Root and Branch
The tree of life is one of our most ancient metaphors, recurring and profound. There it is in Eden, firmly planted in Genesis 2:9. It was initially free for all, unlike the infamous tree of knowledge, which grew beside it. Much earlier it was carved in stone on Assyrian monuments. The menorah is a tree with seven branches. The cross itself is a tree of life: Made from the wood of dead trees, it became the symbol of eternal life.
We have been working out uses for tree pictures forever. Family relationships were represented by trees in the sixth century. Genealogical trees had to wait another 300 years. But then the Tree of Jesse, showing the ancestry of Christ, grew into vivid displays on medieval glass and illustrated texts. In the eighteenth century Linnaeus and his fellow naturalists classified species and genera of plants in a hierarchical way that can lend itself to representation by a tree. Nineteenth-century Victorians were obsessed with family trees, perhaps because the prosperous ones were worried that their interlocking marriages made their pedigrees all too close to incestuous. One of those gentlemen, Charles Darwin, put it all together: "All true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking."
A genealogical tree of life thereby became the symbol of the theory of evolution by natural selection. We used to rely on the shapes of organisms and their parts in order to construct an ancestral tree. We now use molecular genetics to trace relations. Four years ago Richard Dawkins, Darwin's eloquent though abrasive champion, announced that by 2050 we should have completed "the one true tree of life, the unique pattern of evolutionary branchings that actually happened." That is exactly what sticks in the craw of all those who doubt the theories that descend from Darwin.
I myself take the tree of life in Genesis as a wonderful intimation of things to come. Like all the ancient commentators, I read it as allegory. Those who read it as revealed truth should see that it laid out the road to follow, right from the start, in human speculation about the mysteries and miracles of life. Those who think that Genesis is just another old book should marvel that its authors got it right, in the very beginning, planting the tree of life in the human mind.
Not surprisingly, the central chapter of Kitcher's book is "One Tree of Life." He does an exemplary job of showing how apt the metaphor is, as a way of representing knowledge about the origin of the species. Nevertheless, it is useful to reflect on difficulties in the present, rather than successes in the past. They arise precisely because the evolutionary program is "progressive" in Lakatos's sense. Anti-Darwinists love to repeat news of difficulties. They say, "We told you so; it is just a bunch of guesswork." Hence defenders of the faith, like Kitcher, do not like to dwell on present problems, for fear of giving succor to the foe.
I wonder if they should not instead celebrate the difficulties, making plain that evolutionary theory is a living, growing, vital organism, while anti-Darwinism is lifeless, if not, in Kitcher's word, dead. In my opinion the arrogant religion-baiters--yes, Richard Dawkins comes to mind, but others are worse--do a disservice to their cause by making evolutionary theory seem so cut and dried (viz. dead), when it is a blooming, buzzing, confusing delight, finding out more about the world every day. With anti-Darwinians fabricating a "controversy," it helps to see what a real scientific controversy is like, with each competing conjecture piling on new research methods, new explanations, new questions, new failures and new successes.
It is, for example, splendidly difficult to draw a definitive tree of life. I shall mention just two open questions. First, man and monkey. Kitcher well explains that at the species level, chimpanzees are the closest, genetically, to humans. But it is not wholly clear how to put gorillas, chimpanzees and humans on a tree. On balance, at present, the most probable tree has species X splitting into gorillas on one branch and species Y on another; species Y splits into chimps and people. But some molecular evidence suggests that X splits into Y and chimps, with Y splitting into gorillas and people. Another tree suggested by other molecular evidence is X evolving into Y and humans, with Y evolving into gorillas and chimps. A paper in Nature in 2006 proposed an unusual resolution to the conundrum: Early on, there was a lot of hybridization between humans and chimpanzees!