The thread of human evolution over 1.8 million years ago has therefore been one of adapting to an increasingly arid world while being tied down to the need to drink water regularly. Biologically, this has been expressed by the enhancement of those existing features that made this possible: bigger brains, lighter bodies, longer hind limbs. Behaviourally, it has been expressed through the development of an increasingly multi-purpose and lightweight kit. That is the thread but there were many variations and reversals along the way. The indigenous people of Australia or the westerners that first met them were no more modern than the first populations of Homo sapiens. Each did very well in the context of its time. The western settlers who made contact with the indigenous people of Australia made the basic mistake of confusing them for backward savages. I cannot help feeling that many contemporary archaeologists and palaeoanthropologists have made the same mistake when judging the peoples of the past.
As I was finishing this book, a paper was published8 that reported an ancient African paternal lineage which had not been previously detected by geneticists. The time of the most recent common ancestor of this African-American Y chromosome lineage is estimated at 338 thousand years ago. The paper reported that the date exceeded the oldest anatomically modern human fossils by well over 100 thousand years ago. In palaeoanthropo-logical language, the new date would put this ancestor within the realm of Homo heidelbergensis and not sapiens, a real problem of interpretation. I do not have such a difficulty. On the contrary it serves to confirm the unity of Homo sapiens and that the classification of Middle Pleistocene humans as a separate species—Homo heidelbergensis—was wrong.
As this book was going to print, the sensational fifth skull from Dmanisi, Georgia, was published in the journal Science.9 The 1.8-million-year-old skull, attributed to Homo erectus, was significantly different in shape from the other skulls from the same site. Put together, the five skulls were as variable as African fossils that have been traditionally classified as three distinct species: H. erectus, H. habilis, and H. rudolfensis. The authors concluded that 1.8 million years ago there had been, in fact, only a single species of Homo, for them erectus, on the planet. This conclusion supports the idea put forward in this book that we can regard the observed variability of available specimens as representative of variation within a single biological species. If this was by now clear for the most recent branches of the human tree (sapiens and neanderthalensis), the latest findings show that it was also applicable to the earliest Homo.
The ideas presented in this book are my own but they have benefited from discussion with many colleagues and friends. I am most grateful to all of them. My wife, friend, and colleague, Geraldine, has been my prime assessor; she has always had time to discuss and debate an insight, an idea, or a comment, often at the drop of a hat. This might have been while excavating inside a cave, while driving on a motorway, or simply over dinner. She has kept me on track too, warning me of the pitfalls of some of my arguments. Her unique knowledge of habitat structure has been critical for this book.
My son Stewart is my natural history companion, sharing many hours in remote locations with me. His fresh views on animal behaviour and ecology and his passion for caves have kept my own spirit alive. He understands the importance of a good grounding in natural history before even hoping to try to relate to those great naturalists who were our ancestors, often commenting with incredulity on unrealistic remarks that he has read in scientific papers.
Darren Fa, my former PhD student, is a friend and colleague who has also been deeply involved in our research programme for a long time and who, with Geraldine, participated in an early presentation of our understanding of human biogeography, back in 2000. As a marine biologist, his knowledge and insights of human activity along the intertidal zone are making an important contribution to this field.
I would like to thank all friends and colleagues who have worked with me in the study of our origins and have in some way contributed to the ideas put forward in this book: Joaquín Rodríguez Vidal, Francisco Giles Pacheco, Larry Sawchuk, José Carrión, Juan José Negro, Richard Jennings, Jordi Rosell, Ruth Blasco, Marcia Ponce de León, Christoph Zollikofer, José María Gutierrez López, Alex Menez, and Antonio Sanchez Marco.
Finally, I thank Latha Menon and Emma Ma at OUP for their support, patience, professionalism, and hard work in the preparation of this book.
1
The Inverted Panda
If we were able to go back to the Middle Miocene world of 16 million years ago, when apes were widespread across large areas of the Old World,1 we would be forgiven for not predicting the future existence of a creature that would one day call itself Homo sapiens. We might have predicted something like a gorilla, an orang-utan, or a chimpanzee but not a human. Yet this improbable primate’s heritage is in the deep forests of the Miocene apes and it is here where we should start looking for the antecedents to the path that led to humans.
Let us start with the brain. There is, after all, no other organ that defines us better. Katherine Milton at Berkeley gave us a great and convincing insight into the function of the primate brain in a forest context.2 She highlighted the complexity of the rainforest world that the early insect-eating ancestors of the primates entered at the end of the Cretaceous, some 70 million years ago. This was a world that was coming under the dominance of the flowering plants and some of these early insectivorous mammals were probably drawn up into the trees where insects gathered round flowers. Once up there, flowers and young leaves may have been added to the diet. We can imagine a scene with shrew-like mammals scurrying among the branches of an ancient forest, snapping away at juicy insects clustered around ancient flowers. Occasionally one might take a bite at an insect and accidentally swallow a petal. If it liked the taste, and there was no ill effect, petals might have been added to this individual’s diet. Let us imagine that these animals with a wide tolerance in the recognition of insects, which may have allowed for such mistakes, could have gained some advantage from consuming petals. In time the forest might have been swamped with petal-snapping descendants and any genetic novelty improving petal-snapping would have been favoured. I shall say more about how behaviour can predispose animals to particular genetic novelties in Chapter 4. The traits that characterized the primates thus evolved as plants became increasingly important in the diet, the grasping hand being an early innovation that has stayed with us until today.
