The book offers a popular account of a project sponsored by the British Academy from 2001 to 2010: “Lucy to Language: The Archaeology of the Social Brain”. Lucy is the name given to a 3.2 million years old, uncommonly complete fossil of Australopithecus afarensis. The authors are two archaeologists (Gamble and Gowlett), and the psychologist Dunbar from Oxford.
They undertake to tell the tale of human evolution from the first hominins to modern humans. (The taxon Hominini comprises the sole extant species Homo sapiens, and all extinct antecedents, after the split from the chimpanzees. The great apes and the Hominini together constitute the Hominidae.)
The basis of the grand tale is the hypothesis that the increase of brain size during hominin evolution was directly connected with the extension of the social system. Starting point was the group size of 12 to 20 individuals known from chimpanzees. Today, the number of people we maintain (albeit often loose) personal contact with, is supposed to be about 150. This is Dunbar's number, which was introduced by him as the theoretically possible number with whom one can maintain relationships in which an individual knows who each person is and how each person relates to every other person. Dunbar's number is based on an extrapolation of the correlation between the neocortical volume of non-human primate species, and the respective number of group members (book’s Fig.1.1). The authors claim that it is confirmed by ethnographic data, and by the social interactions of modern people. During their evolution, the complexity of the social systems increased as well. A hierarchy developed from the family (about 5) to a small number of direct supporters (about 15), to bands (30 to 50), and cultural lineage groups (100 to 200). Tribes (from 500 members upwards) and proto-nations exceed the numbers one can maintain personal relations with; the affiliation has to be affirmed by symbols. Prerequisite is a sufficient level of social intelligence.
The evolutionary expansion of the social structure corresponded to a “mentalization” of social behaviour. The term signifies the ability to perceive and interpret the acts of social partners in terms of their mental states. The larger neocortex enabled the hominins to include the intentionality of others, as well as their own, in an increasingly complex manner into the control of their social behaviour. A “Theory of mind” (ToM) developed, beginning with the hominids, but much enhanced on the way to Homo sapiens. The larger the social group, the authors state, the more time is necessary to stabilize it by positive interactions, by “grooming”. In that way, human society soon would have reached an unsustainable size. But the evolution of language, of joint laughter, singing, music, and other group rituals, allowed to cultivate social relations much more effectively. Religion (which the book treats only cursorily) has to be regarded as one of those secondary developments to stabilize society. Especially language, according to Dunbar, may have arisen first as a simple form of social grooming. All such behavioural patterns are internally sustained by the endorphin system of the diencephalon. Therefore, all activities which stabilize the social group are sensually rewarding, like grooming among apes.
The picture of human evolution the book offers has merits. It is probably correct to focus on the evolutionary interaction between increasing “mentalization”, and an increasingly complex social behaviour, including a dynamic cultural development. Its view is supported by the results of comparative developmental psychology, e.g. from the Leipzig Max Planck Institute for Evolutionary Anthropology, which are not mentioned in the book. For a scientific appraisal of the project “From Lucy to language”, one would have to consult, beside the popular book, its “benchmark papers” published 2014.(J. A. J. Gowlett, Lucy to Language -The Benchmark Papers, Oxford 2014) But as prolific researchers are wont to do, the authors take the hypothesis of the social brain too far. They postulate first a linear, and then a calculable relation between the size of the neocortex, and group size in primates. That allows them to determine the group size for a fossil species like Homo erectus (Fig.3.5). The transfer of the (statistically rather weak) correlation between the neocortex of extant primates and their group sizes (Fig.1.1), to fossil hominins is speculative, as the social structure depends not only on what is cognitively possible, but upon ecological factors too. Also, the growth of the hominin neocortex presumably had more functions than to increase social intelligence. It is reasonable to suppose that language and religion promoted social cohesion. But they certainly had more complex social functions as well, like the detailed verbal coordinating of cooperation. And the proposition that the selection pressure which propelled the evolution of language skills was the mounting demand for more and more grooming time is even more speculative. A mounting demand for an effective coordination of group activities, and for the sharing of individual experiences, is a much more plausible candidate.
