Why do we seem to have so much intelligence and curiosity compared to other creatures? Why are you avidly reading this book, while a wildebeest wouldn't even give it a second glance? Why are we so different from even our very closest animal relatives, the chimpanzees, bonobos and gorillas? As part of our effort to answer these questions let's look more closely at those chimpanzees, bonobos and gorillas, and see exactly how we're related to them.
Figure 71 shows our branch of the family tree of all living things: the tree of life. Our branch contains all of the monkeys and other apes: all of the creatures that are in the order known as the primates.
Here's a quick rundown (or climb up) our part of the tree.
The tree grows through time. At the very bottom of the primate branch, over 50 million years ago, is the common ancestor of all of the primates. Each fork of the tree above this point shows where different groups of primates branched off from this primogenitor to follow different evolutionary routes, resulting in different species. The lowest fork shows when primates such as lemurs and tarsiers (collectively known as prosimians, or pre-apes) split off on their own evolutionary branch about 50 million years ago, evolving in a separate direction to the rest of the primates. Similarly, the second branch shows when the monkeys of the New World split off. These American monkeys branched off from the remaining primates on their own evolutionary route quite a long time ago, so they differ in a number of ways from Old World monkeys and the other higher primates - for instance they don't have opposable thumbs (which the Old World monkeys and the great apes have), they are less intelligent, and they have prehensile tails that they can wrap round branches and use as aids to climbing and dangling. The tails of Old World monkeys lack this useful ability, and the great apes don't have tails at all.
You can see human beings perched on the branch at the far right of the tree, close to our nearest relatives, the chimpanzees, bonobos and gorillas, with the orang-utans a little further away.
This diagram shows all of the creatures perched on the ends of branches of the family tree, and indeed all of the species depicted spend a great deal of their lives perched in the branches of real trees. Except for the humans.
We moved out of the trees about five million years ago.
(Perhaps one of the last vestiges of our arboreally-centred past is manifested in the way that even in the modern world small children feel compelled to climb trees. This activity is suddenly abandoned in later childhood when they decide that it's ludicrously infantile and they adopt more sophisticated pastimes such as standing around on street corners. I wonder if any sociological studies have ever been done to test whether or not the lack of tree-climbing opportunities for modern youth has a detrimental effect on the children's development. It may explain a lot.) Before I deal with how our ancestors fared when they first left the trees all of those millions of years ago, let's have a look at how our cousins, the chimpanzees and bonobos, are doing right now, today.
Chimps are found in the forests of west and central Africa, living in communities of between about ten and a hundred individuals. In any particular community all of the males will be related, while the adult females will have joined the community by leaving nearby communities that they were born into.
Once chimps have eaten the food they need, and have avoided any ground-lurking predators by climbing to inaccessible spots in their trees, they'll often be found in small groups sitting back and doing nothing very much at all.
Any humans who sat around as much as a chimp does would be strongly disapproved of.
Chimps don't sit back doing nothing all of the time though. The male members of a community will sometimes suddenly all assemble together on the ground and then walk away from their community in a purposeful line. Their mission is to actively patrol their territory in a gang, and to attack (and sometimes kill) members of other chimp communities that they encounter. Again, if humans acted like that we'd strongly disapprove.
The chimp's closest relatives, the bonobos, act rather differently. These primates are so closely related to the chimpanzees that until recently they were thought to be a subgroup of chimps that just happened to be slightly smaller than the average chimp. They used to be called pigmy chimpanzees as a result. They weren't given the status of being their own species until well into the first half of the twentieth century. They live on the south bank of the River Congo in the Democratic Republic of Congo (Figure 72).
In recent years studies of the bonobos have found that although they look very much like small chimps they don't act very much like them.
Specifically, they don't indulge in attacks on other groups of their own species that live nearby. Generally they actually avoid contact with neighbouring groups, but when they do by chance bump into each other they indulge in a bit of mutual grooming rather than fighting.
They also don't exhibit much aggression within their own group, unlike the chimps. Indeed the bonobos are famous for the fact that they channel most of their energy into promiscuous sexual activity at the slightest excuse. They are a species for which the expression "Make love, not war" could have been invented. They generally seem to be too good to be true (apart from possibly being somewhat over-sexed) and are in great danger of being over-romanticised by sentimental westerners in search of the perfect primate.
Why is there such a difference in behaviour between the chimps and the bonobos? After all, they are very close relatives, and if you look on the map you'll see that they are very close neighbours too.
Looking at the map will also give you a clue as to why they are different. Significantly, although they are neighbours, the bonobos live on the south side of a very large river, the River Congo, while the chimps live on the north side. The theory goes that, as luck would have it, the bonobos live on the side of the river where food is extremely plentiful, so there's little competition for it. As a result there's just no need to fight. Meanwhile, the less fortunate chimps on the north bank of the river live in far from ideal circumstances, with significant competition for food and territory: hence their need to be more aggressive in defending what they've got.
