carnivory, on the other hand, requires a certain skill, as one’s next meal might have a clue that you are chasing it, and will therefore be prepared to take evasive action. This alone suggests that carnivores require proportionately larger brains than herbivores. In addition, meat is much more easily digested than vegetation, meaning that much less time needs to be spent eating—and much less investment is required in digestive machinery.

  It is possible that hominins started to incorporate meat as a large proportion of diet with the evolution of Homo erectus. This is evident from gross anatomy. Australopiths had cone-shaped, flared rib cages suggestive of a large, pot-bellied gut. Homo erectus and later hominins had a cylindrical rib cage, suggesting a trimmer physique and a smaller gut. The evolution of carnivory allowed hominins to invest more in energetically expensive brain tissue at the expense of the gut.

  This “expensive tissue hypothesis” makes intuitive sense.13 It has, however, been dented by recent work showing no strong correlation between massive brains and lightweight guts in a large variety of mammals.14 What the data do show, however, is a link between brain mass and mass of body fat. Animals with small brains tend to have more fat reserves. Animals with larger brains tend to make do with less fat. The rationale is that having a store of fat is a hedge against thin times ahead, especially if you don’t have the smarts to go out and find more food. Animals with smarts, however, can presumably think about how and when to get their next meal, and therefore gamble on carrying less fat.

  The exception to the rule, as it happens, is humans, which have both large brains and large stores of fat. This could offer a belt-and-braces solution to the prospect of starvation, but there are other possibilities. Advocates of the “aquatic ape” hypothesis discussed above might note that whereas humans seem anomalous among land animals, having large brains and large fat stores together is the rule for marine mammals such as seals and whales. The “aquatic ape” hypothesis, however, doesn’t exclude the very gender-specific patterns of fat storage seen in humans, which might indicate that it has more to do with sexual selection than natural selection.

  The tendency to carnivory might have been enhanced by cooking, which does a number of remarkable things to food and might have exerted some influence on human evolution. So, what does cookery do, apart from offering a stream of entertaining TV programs from celebrity chefs?

  The first and perhaps most important thing is that by breaking down tough proteins and fibrous materials in food, cooking releases more nutrients per unit of mass than one can extract by eating a morsel raw. In short, cooking is a kind of predigestion. If we cook food before we eat it, we need devote less time and energy chewing and digesting it once it has passed our lips. This has a number of important implications. By using a given amount of resource more efficiently, cooking allows people to be bigger—that is, more massive—than they would be otherwise. Some increase in brain mass would be a simple side effect of an increase in body mass, irrespective of any other cost or benefit. Cooking also neutralizes any toxins and kills parasites in food, so that people eating cooked food might live longer and be healthier than those subsisting entirely on raw food.15

  There would be changes in shape as well as size. Modern humans have smaller teeth than many earlier hominins, and this has also been related to cooking. If food has been softened by cooking, one needn’t expend any more resources than necessary building a big and complicated digestive system. This applies to teeth and jaws as well as guts, and to the muscles that open and shut our jaws. Much of the force of the bite inflicted by a dog, say—or a chimp—comes from the chewing muscles on each side of the head. These muscles run from the sides of the jawbone, thread beneath the zygomatic arch (cheekbone), and fan out on the sides of the head, anchoring at a crest at the very top. Animals with big chewing muscles—such as dogs and gorillas—often have such a crest running along the midline of the skull. Hominins such as Paranthropus had teeth like tombstones, powered by big, thick chewing muscles—we can tell this from the prominent head crests and the widely flared cheekbones that made way for the muscle mass. Modern humans, though, have none of these things, and the relatively weak chewing muscles get only as far as the sides of the head before petering out. The skull roof is smooth and not clothed in muscle. This is one reason why modern human skulls are globular, with no crests or other prominent signs of muscle attachment.

  The smallness of human chewing muscles has been linked with a particular genetic mutation found in humans but not other mammals.16 Could this mutation have played a part in the expansion of the human skull and the weakening of these muscles? Could natural selection against this mutation have been weakened in a population of novice chefs and aspiring gourmets, allowing it to spread? Irrespective of the mechanism, the reduction in muscle mass, and the exposure of the skull, would have removed a possible external constraint on skull growth. The growth of the skull in babies and small children is intimately linked with brain growth.17 The brain drives the
expansion of the skull roof, from which it seems possible that the reduction of jaws, teeth, and their associated musculature might have been connected with the further expansion of the human brain, beyond any expansion that would have accrued as a simple increase in body mass as a consequence of the nutritional benefits of cooking.

