The Science of Discworld Revised Edition

changed. So in the mid-Eocene we find the earliest fossils of whales, such as the sixty-foot (20 m) long
Basilosaurus
, which had a pair of tiny legs at the base of its long tail. We’ve found fossils of its ancestors, and they really did look like small dogs.
    The Mediterranean sea was dammed, Africa came into contact with Europe, and creatures previously confined to Africa spread into Europe, among them elephants – and apes. Horses evolved, as did true cats (such as the famous sabre-toothed tiger). By five million years ago, most of today’s mammals were represented in recognizable form, and the climate had become similar to today’s.
    The scene was set for the evolution of humans.
    Not that it had all been set up
in order
to lead to us, you appreciate. Our early ancestors just happened to be in a position to take advantage of the world as it then was. They did so.
    We can trace the ancestry of modern mammals – indeed all living creatures that still exist today – by mapping out changes in their DNA. The rate at which DNA
mutates
– acquires random errors in its code – leads to a ‘DNA clock’ that can be used to estimate the timing of past events. When this technique was first discovered, it was widely hailed as a precise and therefore uncontroversial way to resolve difficult questions about which animals’ ancestors were more closely related to what. It is now becoming clear that precision alone cannot provide definitive answers to such questions.
    The issue of interpretation – what does this result
mean
? – can still be controversial, even if the result itself can be made precise. For example, S. Blair Hedges and Sudhir Kumar have applied the DNA clock to 658 genes in 207 species of modern vertebrates: rhinos, elephants, rabbits, and so on. Their results suggest that many of these lineages were around at least 100 million years ago, coexisting with the dinosaurs – though no doubt the early elephant and rhino ancestors were rather small. The fossil record agrees that there were mammals then – but not those. The molecular biologists claim that the fossil record must be misleading; palaeontologists are convinced that the DNA clock sometimes ticks faster and sometimes ticks slower. The debate continues – but for what it’s worth, our money is on the palaeontologists.
    One big surprise about mammal DNA is how much of it there is. You might expect a sophisticated creature like a mammal to be ‘hard to build’ and therefore require more DNA, just as the blueprint for a jumbo jet has to be more complicated than that for a kite.
    Not so.
    Mammals have
less
DNA – shorter genomes – than many apparently simpler animals, for example frogs and newts.
    There’s a good reason for this apparent paradox, and it illuminates the difference between DNA and a blueprint. DNA is more like a recipe – and a recipe that makes a lot of assumptions about what else you have in your kitchen, so that none of that needs to be spelled out in the recipe book. In essence, the kitchen for mammals has a really well controlled oven, capable of ensuring nice, even cooking temperature, so a whole lot of tricks about what to do if the temperature changes need not be mentioned. 2 In the frog kitchen, on the other hand, the temperature goes up and down depending on the time of day and the weather, so the recipe has to deal with all contingencies, requiring more DNA code. By ‘kitchen’ here we mean the environment in which the embryonic animal has to develop. For a frog, the kitchen is a pond. For a mammal, the kitchen is mother.
    Mammals evolved good temperature control – unlike the reptiles, they are warm-blooded, but what matters is not so much being
warm
, as being controllable. Frog DNA is full of genes for making lots of different enzymes, together with instructions along the lines of ‘use enzyme A if the temperature is lower than 6°C, use B if the temperature is between 7°C and 11°C, use C if the temperature is between 12°C and 15°C …’ Mammal DNA just says ‘Use enzyme X’, knowing that mother will take care of temperature variations. Frog DNA is a rocket: mammal DNA is a space elevator.
    How did this change take place? Perhaps when mammals first evolved, their DNA gained extra instructions, but after temperature control evolved, a lot of the DNA became redundant, and it either got dumped or got subverted to other uses. On the other hand, we have no idea what the DNA of early mammals actually