Страница 72 из 83
ANTHILL INSIDE
You KNOW WHAT'S GOING TO HAPPEN TO THE APES -they're going to turn into us. But why do we have them playing in the surf? Because it's fun? Yes ... but more significantly, because the seashore is central to one of the two main theories about how our ape ancestors acquired big brains. The other, more orthodox theory places the evolution of the big brain out on the African sava
Our closest living relatives are two species of chimpanzee: the standard boisterous 'zoo' chimp Pan troglodytes and its more slender cousin the bonobo (or pygmy) chimp Pan paniscus. Bonobos live in very inaccessible parts of Zaire, and weren't recognized as a separate species of chimpanzee until 1929. We can to some extent unravel the past evolutionary history of the great apes by comparing their DNA sequences. Human DNA differs from the DNA of either chimpanzee by a mere 1.6%, that is, we have 98.4% of our DNA sequences in common with theirs. (It is interesting to speculate on what the Victorians would have made of this.) The two species of chimpanzee have DNA that differs by only 0.7%. Gorillas differ from us, and from both chimps, by 2.3%. For orangutans, the difference from us is 3.6%.
These differences may seem small, but you can pack an awful lot into a small percentage of an ape genome. A big chunk of what we have in common must surely consist of 'subroutines' that organize basic features of vertebrate and mammalian architecture, tell us how to be an ape, and tell us how to deal with things we've all got -like hair, fingers, internal organs, blood ... The mistake is to imagine that everything that makes us human and not a chimpanzee must live in that other 1.6% of 'special' DNA, but DNA doesn't work that way. For example, some of the genes in that 1.6% of the genome may organize the other 98.4% in a completely new way. If you look at the computer code for a wordprocessor and a spreadsheet, you'll find they have an awful lot in common, routines for reading the keyboard, printing to the screen, searching for a given text string, changing fonts to italic, responding to a click on the mouse ... but this doesn't mean that the only distinction between a spreadsheet and a wordprocessor lies in the relatively few routines that are different.
Since evolution involves changes to DNA, we can use the sizes of those differences to estimate when various ape species diverged from each other. This method was introduced by Charles Sibley and Jon Ahlquist in 1973, and while it needs to be interpreted with caution, it works well here.
A convenient unit of time for such discussions is the 'Grandfather', which we define to be 50 years. It's a good human length, being about the age difference between the child and the grandparent who says 'When I was young ...' and passes on a sense of history. In these terms, Christ lived 40 Grandfathers ago, and the Babylonians go back about 100 Grandfathers. That's not a lot of grandads, passing down through recorded human history recollections like '... we never had any of this modern cuneiform when I was a lad ...' and'... bronze was good enough for me'. Human time is not very deep. We've just been good at packing a lot into it.
DNA studies indicate that the two chimp species diverged about 60,000 Grandfathers ago, three million years. Humans and chimps diverged 80,000 Grandfathers earlier, so a chain of only 140,000 grandfathers unites you and your chimplike ancestor. Who was also, we hasten to point out, a modern chimpanzee's manlike ancestor. Humans and gorillas diverged 200,000 Grandfathers ago; humans and orangutans diverged 300,000 Grandfathers ago. So among these animals, we are most closely related to a chimpanzee, and least closely related to the orangutan. This conclusion is borne out by physical appearance and habits, too. Bonobos really like sex.
If those times seem rather short for all the necessary evolutionary changes, bear two things in mind. First, that they were estimated by using a realistic rate for DNA mutations; second, that according to Nilsson and Pelger an entire eye can evolve in a mere 8,000 Grandfathers, and lots of different changes can, should, and did evolve in parallel.
The most striking feature of humans is the size of our brains: bigger, in comparison to body weight, than any other animal. Strikingly bigger. A detailed story of what makes us human must be extraordinarily complicated, but it's clear that big, powerful brains were the main invention that made it all possible. So we now have two obvious questions to think about: 'Why did we evolve big brains?' and 'How did we evolve big brains?'
The standard theory addresses the 'why'. It maintains that we evolved out on the sava
Nine years earlier Elaine Morgan had developed Alister Hardy's theory of the 'aquatic ape': we evolved not on the sava
Brains are fascinating. They are the physical vehicle for minds, which are even more interesting. Minds are (or, at least, give their owners the vivid impression that they are) conscious, and they have (or, at least, give their owners the vivid impression that they have) free will Minds operate in a world of 'qualia', vivid sense impressions like red, hot, sexy. Qualia aren't abstractions: they are 'feelings'. We all know what it's like to experience them. Science has no idea what makes them the way they are.
Brains, though ... we can make progress on brains. On one level, brains are a kind of computational device. Their most obvious physical components are nerve cells, arranged in complicated networks. Mathematicians have studied such networks, and they find that what networks do is to carry out interesting processes. Give them an input and they will produce an output. Allow their interco
What makes the human brain unique, as far as we can tell, is that it has become recursive. As well as detecting a banana, it can think about detecting a banana. It can think thoughts about its own thought processes. It is a pattern-recognition device that has turned its attention to its own patterns. This ability is what lies behind human intelligence. It probably underpins consciousness, too: one of the patterns that the pattern-recognition device has learned to recognize is itself. It has become 'self-aware'.