Life changes: To understand how organisms evolve, biologists start at the beginning: The San Diego Union-Tribune
Dances with fruit flies: US News and World Report
Evo Devo is the new buzzword for the 200 year old search for links between embryos and evolution: Scientific American
Evolution Revolution: Toronto Globe and Daily Mail
Evolution today is highly controversial, particularly in the United States, where there are ongoing battles about whether it should be taught in schools. Yet, when Charles Darwin published his Origin of Species in 1859, very many people in Britain, in Europe and even on this side of the Atlantic at once agreed that all living organisms, including us humans, are the end results of a long, slow process of natural development, from very simple forms, perhaps even originally from inorganic substances. A major reason why Darwin was so effective was that he showed that the idea of evolution explains so many different things: paleontology, geographical distributions, anatomy and more.
One area covered by Darwin was embryology, the development of the individual organism from conception to adulthood. Organisms are basically alike in their earliest stages and then diverge along different pathways. Darwin was always very proud that he could show that this is all a matter of evolution, especially evolution by his own mechanism of natural selection, where only the fittest survive and pass on their features to subsequent generations. Darwin's point was that the earliest stages of development show shared origins, and adult diversity occurs -- and only occurs -- because natural selection works on organisms to fit them for different adult conditions and environments and lifestyles.
After Darwin, embryology almost took over evolutionary studies, but not in the way he intended. Drawing on ideas dating back to the poet and polymath Goethe, biologists ignored natural selection and used embryology to work out individual histories. German scientist Ernst Haeckel argued that the history of the individual reveals the history of the race -- "Ontogeny recapitulates phylogeny" -- and in the subsequent spinning of stories of origins, Darwin's own contribution to embryology was quite forgotten. More unfortunate was the fact that the foundations of the new evolutionary embryology were desperately insecure, for too often the history of the race is not revealed by the history of the individual.
Hence, it is not surprising that when, in the 1930s, evolutionists rediscovered natural selection, combining it with the new theory of heredity -- then Mendelian genetics, later molecular genetics -- almost to a person they rejected or ignored embryology. The great theorists of the day -- Ronald Fisher in England and Sewall Wright in the United States -- treated organisms as black boxes. They were interested in the genes; they were interested in the fully fledged organisms; and they were not interested in much in between. One has a gene for brown eyes, one has a Guernsey cow, and no one cared too much about the processes taking life from the one level to the other.
Of course, there had to be something wrong or incomplete here. The great evolutionist Ernst Mayr, who died earlier this year at the magnificent age of 100, used to speak contemptuously of "beanbag genetics." He implied that simply treating the units of heredity as isolated entities separate from the whole living being had to be wrong. Darwin was right. Any adequate theory of evolution simply has to consider the whole organism, especially the developing whole organism.
About 25 years ago, there was a veritable revolution, and embryology came rushing back into the evolutionary story. By then, the molecular biologists had done their easy bits -- working out the double helix, the genetic code and those sorts of things -- and were now ready to tackle the really hard jobs. These included development, and overnight one had the beginning and growth of a whole new sub-discipline, looking at embryology and then trying to relate it to the evolution of organisms: evolutionary development, or as it is more popularly known, "evo devo."
As explained in Endless Forms Most Beautiful, by U.S. geneticist Sean Carroll, one of the leaders of this new field, immediately there were very exciting discoveries. Darwin himself had always argued that the way of evolution is rarely to build the entirely new, but to take the already existing and modify and reuse. The forelimb of vertebrates is a classic example. Don't build a whole new structure for flying. Take what you have -- something used for walking and grasping -- and shape it accordingly.
What no one realized is the extent to which this would be going on at the genetic level. Who would dream that there could be something significantly in common between humans and the little fruit fly, Drosophila? But it turns out that there is. Basically, what we now know is that organisms are built on a modular fashion. It is not a question of everything being done at once. Rather, bits and pieces are built, and if you want something more, then you repeat one of the pieces that you have and then modify it. Or you drop one piece and add another kind of piece and so forth. Snakes are obviously built this way -- think of all of those vertebrae -- but the same is true of other organisms. Start with a pair of legs, double them, turn one set into wings -- good idea -- get another set, and turn those into wings as well. And so it goes.
But how does it go? It turns out that the answer is at the level of the genes, but not just any genes. Some genes produce substances (proteins) that do not make things, but control the making of things by other genes. They turn on (or off) the DNA. This is the secret to repeating things -- double your DNA and your switches -- and to transforming things -- alter the order of switches or their operating, and you have your desired end product. It is these controlling genes -- what Carroll calls the "genetic toolkit" -- that are the secret to building an organism or of transforming one organism into another kind of organism.
What was totally unexpected and incredibly exciting is that these switch genes are the same in animal after animal. Humans and fruit flies share virtually identical genes doing exactly the same things. Some of the best studied are so called "Hox genes." These order development in the fruit fly, and explain why one part comes before another and why one part (say a wing) comes in the wing position and not in the position of another part (say a leg). There are almost exactly the same genes doing almost exactly the same things in humans.
The revolution is not yet over. Carroll and his fellow workers are committed Darwinians. They think that the Darwinian mechanism of natural selection is the ultimate force -- the sculptor of form -- but there is not yet a full picture of how selection works to preserve and multiply and order the switch genes. In Endless Forms Most Beautiful -- a phrase, incidentally, taken from the closing paragraph of the Origin -- the major emphasis is on following the developments of individual organisms: butterflies getting spots, zebras getting stripes and humans getting brains. There is no linking vision, like that of the Origin, bringing in paleontology, biogeography, anatomy and everything else. Building this big picture lies in the future.
Sean Carroll is a scientist who loves his work and who wants to share his joy of discovery. His very existence is a major reason why creationism is so wrong, so sterile. Science like this is the proof that, although we may be modified monkeys, the Christians were not entirely wrong when they argued that our intelligence and our imagination is the proof that we are made in the image of God.
Michael Ruse daily thanks his maker that he is a philosopher and not as other men. However, this book almost makes him wish that he had started his academic career as a biologist, around 1980.
Book Review: Endless Forms Most Beautiful: Discover