Natural Selection equips every living creature with the genes that enabled its ancestors—a literally unbroken line of them—to survive in their environments. To the extent that present environments resemble those of the ancestors, to that extent is a modern animal well equipped to survive and pass on the same genes. The ‘adaptations’ of an animal, its anatomical details, instincts and internal biochemistry, are a series of keys that exquisitely fit the locks that constituted its ancestral environments.
Given a key, you can reconstruct the lock that it fits. Given an animal, you should be able to reconstruct the environments in which its ancestors survived. A knowledgeable zoologist, handed a previously unknown animal, can reconstruct some of the locks that its keys are equipped to open. Many of these are obvious. Webbed feet indicate an aquatic way of life. Camouflaged animals literally carry on their backs a picture of the environments in which their ancestors evaded predation.
But most of the keys that an animal brandishes are not obvious on the surface. Many are buried in cellular chemistry. All of them are, in a sense which is harder to decipher, also buried in the genome. If only we could read the genome in the appropriate way, it would be a kind of negative imprint of ancient worlds, a description of the ancestral environments of the species: the Genetic Book of the Dead.
Naturally the book’s contents will be weighted in favour of recent ancestral environments. The book of a camel’s genome describes recent millennia in deserts. But in there too must be descriptions of Devonian seas from before the mammals’ remote ancestors crawled out on the land. The genetic book of a giant tortoise most vividly portrays the Galapagos island habitat of its recent ancestors; before that the South American mainland where its smaller ancestors thrived. But we know that all modern land tortoises descend earlier from marine turtles, so our Galapagos tortoise’s genetic book will describe somewhat older marine scenes. But those marine ancestral turtles were themselves descended from much older, Triassic, land tortoises. And, like all tetrapods, those Triassic tortoises themselves were descended from fish. So the genetic book of our Galapagos giant is a bewildering palimpsest of water, overlain by land, overlain by water, overlain by land.
How shall we read the Genetic Book of the Dead? I don’t know, and that is one reason for coining the phrase: to stimulate others to come up with a methodology. I have a sort of dim inkling of a plan. For simplicity of illustration, I’ll stick to mammals. Gather together a list of mammals who live in water and make them as taxonomically diverse as possible: whales, dugongs, seals, water shrews, otters, yapoks. Now make a similar list of mammals that live in deserts: camels, desert foxes, jerboas etc. Another list of taxonomically diverse mammals who live up trees: monkeys, squirrels, koalas, sugar gliders. Another list of mammals that live underground: moles, marsupial moles, golden moles, mole rats. Now borrow from the statistical techniques of the numerical taxonomists, but use them in a kind of upside-down way. Take specimens of all those lists of mammals and measure as many features as possible, morphological, biochemical and genetic. Now feed all the measurements into the computer and ask it (here’s where I get really vague and ask mathematicians for help) to find features that all the aquatic animals have in common, features that all the desert animals have in common, and so on. Some of these will be obvious, like webbed feet. Others will be non-obvious, and that is why the exercise is worth doing. The most interesting of the non-obvious features will be in the genes. And they will enable us to read the Genetic Book of the Dead.
In addition to telling us about ancestral environments, the Genetic Book of the Dead can reveal other aspects of history. Demography, for instance. Coalescence analysis performed on my personal genome by my co-author (and ex-student) Yan Wong has revealed that the population from which I spring suffered a major bottleneck, probably corresponding to an out of Africa migration event, some 60,000 years ago. Yan’s analysis may be the only occasion when one co-author of a book has made detailed historical inferences by reading the Genetic Book of the other co-author.