Once in the difficult, three-dimensional forest canopy any improvements to the visual apparatus would have been favoured. This is because visual discrimination—colour vision, sharpened acuity, and depth perception—would have helped to detect ripe fruits and tiny, young, and succulent leaves from among the vast expanses of green. The green world of the forest canopy may seem luxuriant to us viewing it from the outside but this is a false impression. It is not an easy world at all for a primate. High-quality foods, particularly fruit, are distributed in patches with lots of unsuitable trees in between. These foods are often seasonal so a knowledge of when, as well as where, to locate the good patches is critical to survival. The canopy actually resembles an open ocean or a plains environment in the sense that life is boom or bust: find a good patch at the right time of year and you will flourish but the consequences of not finding such bonanzas can be fatal.
Forest primates face two dietary choices: eat large quantities of low-quality food, such as leaves, which are widespread and abundant but low in energy and high in fibre content or, instead, focus on the high-quality foods, such as fruit, which are rich in carbohydrate and relatively low in fibre but which require time and effort to find. Those eating fruit need protein supplements which they often obtain by eating insects or other animal matter. Primates that have taken the abundant, low-quality food route have evolved specialized guts that allow them to process bulk and extract as much energy as possible from the mature leaves which they eat. In general they tend to be relatively inactive, to conserve energy, in comparison with those primates that take the
high-energy foods. Think of a group of gorillas and you will get the picture. The highly specialized giant pandas of China or the tree sloths of central and South America, mammals unrelated to the primates, paint a similar picture.
Those that have gone for the scarcer and patchier high-quality foods have, instead, relied on behaviour, especially a good memory that allows them to remember where and when to go for the choice items. Among these primates we could include the spider monkeys of Panama3 and chimpanzees.4 Improved visual and cognitive skills gave direct foraging benefits to the early primates, promoting large brains—the hallmark of primates from the very beginning. That brain was put to good use by a wide range of primates, in particular those which went down the path of feeding on high-energy, difficult-to-find foods. So our brain is also probably an outcome of the fruit- and foliage-seeking primates’ behaviour, tweaked and enhanced along the 70-million-year journey to the present. The brain expansion that our ancestors experienced over the last 2 million years as they left the forests would not have happened without the head start that the forest primate brain gave them.
One of my earliest recollections of watching birds in an English forest, when I was a student in Oxford, is of the mixed winter flocks of tits and other insect-seeking birds in the nearby Wytham Woods. I would spend time searching what appeared to be an empty wood only to suddenly hear the call of a blue tit or a goldcrest. Soon I would be overrun by a noisy cacophony as several species of birds swept past me at canopy level, taking what they could find on the way. At the scale of the insects which were their prey, I can only imagine that this innocent-looking and quaint flock of colourful birds would have been equivalent to a mixed pack of hyenas, lions, and cheetahs flushing prey across a savannah. Many years later I observed the same winter-bird phenomenon in a mountain forest in southern Spain and a similar concert of nomads among desert birds on the island of Fuerteventura.
For a number of years now I have been lucky to go out to sea to observe marine birds in the Strait of Gibraltar. September is an excellent time because it is when the flying fish are migrating. You can spend hours searching the horizon for birds and you can travel many miles of empty sea but from time to time you hit a feeding frenzy. A pod of dolphins has found a shoal of flying fish and they are after them. The fish make their escape by darting out of the water and gliding to safety. But the seabirds that have been absent in our journey suddenly appear from all directions. Shearwaters, gannets, gulls are all around us, chasing the fish on the surface or diving after them. In a matter of minutes hundreds, even thousands, of birds are all around us making the most of the bounty (Fig. 2).
FIGURE 2. Griffon vultures feasting on a carcass, typifying the boom-or-bust world of species, including humans, dependent on patchily distributed resources in space and time.
I have also spent many days in a remote mountain in the Pyrenees, sitting inside a cold wooden hide waiting for vultures to come to a carcass. Sometimes it is a dead deer, at others a goat or a pig, and they have been left there as bait by park wardens. My aim has been to photograph the vultures. Here we can get all four European species together including the now rare lammergeyer—the ‘bone breaker’. I have spent days seeing nothing; staring at an empty place, desolate and freezing. But it has been worth it when the birds have come. As with the tits and the shearwaters you go from nothing to a frenzied mass of vultures in minutes. Once an entire deer was reduced to a skeleton in 17 minutes and then the birds were gone once more, leaving the cleaned bones as souvenirs.
