A Different Kind of Animal: How Culture Transformed Our Species, by Robert Boyd, Princeton University Press, 248 pp, $27.95, ISBN 978-0691177731
Are humans ordinary, or extraordinary animals? Is Homo sapiens an outlier in the animal kingdom? If you do not subscribe to scientific theories of the evolution of organisms, read no further. You will consider the answer to these questions to be self-evident: humans are closer to angels than to beasts. This review is written from the point of view of a palaeobiologist interested in evolution who considers the questions legitimate and worth discussing.
What is the evidence that humans differ fundamentally from other animals? It is commonly suggested that our large brains set us apart. However, animals such as elephants have a greater mass of brain tissue than humans and it has long been realised that it is not only the absolute size of the brain that is important, but also its size relative to the size of the organism. Aristotle wrote: “Of all the animals, man has the brain largest in proportion to his size.” It turns out that even this statement is not true: a shrew has a substantially larger brain relative to its size than a human. However, if the mass of brain is compared with body mass in a wide range of mammals, it is found that when the data are plotted on a graph where the units on both axes are logarithmic (that is, 0.01, 0.1, 1, 10, 100, etc.), they lie within a narrow envelope which has a clear linear trend. The distance above or below this mathematically computed trend line of each kind of mammal is the amount by which it exceeds or is less than the value that would be expected for a mammal of its mass predicted from the trend line. This value has been termed the encephalisation quotient. When encephalisation quotients are compared, humans exhibit the highest values and, perhaps significantly, values substantially greater than those of other primates such as gorillas and chimpanzees, with whom we share a common ancestor.
But does having larger brains than expected for our size necessarily imply that we are special? After all, a giraffe’s neck is very considerably longer than that of most other mammals, while dogs have a much more developed sense of smell than humans. Does that make giraffes or dogs any more extraordinary than other animals?
We tend to place more emphasis on the differences between organisms than on their similarities. Biologists commonly assess the overall morphology of a group of animals using a metric of similarity and difference termed disparity. If measurements are made of two characters, for example the diameter of the eye socket and the width of the skull, in two species, the two species can be represented by two points on a graph in which the two axes correspond to the two characters. Using Pythagoras’ Theorem, the distance between the two points representing the two species can be calculated. If a third character is added to the analysis, the two species can be placed in three-dimensional space and, just as in the case of the two-dimensional graph, the distance between the two species can be calculated. Although it may be difficult to envisage, if many characters are measured, the two species can be positioned in a multi-dimensional space, commonly termed morphospace, and the distance – the morphological distance – between them can be measured in the same way. When this analysis of disparity is carried out for a large number of species, the degree to which the species are clustered or separated in the morphospace can be assessed. If a study was carried out of the morphospace occupied by all living terrestrial animals (and I am not aware that such a study has been performed), the mammals might be expected to form a distinct group. The primates (humans, great apes, gibbons, monkeys, lemurs and tarsiers) would group together and within the primates, the humans and great apes would form a cluster. However, in the morphospace occupied by all animals, or indeed all mammals, humans would not form an obvious outlier. In other words, humans do not stand out as extraordinary from the point of the view of their anatomy if both similarities to and differences from other animals are taken into account.
The wealth of information that has been revealed by the human genome project and gene sequencing of other primates allows us to gauge how similar are the genomes of humans and the great apes. Our nearest relatives are chimpanzees and bonobos (formerly called pygmy chimpanzees), two species which diverged approximately one and a half million years ago. The hominid lineage which led to Homo sapiens and the shared ancestor of chimpanzees and bonobos diverged approximately four million years ago. The genetic difference between individual humans is on average 0.1 per cent. It is commonly stated that the human genome differs from those of chimpanzees and bonobos by slightly more than 1 per cent, but this figure refers only to differences in genes that code for proteins, whereas there are significant differences in other parts of the genome which, among other functions, control regulatory networks that are important in development. Nonetheless, the genetic differences between Homo sapiens, chimpanzees and bonobos are no larger than between other closely related species of mammals, for example, horses and zebras.