  The expansion of the human brain starts early, well before birth, so that the size of the fetal brain is so great that delivering babies is a serious health risk for human mothers. The brain size of human newborns seems to be at the top of the permissible range. We can tell this because the brain continues to grow and develop in the human infant for far longer after its birth than in apes. As a consequence, human babies are born in a relatively helpless, premature state compared with the babies of apes and many other mammals, and take a very long time to mature. As every parent knows, bringing up children requires a vast amount of effort and resources, and is far less onerous if one can share the burden. It is known—or at least strongly suspected—that women who can call on relatives to help will raise children more successfully than those for whom help is unavailable. In traditional societies, at least, a great deal of help comes from a mother’s older female relatives, especially her own mother. This so-called grandmother hypothesis might explain another otherwise puzzling feature of human biology—the menopause.18

  In virtually all creatures, the evolutionary imperative means that having been born, one should grow up as quickly as possible, reproduce early and often, and then die. Very few creatures live for very long past reproductive age, but humans are a marked exception. In human females the process of menopause marks a definite shutdown in reproductive capacity—after which the individual can expect to live for several more decades.19 An explanation, which seems to have some traction, is that by ceasing to reproduce herself, a female is then better able to assist the reproduction of her daughters.20 Because human babies take so long to mature, one can imagine selective value in both prolonging human female life span and introducing a definite cessation of reproductive effort in order to care for one’s grandchildren, as well as one’s own children—rather than producing more children who will compete for resources with one’s grandchildren. And, given the high risk of death in childbirth as a function of the large brains of babies, the odds might be stacked in favor of ceasing reproduction to care for grandchildren rather than incurring the risks entailed in producing more young of one’s own—risks that might become greater with age.

  The menopause has other effects, too. It automatically provides a stratum of society that other animals simply do not have, which is a cadre or caste of older individuals whose existence is not predicated solely on their own reproduction. By offering stores of knowledge that can be passed on to younger members of a group, elders make societies more cohesive and offer the potential for such groups to become more complex.

  Could all this—the expansion of the human brain, the helplessness of human infants, and the growth of society—all be related to the invention of cooking? Even were one to regard the above with a laudable skepticism, the fact remains that cooking is in itself a social and sociable activity. People gather round the hearth, and there can be few social, ritual, or religious occasions that do not revolve around the provision of food and drink, sometimes after specified periods of abstinence and fasting. As religious offerings were once edible (and suitably cooked to send the savor of cooked food to heaven), no birthday party is complete without a cake, just as Christmas isn’t Christmas without a Christmas pudding, Thanksgiving without its turkey, nor Easter without its eggs.

  And where people come together to eat and drink, they gather to do deals, choose mates, play music, sing, dance, and swap stories. In humans, at least, cooking facilitated our need to be social. Therein might lie another clue about brain expansion. All the animals we know that have large brains, large EQ, and behavior that we humans recognize as “intelligent” are also social. Brain size, intelligence, and social life go together.

  Or do they?

  Before leaving this chapter, I’d like to take a quick look at Neanderthals. These were hominins with brains larger, on average, than those of Homo sapiens, and of a comparable EQ. They were capable chefs, made tools, and perhaps even had an inner spiritual life. Neanderthals lived on Earth for around 300,000 years but left without making a mark. Homo sapiens has been around, so far, for around 200,000 years, and in just the past 10,000 years has come to dominate the planet’s ecology and resources. One is entitled to ask why.

  Clearly, brain size isn’t everything. But what several recent lines of research have indicated is that Neanderthals were a lot less sociable than modern humans. They tended to live in smaller groups, had smaller home ranges, and were therefore less likely to come across other members of their own species.21 Neanderthals were, in this respect, rather like modern great apes. Perhaps the earth was inherited not by the creatures with the biggest brains, or the most intelligence, but with the busiest social calendars.22 The invention of agriculture between around 10 and 20,000 years ago created a situation in which humans were forced to live in fairly large, concentrated groups. The first villages were less like enlarged nests of gorillas or the expanded ranges of chimps, and more like rookeries.

