Throughout most of the time that life has existed on Earth, the only inhabitants have been bacteria. The arising of complex multicellular organisms, including ourselves, is a major puzzle; there seems to be no inevitable trend towards complexity. Life appears to have begun as soon as conditions on earth permitted it to do so, and is therefore a very probable event, but complex life took much longer to evolve and the first vertebrates (fish) are not found until 500 million years ago. The development of complex organisms is therefore improbable, and although life may be widespread in the Universe, complex life is likely to be rare and intelligent life even more so. In this book Ridley, who is a leading evolutionist and the author of one of the standard textbooks on evolution, considers how and why complexity may have evolved.
In part, the explanation may be ecological: complex life has evolved because ecological opportunities existed for it. But Ridley doesn't think this is the whole story. In order to take advantage of new ecological opportunities organisms need the ability to change, and Ridley connects this with genetic complexity; more genes equals more possibilities. Simple bacteria have 1000 to 5000 genes; single eukaryotic cells have up to 15,000 genes, and simple multicellular organisms have more. However, life appears to have been stuck at about 60,000 genes for the last 500 million years. What determines the possible evolution of complexity, Ridley says, is the rate at which copying errors accumulate, and this question is what mainly concerns him throughout the book.
Copying errors may not necessarily always be a bad thing for organisms, of course, since they allow for variation. This is particularly true of RNA viruses, such as the influenza virus and HIV, which are among the simplest life forms on Earth and may have error rates like those of early life forms. But even here, things may not be what they seem. We are generally told that the high rate of mutation of HIV gives it an advantage, because it can evolve faster than its host can adapt to deal with it; but Ridley thinks that this may really be the Achilles' heel of the virus and may even offer us a way of attacking it, by artificially increasing its mutation rate until it breaks down under its own copying errors. This is just one of the fascinating ideas that he throws off in passing.
There are other potential sources of trouble in addition to copying errors, notably selfish genes. A selfish gene is one that has some attribute enabling it to copy itself better than some alternative gene. Such genes are important in organelles such as mitochondria, which are bacteria whose incorporation into the cell led to the development of the eukaryotic cell. This was a major step in evolution, but it was attended with hidden costs. Accumulation of errors in the mitochondria is thought to be an important cause of aging. In addition, the potential for conflict during reproduction exists between maternal and paternal organelle genes, and between nuclear genes and organelle genes. But we have evolved a means of controlling this conflict; male organelle genes are not transmitted by the sperm to the ovum.
This brings us to the much-debated question of why sex exists. One popular theory is that it gives organisms an advantage in the arms race against parasites, and Ridley acknowledges that this is a plausible alternative to the one he advocates, which is that sex exists in order to purge bad genes. (Actually, I don't see why both theories might not be true.) Selfish organelle genes are only one part of the problem faced by complex organisms; the nuclear genes are involved as well. For example, trisomy (two chromosomes from one parent and one from the other) are a cause of Down's syndrome, and many miscarriages are due to trisomies. We usually think of these as accidents, but Ridley suggests that they may be due to what he calls "law-breaking chromosomes": that is, chromosomes that cheat by being inherited in more than 50 per cent of offspring. Such events do occur but they are the exception, not the rule.
A similar case of law-breaking may explain the huge production of an anti-insulin hormone by the human placenta, which raises the mother's blood sugar level, producing a diabetes-like effect. The existence of this hormone may be due to a past conflict between maternal and fetal genes. A fetus containing a gene for the hormone would grow up better nourished and more likely to survive, but this would be harmful to its siblings because the mother's total reproduction would be diminished. Mothers would then react by evolving means to restore their reproductive ability. The present state of relative balance is thus, on this theory, the outcome of millions of years of genetic conflict; an idea which Ridley describes as one of the weirdest in the modern theory of evolution.
Sexual reproduction, including the curious process of meiosis, in which the number of chromosomes in the gametes is halved, is, according to Ridley, a means whereby copying errors are controlled, potential conflicts are resolved, and information is randomized effectively enough to allow the evolution of complex organisms such as ourselves. This is Mendel's demon (the expression derives from Maxwell's demon, meaning something that biases random processes in a particular direction); Mendel's demon has shifted evolution in the direction of complexity. Meiosis increases the uncertainty about which other gametes, or offspring, contain copies of a gene, and this stops, or almost stops, the evolution of the most damaging forms of selfish genes.
All these ideas are discussed in considerable depth in the book, and due weight is given to counter-arguments and to uncertainties. There is a change of tone in the concluding chapters, where Ridley lets his imagination rip, as he speculates on the possible evolution, here or elsewhere, of organisms with far more genes than our 100,000. Could there be organisms with 200,000, or 2 million, genes, and what would they be like? They might have better anti-error techniques than we do, and this might offer fascinating possibilities. For example, a creature with 150,000 genes might be able to choose among four different developmental possibilities, according to the environment. The embryo could metamorphose into a lobster, an oak tree, or a winged form, switching on and off different sets of genes as required.
This is a rich and fascinating book, bubbling with ideas. And Ridley is a lively writer, with a delightfully humorous turn of phrase. I particularly liked the image of the mitochondrial genes in the ovum cheering as they see an X-bearing sperm swimming towards them and booing and making rude signs when they see a Y-bearing sperm; there are many asides of this kind. Nevertheless, this isn't an elementary introduction to evolution theory, and non-specialists will have to concentrate to absorb the ideas. A glossary is provided, which is useful, but the elementary nature of some of the terms it defines (placenta, hormone, arthropod) seems to reflect a degree of uncertainty about who is expected to read the book. Anyone who needs to be told what the placenta is will probably find Ridley's arguments pretty hard going. As an up-to-date review of one of the deepest questions in evolution, however, it would be hard to beat.