Quite why the hard-pressed chimps don't cross the river and move into bonobo territory I'm not sure. Maybe the river really is very wide indeed. Or maybe the theory's wrong.
(It'd be tempting to say that the bonobos have an almost perfect existence. Unfortunately this is no longer true. In recent years human incursion into their territories has significantly reduced the number of bonobo groups, and unless something is done about it very soon they will become extinct.) That's a quick run-down of what the chimps and the bonobos are doing at the moment. I think that it's safe to say that they're living in ways that are very similar to the ways that their and our common ancestors did millions of years ago, at the time immediately before humans set off to climb their own separate branch of the evolutionary tree.
Since the days of those common ancestors we humans have moved on. We aren't living in the trees any more - in fact we're now busily engaged in chopping them down rather than climbing them. That's progress for you.
Why have we progressed so far, while our ape cousins still live in the trees? The answer is generally assumed to lie in the fact that we, alone amongst the primates, left the trees in the first place, about five million years ago, never to return to them.
But why did we move away from the trees? It seems like a strange thing to do, as the branches of trees were, amongst other things, a safe haven from the predators that generally roamed around on the ground. Why didn't we just stay up there in the branches, out of harms way? Did we jump out of the branches, or were we pushed? It's quite likely that we were pushed - after all, what's the point of spending millions of years becoming highly adapted to life in the trees only to go and abandon them for a life on the ground? It's a common conception that when our ancestors left the trees they wandered straight out into the open grasslands of the African savannah. However, there's evidence that they may have actually lived for a considerable period of time on the ground within the forests themselves before venturing out onto the plains. This is deduced from the fact that the remains of forest animals have been found accompanying early hominid skeletons. It's not surprising that our tree-dwelling ancestors may have spent considerable amounts of time on the ground - after all, most modern apes such as chimps and gorillas spend a certain amount of time there themselves. It's quite possible that our ancestors simply spent more time on the ground than other apes, thus becoming reasonably proficient at living there.
The reason that we eventually abandoned the trees for good however is unknown.
One possible reason for our abandonment of the arboreal lifestyle is that in the region where our ancestors lived there was a sudden decrease in the number of trees, possibly due to a period of climate change..
It's also possible that we were elbowed out of the trees by some of our close relatives, perhaps the antecedents of the chimpanzees and bonobos, who may have been more adept in the climbing department and/or been more aggressive than we were, so they just muscled us out by monopolizing the best trees. (Because our ancestors had started to spend more time on terra firma than the rest of the apes they may have lost some tree climbing skill or may have just become accustomed to being on the ground - making elbowing them out a relatively easy task.) Pressure from chimpanzee and bonobo ancestors would be especially significant in a world of ever decreasing tree availability, due to the increased competition for trees. But our ancestors may have been elbowed out even if there were no shortage of trees. If our ancestors and the chimp/bonobo ancestors lived in geographically distinct regions (perhaps separated by a wide river as in the case of the modern day chimps and bonobos) all that would be needed would be for chimp/bonobo ancestors to gain a foothold in the geographical area that sustained our ancestors, allowing them to spread due to their superior skills or greater aggression, forcing our ancestors out.
This scenario of a more adept/more aggressive species forcing another species out of a habitat is a dynamic that has been repeated over and over again since the dawn of life. It still goes on today, often facilitated by the agency of human activity.
It is how, in Britain, Ireland and parts of Italy, the grey squirrel - a rodent introduced from America - has forced the native red squirrel to the margins of the woodlands and (especially in Britain) to the margins of existence. Similarly, as I write this very paragraph there is a ladybird sitting on the wall of my room here in London: it is a harlequin ladybird, a species unknown in Britain until recently, but one which is currently in the process of pushing the less aggressive native two spot ladybird out of its habitat. In a few years time the two spot may be a rarity in Britain.
Most species that are forced to the periphery of their natural habitat can't adapt and either survive in depleted numbers on the margins or become extinct. Fortunately for us, when our ancestors jumped or were pushed out of the trees they found that they were just about capable of surviving, due to their relatively versatile anatomy and psychological makeup that had evolved during their time in the branches. (To understand how difficult it is for most species to adapt to a new environment, try imagining how some species would have coped if they had been forced in the opposite direction to the one that we took - off the grassland and into the branches of the trees. How would a zebra cope in the treetops for instance?) Despite all this talk of our ancestors being forced out of the trees due to aggressive relatives or depletion of forests, it is vaguely possible that they simply left the trees of their own volition. It's possible that something in their psychological makeup made them particularly restless and dissatisfied, and made them want to go and explore pastures new. We still do that today. That's why we're busy planning trips to Mars at this very moment. Mars may be the new savannah.
Whatever the reason that we left the trees and moved out into the plains, our ancestors ended up on the open grassland while the ancestors of the chimps and bonobos remained in the place that they were perfectly adapted to - the trees.
And they've been sitting in those trees ever since.
Lucky chimps and bonobos.
And poor us.