In this book Robert Boyd, who is Origins Professor in the School of Human Evolution and Social Change at the University of Arizona, makes the case that humans are indeed a different kind of animal and discusses the evidence which underlies this claim. The first half of the book consists of an introduction by Stephen Macedo, professor of politics at Princeton University, and two chapters by Robert Boyd, based on the Tanner Lectures on Human Values which he gave at Princeton in April 2016. The second half contains discussion of Boyd’s ideas by H Allen Orr, professor of biology at Rochester University; Kim Sterelny, professor of philosophy in the Research School of Social Sciences at Australian National University; Ruth Mace, professor of evolutionary anthropology at University College, London; and Paul Seabright, professor of economics at the Toulouse School of Economics. In the final chapter, Robert Boyd responds to the four sets of discussion. There is an extensive list of references and an index.
Robert Boyd bases his claim that modern humans are a different kind of animal not on the basis of anatomical or genetic differences but rather on the evidence of a set of parameters that are measured by ecologists, such as species and ecological range. The former is the geographical range of a species, that is the area it occupies, and the latter, the range of environments that it inhabits. Humans now occupy a greater geographical range and a spectrum of habitats wider than those of any other terrestrial animal species. We inhabit every continent except Antarctica, most oceanic islands and a wide range of terrestrial habitats from Arctic tundra to tropical swamps and from rainforest to desert. It could be argued that through the use of technology we have also invaded aquatic and aerial habitats. As Robert Boyd points out, our extraordinary species range suggests that we are able to adapt to variations in the environment better than any other living terrestrial animal. It might have been expected that this expanded species range postdated the evolution of agriculture, which developed independently in different regions within the Holocene (the last 11,700 years). In fact, hunter-gatherer societies of Homo sapiens had moved out of Africa to Asia by at least seventy thousand years ago and possibly substantially earlier, had reached Australia fifty-five thousand years ago, had spread across Europe by thirty-five thousand years ago and had crossed from Asia to Alaska about twenty thousand years ago, and from there to the remainder of North and South America before the Holocene.
A second ecological parameter that identifies Homo sapiens as an extraordinary animal is species biomass, the total mass of living material of the species on the globe. At the present day, the biomass of the estimated population of 7.6 billion humans exceeds that of all other species of vertebrates, except for those, such as cattle, that we have domesticated. A corollary of this is that we also process more energy than any other species. Analysis of DNA of a large sample of modern humans suggests that the populations of the earliest Homo sapiens consisted of ten to several tens of thousand individuals, considerably smaller than the population of chimpanzees and gorillas living today. Population size may be important in determining the degree of social learning in a group: as populations expanded, so the opportunities for social learning also increased.
If, as discussed above, modern Homo sapiens really is an outlier amongst terrestrial animals, what evolutionary or other factors have led to this situation? A popular set of answers to this question might include our bipedal stance, our relatively large brain, our use of language, our ability to make and use tools and our control of fire. Obviously, many if not all of these characteristics are linked in some way to the evolution of our cognitive abilities, which would have been coupled with the evolution of our brains. However, Robert Boyd, in the first chapter of the book, titled “Not by brains alone”, argues that the key change in the transformation of human ancestors nto their modern counterparts was the development of social learning, transmitted over many generations, so that human populations accumulated adaptive information which lead to the evolution of new behaviour and technology. In a seminal publication in 1985, he and Peter Richerson referred to this as “cumulative cultural evolution”. The word culture has come to have many different meanings. In the present context, the definition in the Cambridge English Dictionary – “the way of life, especially the general customs and beliefs of a particular group of people at a particular time” – is appropriate.
A telling example cited by Boyd in support of his “not by brains alone” argument is the ill-fated Burke and Wills expedition of 1860, which had the aim of crossing Australia from Melbourne to the Gulf of Carpentaria. Burke, who was born in Galway, and Wills, an Englishman, both perished near Cooper Creek, South Australia on the return leg of their journey when they ran out of food, even though they had been given succour by an aboriginal tribe, the Yandruwandha, who supplied them with fish and bread cooked from flour made by grinding the sporocarps (seed-like spore cases) of an aquatic fern (nardoo). Burke and Wills, after the Yandruwandha had left them, eventually were able to identify nardoo as the source of the flour that they had eaten, but did not know how to process it to remove what was effectively a toxin that it contains. Nardoo is thought to have been a food source for aboriginal people in central Australia for several thousand years. Burke and Wills were not stupid but they lacked the many cultural adaptations, including the processing of nardoo, evolved over many generations, that allowed the Yandruwandha to survive in the desert around Cooper Creek.