The book, but not the original research papers, contains quite a number of sloppy remarks, which seem to have slipped into the popular text. In Ch. 2, for example, we are informed that the primates “invented” social life. Intended is the closed, stable, individualized group of primates, for many mammals have some form of social life beyond the family, from the reindeer herd to the eusocial naked mole-rat. Even the individualized group was “invented” by dolphins, elephants etc. as well. The book’s Fig. 2.1 shows an advantage of social life: Baboons defend themselves against a leopard. Unfortunately, the cat on the photo is a cheetah, which normally does not prey on baboons.
Ch. 4 tells us that, 7 million years ago, the climate became drier, and the ancestors of horses, together with the grass families which constitute their nutriment, invaded Africa. This triggered the hominin evolution. Of course, grass and grassland existed in Africa long before the late Miocene. And the ancestors of horses proper (of the genus Equus) arrived only 4 million years later. Meant is the so-called savannah hypothesis, that grassland and riparian forest expanded at the expense of forestland, and that induced the first hominins to develop their upright stance, their manipulative skills etc. Such fuzzy assertions don't invalidate the overall picture the book offers, but they could have been avoided with little effort.
The overall picture aspires to unify the explanations of human evolution from archaeology, palaeontology, and biology. It does so, however, on the basis of too many speculations. In anthropology, that might be the rule, not the exception, as Ina Wunn and Davina Grojnowski describe in their recent book. Gowlett, Gamble, and Dunbar continue this tradition of speculative narrations. For example, they propose that Neanderthals stabilized their society by singing and dancing, and refer to Steven Mithen (2005). There are no data to support the notion. Anyhow, where should they come from? Another example will be analysed in greater detail, mainly because the professional past of the reviewer includes research into the human visual system.
In Ch. 6, one finds the hypothesis that the neocortex of the later Homo neanderthalensis, despite the roughly equal cranial capacity, was organized differently from that of Homo sapiens. Neanderthals lived farther north. As an adaptation to the lower mean luminosity in higher latitudes, their eyes became larger, to improve their light sensitivity as well as the visual acuity at low luminance levels. The authors propose that this demanded a larger visual cortex (occipital cortex) for the processing of additional visual information.
Accordingly, the capacity of the “social brain”, and other cognitive functions, were diminished. That caused a certain mental inferiority of the Neanderthals, and finally their extinction. A look into the original paper10 shows that craniums of 21 fossil Neanderthals were examined. Only 8, however, are young enough to compare them with near-contemporary Homo sapiens skulls. The mean volume of the Neanderthal orbital cavities was about 13% larger than that of fossil Homo sapiens. But the interpretation as an adaptation to higher latitudes is questionable. The heartland of the classical Neanderthal (150 000 to 40 000 BP) was South-Western Europe, not the Arctic. Our own eye, with its ability to adapt to a wide range of luminances, is more than adequate to deal with light conditions in Spain and Southern France. A size increase of 13% could have, at most, slightly increased the visual acuity at very low luminance levels, e.g. at night under a cloud cover. For that reason, many nocturnal animals have rather large eyeballs. But their retina is organized differently to optimize night vision. The human retina has no rods (the receptors responsible for scotopic vision) at all in the region of its highest visual acuity, in the fovea centralis. Human scotopic vision relies on the retinal periphery. To concentrate some more rods there on a slightly increased surface, would have had only a marginal effect. And it almost certainly would not have demanded more cerebral effort to process the information. The human eye specializes in recognizing shapes and colours as effectively as possible through a wide range of luminances during the day, from early morning light to glaring midday sunlight. No wonder, as the Hominins descend from diurnal Hominids, and these from diurnal primates. By the way, the orb size of people today varies as much as in the above study, or more. The authors themselves confirm that in another paper. There is no evidence that slightly larger orbs are more light-sensitive, or offer an acuity advantage. And we know for certain that the neocortex of all these people is organized in exactly the same way. Why should Neanderthals have been different? The obvious explanation for their larger orbs is isometry. Because of their general robustness, they had larger facial skulls, and larger orbs respectively. The authors concede themselves that their conclusions are highly speculative. And the above example is not the only one. There are similar speculations about the sexual behaviour of extinct hominins, based on the so-called digit ratio, the ratio between the length of the index finger, and the ring finger (Ch. 3). The examination of Ardipithecus, Australopithecus, and Homo fossils is supposed to show whether these species tended to polygamy or to homogamy. Neither the data, nor the inferences from them, are convincing.
