The Web of Life
IN RETRACING THE HISTORY of life on Earth (see figure 23.1), we have looked mostly at the core structure of the tree, the line traced by the successive appearance of living forms of rising complexity. But each major step in this progression has also produced side branches that extend their ramifications to the present day. The history of life is not just vertical growth in the direction of complexity; it is also horizontal expansion in the direction of diversity. Each cross section of the tree becomes more varied with advancing time, recapitulating the tree's previous history by means of what were the terminal twigs of the branches at the time considered. A cross section at three billion years ago would show two sturdy, moderately diversified clusters bearing the archaebacteria and eubacteria existing at that time, almost hiding a tiny, isolated bud, which no observer could have suspected would one day turn into the massive eukaryotic trunk. A cross section at 400 million years ago would show a diversity of bacteria of both types, many kinds of protists, an abundance of algae, some primitive mosses and fungi, a variety of sponges, coelenterates, worms, mollusks, arthropods, and echinoderms, many of them long since extinct, and, sprouting from what we now know to be the main trunk, a number of primitive fish. The display would be richer than the earlier one, yet would include no trees, no flowers, no insects, no amphibians, no reptiles, no birds, no mammals. In such reconstructions, we identify the trunk in retrospect, as the branch that was to lead to the most important innovations in the future. This identification often would not have been evident to contemporaries and is liable to change with time. Today, we place our own species on top of the tree. At least, most of us do so. Ten million years from now, however, we could be on a side branch, or nowhere at all. The new trunk could prolong what looks today as a side branch; it could bear a form of life more complex than the human and beyond the power of our imagination.
Cross sections through the tree of life do not just consist of separate dots, as do cross sections through a real tree. In the tree of life, the dots are interconnected by an intricate network of relationships; they form a web. As the dot pattern increased in complexity, so did the web. In this chapter, we shall look at some critical aspects of the development of this web.