Out of Eden: The Peopling of the World (4 page)

Others have suggested alternative theories of why it was advantageous to adopt a two-legged posture, such as literally keeping a cool head
²
or, like the African meerkats, keeping an eye out for predators on the plain. However, our ancestors’ brains, although larger than those of most other land mammals, were no bigger than that of our cousin the chimpanzee, so there was less danger of them overheating. Nor is standing upright – which many mammals do, including monkeys, chimpanzees, bears, and meerkats – the same thing as habitually walking on two legs for long periods. The idea of leaving hands free to do other mischief such as wielding heavy sticks for hunting (or more likely for defence against predators, since our ancestors were mainly vegetarian) is attractive as an evolutionary force. Unfortunately we have no direct proof, since wood is perishable and stone tools are not found from that time.

Those early walking apes, for whom there is still only fragmentary evidence, were followed by the famous ‘Lucy’ family,
Australopithecus afarensis
. Lucy’s partial female skeleton was discovered by Donald Johanson in 1974 at Hadar, in Ethiopia. Living between 3 and 4 million years ago, her kind were 1–1.5 metres (40–60 inches) tall, more clearly upright and bipedal, with a pelvis
more similar to ours. Above the neck the similarity ceased, for their skulls and brains were like chimpanzees’ (375–500 cm
³
in volume), although they had smaller canine teeth (see
Plate 1
). As with gorillas, their females were much smaller, suggesting a harem society. A different two-legged version (
Australopithecus africanus
) lived between 2 and 3 million years ago and, although the same size, had a slightly larger average brain size than chimps at between 420 and 500 cm
³
. Their teeth were also smaller and more like ours. It should be said that these two particular bipeds are not necessarily steps on a direct evolutionary sequence towards ourselves: our ancestor at this time could easily have been a sister species as yet undiscovered. For instance, our nearest ancestor could have been a recently discovered species from about 3.6 million years ago called
Kenyanthropus platyops
– quite literally, ‘flat-face’ (
Figure 0.1
). The flat face, a feature of humans, might represent a bridge between the walking apes and us. What is certain, though, is that over the few million years in which the australopithecines (‘southern apes’) and their immediate ancestors walked Africa’s grasslands, we see only a moderate, not a dramatic, increase in brain size.

Figure 0.1
   The ‘untidy tree’. Over the past 8 million years of hominid evolution (including here only our nearest living relatives, chimps), several species usually co-existed at any one time, so drawing a tree of direct descent on the basis of chance fossil finds may be misleading – hence the lack of branches on this tree.

Growing brains in the big dry

Things were about to change, because 2.5 million years ago the world started getting colder. Within a million years, the wet and warm Pliocene geological period gave way to the Pleistocene ice epoch. This was a grinding cycle of repeated dry ice ages, with alternating advances and retreats of African grassland lasting right up until the most recent glaciation, which climaxed 18,000 years ago. Soon after the start of this unstable, icy, and dry period, the first humans (the
Homo
genus) with their stone tools and larger brains made their entrance on the African savannah stage. As had happened a few million years before with the split into four-legged and two-legged locomotion, this was a parting of the ways for the descendants of the walking apes. One branch, known as
Paranthropus
, developed larger jaws to cope with grinding up tough vegetable matter. The other branch,
Homo
, made stone tools, developed substantially larger brains, and set off down the road towards becoming better hunter-gatherers and then, finally, us.

Naturally, we are inclined to think of humans as being special and set apart from the other apes. Many think that it is our especially
large brain that makes us what we are. Some even think that an increase in brain size led to tool-making, but this argument seems unlikely. Fashioning
stone
tools, unlike walking on two legs or manual dexterity, may be unique to humans (and possibly to
Paranthropus
³
), but even chimpanzees make crude but effective tools out of wood, and they have smaller brains and branched off much earlier than the walking apes. Although we do not have the evidence in wood from the last 7 million years, chimps still have roughly the same sized brain as our common ancestors who lived at the beginning of that time. This does not seem to constitute a strong link between simple tool-making and achieving a critical brain size. Nor does it rule out the possibility that the common ancestors of chimps and humans were making tools so long ago.

One of the earliest human tool-makers,
Homo habilis
, had an average brain volume of 650 cm
³
, but among the known habilis skulls is one 1.9 million years old with a chimp-like brain volume of only 500 cm
³
, which is at the top end of the range for the earlier australopithecines.
⁴
The small body and brain size and the other apelike features of
Homo habilis
have led some anthropologists to call for their expulsion from the
Homo
genus or ‘human club’, but in spite of the well-argued case, this seems more like size prejudice than scientific reasoning.
Homo habilis
were unlikely to have been our direct ancestors, but that can be said for most hominids; and they made stone tools.