  I’ve used the word “intelligence” several times, but without attempting to define it. I have, however, compared human intelligence with crow intelligence, suggesting that crow intelligence is something we can intuitively recognize. But is it possible to define intelligence on its own terms, without reference to the animals in which it is found? Can it ever be isolated from cultural or developmental context? The application of such things as IQ (intelligence quotient) testing is too parochial for the remit of this book, which aims to take a wider, more zoological view. When applied to the wider world of life, one might propose that intelligence is a rough measure of the speed and efficiency of information retrieval, perhaps combined with a way of generalizing this information so that it can be applied in novel situations—something like Spearman’s original concept of “general intelligence,”23 derived from the observation by the pioneering statistician Charles Spearman, a century ago, that schoolchildren who were good at one subject were likely to be good in others, too, reflecting an underlying intelligence unrelated to subject-specific capabilities.

  Importantly, this rough-and-ready definition of intelligence says nothing about brains, how big they are, what they are made of, or how they are wired up. This is important, for it allows us to look at creatures whose brains are made very differently from ours, and estimate what intelligent creatures have in common concerning their brains and nervous systems.

  What, then, can we make of the comparison between crows and humans? In the main, that the broad definition of intelligence I sketched above is correct. Intelligence is all about the retrieval of information in such a way that its lessons can be generalized and applied to new situations.

  Crows and humans, despite their entirely different evolutionary histories, are intelligent in precisely this way. This suggests that intelligence actually means something—something distinct from the parochial conceit of human evolution, and distinct from brain size—and that we should be able to recognize intelligence irrespective of the nature of organism in which it occurs.

  If there are intelligent aliens, they needn’t, like H. G. Wells’s Martians in The War of the Worlds, have “intellects vast and cool and unsympathetic,” but might be very similar to ourselves.

  They’ll be liars, cheats, hoodlums, and swindlers.

  They’ll also be friendly, sociable, sympathetic, and above all, talkative.

  And when highly social animals get together, they do like to chat.

  10: The Things We Say

  If heard from a long way off—so you can’t hear what’s being said—the conversations between parents dropping their children off at primary school probably sound very like the squawks of crows in a wood at evening. The content of both sets of exchanges would probably be very similar, ev
en if you could hear the words spoken by the humans, or make sense of the squawks of the crows.

  Is this not a scandalous suggestion? After all, human language—the system whereby we communicate information through the rapid modulation of sounds carried on exhaled packets of air—seems something unique, exceptional. The complicated arrangement of the larynx, the resonant chambers in our nose and mouth, the shape of our throat, the musculature that controls our lips and tongue with great precision and delicacy—all seem refined to a degree seen nowhere else, suitable for conveying the infinite subtleties of language. Apes and monkeys do communicate vocally, but they have nothing to match the sophisticated vocal apparatus of humans, nor, as far as we know, do they indulge in communications of a subtlety that might demand such complexity.

  When one looks beyond our immediate primate relatives, we see that many animals have equally unique forms of vocal communication, dependent on equally sophisticated and refined structures. Frogs, for example, display an astonishing range of calls used to attract and respond to prospective mates. Humpback whales have what can only be described as a “culture” of songs, which, like traditional Indian ragas, are built on immensely long sonic structures. Insects of all kinds communicate by chirps, rasps, and buzzes, and the air is full of the songs of birds—created using an organ called the syrinx, every bit as complex and specialized as the larynx, tongue muscles, lips, and so on that we use to create speech. The world is full of the sounds made by animals calling to one another, conveying information.

  But what about the complexities of grammar and syntax? Isn’t this complexity something that only humans can muster, let alone master? To be sure, human language is rich with meaning and intention. The things we say to one another convey meaning about the ever-changing relationship between people and things in times past, present, and yet to come. To marshal such complexities, the atoms of human language are organized into various categories such as nouns (the names of things) and verbs (which indicate actions and the relationships between nouns), both of which can be modified by adjectives (which modify nouns), adverbs (the same, for verbs), and many other particles that indicate gender, person, time (that is, tense), place, and the relationships between all of these.