These apparently disparate examples—tits, dolphins and shearwaters, and vultures—have one thing in common. At the scale at which these different predators are operating they perceive their world as patchy—they are worlds of needles in haystacks. The needles are rich gifts—insects, flying fish, carcasses, trees in fruit. When found they provide much-needed energy that permits continued existence. Many primates also exploit patchily distributed resources and many of them move around in groups. Groups hold the all-important key to finding the coveted metaphorical needles. This key comes with an expensive price tag: cooperation.
In an ideal world, an individual predator would want to take as much energy (food) out of its environment as it can, without having to share it with others. But in many situations an otherwise selfish predator is obliged to cooperate—better to find the cake and take a slice than risk not finding the cake at all. A number of factors will influence the size of a group and the degree to which its members may be prepared to defend a patch of food. The size of a feeding group will depend on the size of the patch of food. This is a simple question of physical limitation—the smaller a patch, the fewer the number of animals that can feed in it. If an area has patches of different sizes, groups may fuse or split into smaller units to take the size of patches into account. This adjustment of feeding-group size is typical of many primates. The density of food inside a patch will also determine how long a group can stay feeding in it. If the density is high enough to keep them there for a while, then the patch may even be worth defending against other groups and individuals within the group may themselves jostle for the best positions within the patch.5
There is one other—all-important—ingredient that I want to add to this soup. That is the actual number of patches within a group’s home range and how that density changes as time passes. An area may have lots of good patches in summer and fewer in winter or the number of these may change from one summer to the next, and so on. All this translates into how easy or difficult it might be to predict and find good sources of energy or other essential commodities. When we come to look at how our ancestors became increasingly terrestrial in habits and spent more and more time in open habitats, we will realize how important the density of good patches of food would have been. Importantly for the thread that I will develop, the density of water patches in an otherwise arid landscape was a vital ingredient of our story. For now, let us understand how the number of patches within an area—their density—can affect the behaviour of primates.
In primates there is a strong relationship between the amount of fruit eaten and the size of a group’s home range. This is because quality fruit patches tend to be very scattered so that primates feeding on fruit have to travel greater distances to secure food than others which feed on easier-to-find foods such as leaves. So it follows that these fruit-eaters spend considerable amounts of time and energy moving about. These patches are high premium and will be defended when other groups are encountered but very often rival groups are so thin on the ground that encounters may be quite rare. It is only when an area holds a high density of animals that aggressive interactions between groups may become a fact of life. When thinly spread on the ground, groups may still want to defend good patches but they may not be able to do so because all the trees within their large home ranges cannot be efficiently monitored at once. The idyllic image of a beautifully balanced Nature masks the underlying reality. Animals constantly face uncertainty and their environment continuously puts them to the test. Humans have been a part of this uncertain world even though we have tried hard to tame it. For now, let us stay with the thought that living in groups is another consequence, like large brains and grasping hands, of a fruit-eating life in the forests.
Living in groups, in regular contact with family, friends, and competitors, poses its own challenges. For each individual, the other members of the same group are part of its environment. If the outside world is complicated, imagine how much more tortuous it becomes when some components of that environment are like you and are constantly changing their minds or are trying to change yours. The game becomes really convoluted in this fluid world. A big brain comes in handy in these circumstances but the problem is that your friends and foes also have large brains. If investment in a life of eating high-quality, highly-dispersed foods promoted large brains and living in groups, then the consequent social life drove primates that adopted this way of life into an arms race. Bigger brains meant better interpreters and manipulators
of society, which in turn encouraged even bigger brains.
All that I have described so far would have happened within the forest. Nothing required a primate to take the first step away from trees, not even to come down from the canopy onto the ground. But we do know that one day a primate did come to ground and eventually stepped away, perhaps nervously at first, from the forest edge. There are a number of views, some strongly conflicting, about how and when all this happened. I am not going to go into that now but we will do so in later chapters. What I want to highlight here is that something happened away from the forest. It may have been a development of something that the forest ancestors had been doing already or it could have truly been a novelty. Either way, it was taken to a new level: it was the inclusion of meat in the diet.
I realize that my last sentence may have sounded somewhat banal but it is very important to our story. In later chapters we will consider how meat-eating took off; if it was gradual, sudden, if meat was scavenged or hunted, if it was a small part of an omnivorous diet or if it was, instead, a major component. That does not matter right now. What matters is that we ended up with a primate walking on two legs and regularly eating meat in an open savannah. Such a primate entered a world that had until then been the exclusive domain of the carnivores—the big cats, dogs, and the hyenas. These predators had an ancient legacy of meat-eating, an inheritance that took them back tens of millions of years to the Late Eocene and Early Oligocene.6 Our ancestors arrived very late on the savannah scene and they brought with them a vegetarian and arboreal heritage. They were interlopers, amateurs in the world of the professional carnivores. So it is quite amazing that they managed to eke out a living in this challenging environment.
The Improbable Primate Page 2