The Science of Discworld Revised Edition

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 savannahs, surrounded by lots of big predators – lions, leopards, hyenas – and without much cover. We had to become smart in order to survive. Rincewind would instantly see one flaw in this theory: ‘If we were so smart, why did we
stay
on the savannahs, surrounded by lots of big predators?’ But, as we’ve said, it fits the fossil evidence. The unorthodox theory addresses the ‘how’. Big brains need lots of brain cells, and brain cells need lots of chemicals known as ‘essential fatty acids’. We have to get these from our food – we can’t build them ourselves from anything simpler – and they’re in short supply out on the savannahs. However, as Michael Crawford and David Marsh pointed out in 1991, they are abundant in seafood.
    Nine years earlier Elaine Morgan had developed Alister Hardy’s theory of the ‘aquatic ape’: we evolved not on the savannahs, but on the seashore. The theory fits a number of human peculiarities: we like water (newborn babies can swim), we have a funny pattern of hair on our bodies, and we walk upright. Go to any Mediterranean resort and you see at once that an awful lot of naked apes think that the seashore is
the
place to hang out.
    Whether the Aquatic Ape story of human origins will displace the ‘savannah’ theory remains to be seen, but the savannah story is in trouble from a very different direction. Phillip Tobias has challenged not the fossil record, but its interpretation. He asked a