One last example which refers to the evolution of the genus Australopithecus: Ch. 4 explains why its “gracile” branch evolved into the genus Homo, and enhanced its brain volume from app. 500 cc to 800 cc, while its “robust” branch (e.g. Paranthropus), until its extinction about 1.5 million years ago, retained the smaller brain. The hypothesis is that upright walking caused a biomechanical difficulty, for the head now had to be carried on top of the body. Therefore, the head could not, with a sustainable cost, have supported both a massive lower jaw, and a larger brain. As the robust australopithecines adapted to coarse vegetable food, they could not, at the same time, develop their cognitive capacity. However, to balance the head above the body mass centre, as humans do, requires less muscular effort than to carry it before the centre, as knuckle-walking chimpanzees do. The development from the typical australopithecine brain to that of early Homo would have increased its weight by about 0.3 kg. One should think that a very modest adaptation of the neck and shoulder muscles of these burly creatures could have dealt with that. The explanation really explains nothing. The story of the pre-human band who discovered language to lighten the burden of nearconstant grooming is as incredible as the story of the Neanderthal who sacrificed part of his social intelligence for the adaptation to northern lights, and as the story of Paranthropus, who sacrificed a larger brain for the adaptation to tough food. Life and death of species, the complex behavioural patterns they develop, the ecological niches they build, maintain, and lose, rarely result from such simple causes. Usually several, partly conflicting, selection pressures influence each trait of a species, and certainly its overall evolutionary trajectory. It is determined by a nexus of interwoven feedback circles between the organism, its behaviour, and its ecology, which seldom allows easy explanations for the course evolution takes, even less for the appearance and disappearance of species (except when we humans wipe them out ourselves).
One last observation: The book does not discuss causal mechanisms
which were operative during the evolution of the social cooperation typical for humans, and of the necessary social intelligence. It consistently assumes that functionally useful features have a selective advantage, and therefore will come about by evolution. Both assumptions are not necessarily true. The dispute about the evolutionary mechanisms which allow “altruism” and “fairness”
to emerge by natural selection, is filling libraries. The authors might
have evaded that dispute to keep the book popular. The result is that human evolution is described uniformly in reductionistic and selectionistic terms.
The well-established concept of human evolution as a process characterized by the interaction of several system levels, from the gene pool to bodily adaptations, to ecological “niche construction” and increasing cognitive abilities, to social learning, and finally to culture, does not show up. Also, the development of culture follows its own dynamic. Culture is culture-productive.
It may evolve from one innovation to the next on a little changed biological basis. There is no need to construct reductionistic explanations for that. The authors have a problem to explain the cultural “explosion” of Homo sapiens 60 000 years ago, about 100 000 years after the human neocortex had already attained its present size. Cultural innovations, nonetheless, were slow to accumulate, until our ancestors “out of Africa” heaped up invention after invention. That is not especially surprising, if one abandons reductionism, and biologism.
On the condition that one keeps its speculative, and reductionistic tendency in mind, the book offers a very useful introduction into what is presently known about human evolution, in form of a colourful picture-book of much information, many stories, and highlights. It does not, and is not meant to, substitute professional research of the topic.
Reprinted from ESSSAT News and Reviews, n. 27:1 (March 2017), pp. 30-35.