Boyd writes: “I hope I have convinced you that the human propensity to learn from others has allowed human populations to create superb cultural adaptations to local environments. Even the simplest hunter-gatherer societies depend on tools and knowledge far too complex for individuals to acquire on their own.” Human culture is both much more complex and clearly cumulative in character compared to the examples of social learning and behavioural traditions that have been described in other animals, even though such examples have been reported in a wide and disparate range of species, including, among others, chimpanzees, orangutans, various monkeys and birds. In particular, it is not clear to what extent behaviour such as the use of tools by chimpanzees is socially transmitted and there is very little evidence of the progressive adaptation of such behaviour over successive generations.
In the second chapter, “Beyond Kith and Kin”, Boyd explains how wide-scale cooperation in humans has evolved and has contributed to our success as a species. Many groups of animals exhibit cooperative and social behaviour to varying degrees. For example, lions live in prides and cooperate in hunting and in the care of their young; however, another big cat, the leopard, is solitary and cooperation is limited to the raising of the cubs by the mother. Our nearest relatives, chimpanzees and bonobos, in contrast to humans, exhibit a very limited range of cooperative behaviour. Boyd points out that studies of chimpanzees have shown that after weaning, each individual provides its own food, whereas in human hunter-gatherer societies, which are our best proxy for the societal organisation of early man, food is provided by certain individuals and shared by other members of the society, who may have other roles.
Of course, so-called eusocial animals, which include termites, ants and bees, have developed cooperation and social behaviour to a high level: they are colonial and have evolved to the point where different individuals carry out different tasks. In the case of honey bees, sterile workers forage for food, secrete the wax, construct the nest and feed the developing larvae, while the few drones fertilise the single queen who lays the eggs. Charles Darwin puzzled over the evolution of eusocial animals because his theory was predicated on the relative fitness of an individual: it was difficult to understand how sterile social insects, such as the worker caste of honey bees, could be selected because reproduction was restricted to the drones and the queen.
With the understanding of the genetic underpinning of the theory of evolution, it has been possible, using mathematical models, to show that cooperative behaviour can be advantageous if the individuals interacting are related and thus share some of their genes. The closer the relationship, the more costly the behaviour can be to one participant and still be advantageous. This mechanism of evolution has been termed “kin selection”. But as Boyd points out, all human societies depend on cooperation involving specialisation and exchange, which is impossible to explain by kin selection. Clearly in our globalised western society, the purchaser of a mobile phone is unlikely to be closely related genetically to all those who contributed to its manufacture. But even in small hunter-gatherer groups, where kin selection might have been anticipated to sustain cooperation, anthropologists have shown that the degree of relatedness of the individuals in most cases is not close enough to provide an evolutionary explanation for cooperation. So, widespread cooperation seems to be a characteristic of humans, which distinguishes them from our closest relations, chimpanzees and bonobos. It is interesting to speculate that using the terminology of evolutionary biology, widespread cooperation may be considered an apomorphy (a derived characteristic) of the genus Homo or of our species, Homo sapiens. This is further discussed below.
If kin selection is not involved, how has human cooperation been maintained and how have human cultures evolved? Cooperation in human societies today occurs at different levels of sophistication. The lowest level is in one-to-one interactions, where both partners share the cost of the cooperation and both gain. Mutual grooming is an example of this in non-human primates. At the other extreme is the production of public goods, an example in Western societies being the provision of public services.
At both ends of this spectrum, the maintenance of cooperation requires a solution to the problem of what Boyd refers to as “free riders”; in the context of public goods, a tax-dodger is an example of a free rider.
One mechanism that has been suggested as dealing with the problem of free riders is contingent cooperation, also termed “reciprocal altruism” or “reciprocity”. This model can be formulated as: “I’ll cooperate with you now, if you cooperated in the past, but if you defect (resulting in a cost to me and a benefit for you), I will not cooperate with you in the future.” While this mechanism can sustain cooperation in small groups, it can be shown mathematically not to be effective in larger groups and certainly cannot explain the maintenance of the highest levels of cooperation, the provision of public goods.