The idea that we somehow grew a big brain first, then decided what it was for, is a negation of Darwinian principles. Any new kind of behaviour always precedes the physical adaptation that evolves to exploit that behaviour. Well before the start of the Pleistocene ice epoch there must already have been some aspect of our behaviour – something to do with the way we faced the climatic challenge, perhaps – that gave large, energy-expensive brains survival value. The problem of finding food in an increasingly dry environment must have taxed our ancestors’ resourcefulness. Larger brains
clearly helped them in some way. That behaviour must still be with us today, because over subsequent major glaciations during the past 2.5 million years, new human species with larger brains and more skills appeared in Africa. As the climate briefly warmed after each glacial maximum, the Sahara would become green for a few thousand years and the new human species would venture out to try their luck in Eurasia. By 1 million years ago, brain volumes of various human species living both within and outside Africa had increased from 400 to 1,000 cm
³
, and even into the modern size range. In other words, human brains had grown to three-quarters of their modern size long before we came on the scene.
⁵

Why did we grow big brains?

There have been various suggestions as to what the key behaviour selecting for big brains might have been. The ice ages forced Africa into extreme aridity and would have inspired the dwindling numbers of savannah humans to greater resourcefulness and cooperation. We can see the value of larger brains in such circumstances, but why should
our
brains have grown, and not those of the other mammals living at the edge of the savannah? One behavioural characteristic that is still very much with us today is our fondness for meat with our vegetables. Indeed, London-based anthropologist Leslie Aiello and her colleague Peter Wheeler (the originator of the cool heads theory) suggested that we needed to eat meat in order to facilitate our brain growth.
⁶
Brains need lots of calories to fuel them, and require high-grade nutrients in order to grow. Yet, as Aiello and Wheeler acknowledged, meat-eating is more a means than a motive for brain enlargement. They also argue that the parallel reduction in size of human intestines, as shown by an alteration of the shape of the ribcage in
Homo ergaster
, is evidence for a change to eating more meat than vegetables. In other words, the oldest true humans had lost the vegetarian pot-belly so characteristic of
australopithecines. But this change in the ribcage happened before the dramatic brain growth.

Early humans such as
Homo habilis
,
Homo rudolphensis
, and
Homo ergaster
may have been more scavengers than hunters. Perhaps they learnt that by arming themselves with sticks and stones they could drive larger predators away from kills, and as their tool-making and cooperative hunting abilities improved they became more confident. But without evidence, all such arguments for the role of climate and meat-eating in the enhanced brain growth of early humans remain largely armchair speculation. We know that, at least in Africa, stone tools were used by
Homo erectus
to butcher meat,
⁷
but to establish a link between meat-eating, worsening weather, and brain growth we would need a comparison with purely vegetarian primates in the same environment over the same period.

Recently, Sarah Elton,
⁸
an anthropologist based at the University of Kent at Canterbury, has provided just that – but her results rather shake our sense of the uniqueness of the human line. She measured brain size in a number of fossil skulls from primate species over the period roughly from 2.5 million years (the start of the cooling) to 1.5 million years ago. She studied the two main branches of hominids that diverged during that period,
Homo
and
Paranthropus
, covering a total of six species. As a comparison primate group she chose several prehistoric species of large, grass-eating, baboon-like
Theropithecus
monkeys that lived in the same environment over the same period. The results were startling. The large vegetarian monkey species showed no trend of increasing brain size over that time period, but hominids from both the
Homo
(
ergaster
and
habilis
) and
Paranthropus
(
boisei
) branches did. So, not only were several new
Homo
and
Paranthropus
species appearing with successively larger brains, but brain size was increasing specifically
within
each species of each genus. The latter observation is even stronger evidence for a shared new behaviour selecting for larger brains held by the common ancestor of the
Homo
and
Paranthropus
branches, but not shared
with other contemporary primates. The relative increase in size in both hominid branches is also surprising since the
Paranthropus
branch, with their huge jaws, were supposed to be specialist vegetarian grinders. Over the million-year period that Elton studied, the average hominid brain size for all species increased from 400 cm
³
to 900 cm
³
.

If we compare this early era of phenomenal brain growth with more recent times in the human line, there is a clear discontinuity between ancient and modern. Between the earliest
Homo habilis
just under 2 million years ago and the first so-called
Homo rhodesiensis
fossils of 1.07–1.3 million years ago,
⁹
a period of roughly 700,000 years, brain volume increased by two and a half times. In the subsequent 1.2 million years, although there were modest trends in brain size increase in individual human types outside Africa, such as Asian
Homo erectus
and European Neanderthals, a net increase of only 6 per cent was required to reach the brain size of today’s humans. (In fact there has been an overall
decline
in brain volume in modern humans over the past 150,000 years – see
Figure 0.2
). So, from a physical point of view, the earliest period of the human tree was the most dramatic.

These results suggest that the earliest period of increased climatic adversity at the end of the Pliocene, and over the Pliocene–Pleistocene climatic changeover, selectively favoured brain growth in the various new hominid species, but not in other primates sharing the same environment. What does this mean? First, it supports the view that all these hominids belonging to the
Paranthropus
and
Homo
branches, and by implication their common ancestor, possessed some new behaviour selectively favouring brain growth, which they had shared from at least the beginning of the cool period. In other words, the behavioural seeds of our extraordinarily rapid brain development may already have been in place in walking apes 2.5 million years ago. Second, it puts the meat theory under some strain, although in her defence of that theory Elton argues that
Paranthropus
were neither strict vegetarians, nor were they incapable of making tools to assist in extracting food from a variety of sources.
¹⁰
Third, the selection for brain size seemed to have its greatest acceleration at the beginning of our genus, over 2 million years ago.