Robert Boyd argues strongly that in contrast to the explanation based on reciprocity, human cooperation generally is dependent on a system of norms enforced by sanctions imposed by third parties. In developed societies, some but not all of these norms are codified as laws and the corresponding sanctions are imposed by the state. But even in numerically small groups of hunter-gatherers, there are norms, in the form of customs and taboos, and sanctions are applied to those who violate those norms. Boyd argues further that the ultimate motivation for norm enforcement is that norm enforcement itself is normative. He speculates that as a result of living in societies in which norm violators suffered serious sanctions, early humans acquired by genetic (and it must follow, physiological/psychological) evolution, moral sentiments that promoted cooperation, trust and the willingness to obey and enforce norms: cultural knowledge accumulates best in situations where there is an inherent motivation in people to adopt the beliefs of the people around them, even when those beliefs conflict with their own experience. This credulity commonly leads members of a group to imitate other members of the group, particularly those who are perceived as being successful.
While norms maintained by sanctions are consistent with what can be seen in human societies, how those norms have evolved is less clear. One possible mechanism proposed by Boyd is “cultural group selection”, that is, competition among culturally different groups. In some ways, this is analogous to Darwinian natural selection: there need to be persistent cultural differences between groups that affect their competitive ability and losing groups must be replaced by winning groups. It differs from Darwinian natural selection in that losing groups do not necessarily become extinct but may be assimilated into winning groups. It also differs in the rapidity with which changes in norms can take place. Not all shifts in norm content, however, are readily explained by cultural group selection. An example from our own society is the change in attitude towards homosexuality in the last fifty years.
Boyd sums up his thesis as follows: “the evolution of cultural adaptation was an essential ingredient in both the ecological success of humans and in their ability to cooperate. It wasn’t the only factor but it was a crucial one.” However, this is not to deny that Darwinian natural selection has been active in human evolution. He concludes: “The morphological, physiological, and psychological changes that make human culture possible evolved over the last several million years as a consequence of the usual evolutionary processes. Culture has made us a very different kind of animal, but without doubt we are still part of the natural world.”
The second half of the book consists of extended comments on Robert Boyd’s thesis by Professors Orr (an evolutionary biologist), Sterelny (a philosopher), Mace (an evolutionary anthropologist) and Seabright (an economist), and his response to them. All the commentaries essentially support Boyd’s thesis, that of Seabright being the most critical. These chapters are particularly useful in that they illuminate aspects of the first two chapters that may not have been fully grasped on a first reading. Irish readers may also find interesting an anthropological study by Professor Mace and a student carried out in Belfast in the early years of this decade.
As a palaeobiologist, the book left me contemplating at what point in the history of hominins our characteristic cultural adaptations evolved, a topic that is not given much attention. The discovery at Lomekwi, northern Kenya of 3.3 million-year-old stone tools, showing evidence of fabrication, suggest that the beginning of this journey of cultural evolution had begun prior to the evolution of the genus Homo, the earliest fossils of which are 2.75–2.8 million years old. From 3.3 million years ago, for approximately another three million years, the development of stone tools appears to have been very slow, with Oldowan crude hand axes, associated with Homo habilis, being replaced approximately 1.7 million years ago by slightly more sophisticated Acheulean hand axes and scrapers, generally associated with Homo erectus. The makers of both Oldowan and Acheulean tools used locally available materials to fabricate their axes. A very recently published and exciting discovery in deposits of the Olorgesailie Basin in Kenya, securely dated to 320,000 years ago, is of more sophisticated blades and points, fabricated from obsidian imported from sites up to ninety kilometres away. Although no hominid remains have been found associated with the tools, recent discoveries of earliest known Homo sapiens in Morocco are dated to approximately three hundred thousand years ago and it seems likely that the Olorgesailie tool makers were Homo sapiens. They were surely also members of a cooperative society. Had our close relatives, the Neanderthals and Denisovans, who overlapped in time with humans, also evolved cultural attributes similar to ours? The confirmation recently that cave paintings in Spain are sixty-five thousand years old and therefore must have been made by Neanderthals (because this predates the spread of modern man to the Iberian peninsula) suggests that Neanderthals, at least, shared some of the facets of human culture.
A final thought: species longevity is a metric of interest to palaeobiologists and many species have persisted for several million years. Will Homo sapiens be among them?
George Sevastopulo teaches in the Department of Geology, Trinity College Dublin.