Lesson 3: The Essence of Life 3.7 Evolution, the Central Concept Figure 3.7.1 A young chimp uses a stick as a tool to reach insects. Humans and chimpanzees share 98% of their DNA. We have seen that Evolution is not some kind of add-on to Life; it is an integral part of it. Without it, there is no explanation for the Tree of Life, the observed patterns in the similarities and differences between the various life forms. Why should it be possible to graft a branch from an orange tree on that of a lemon tree? Why should a donkey be able to breed with a horse? Because they are not very far apart, in terms of their genetic endowment and chemical machinery. Why then should it be possible to take a gene from a bacterium and insert it into a plant to make it unpalatable for an insect? Why should the insect be able to digest the plant in the first place, incorporating molecules produced by the plant into its own body? Because there is an underlying unity in the way different life forms conduct their business, on the level of molecular biology. The obvious implication for this is common ancestry. It explains why living things are similar. Linnaeus' classification of plants and animals opened the path to this realization. The naturalists Jean Baptiste de Lamarck, Geoffrey Saint-Hilaire (1772-1844) and Leopold von Buch, among others working around the end of the 18th and in the early 19th century, realized that common ancestry would make sense of similarities, and a little later Charles Robert Darwin (1809-1882) provided a mechanism for evolution in his book on the origin of species, published in 1859. Darwin's contribution was to introduce the concept of the struggle of survival in the face of adversity and competition, resulting in the survival of the fittest, a culling process he labeled "natural selection." The same idea had occurred to another famous naturalist, Alfred Russell Wallace (1823-1913), a brilliant eccentric with deep insights into the rules of biogeography, but who exempted humans from evolution. Darwin did not. The use of molecular biology in pursuing the path of evolution is simply an extension of the earlier approach, based on similarity of anatomy and morphology, as urged by Geoffrey and Lamarck. Figure 3.7.2 The evolution of myoglobin can be traced through its amino acid sequences in different species. Molecular biology has provided overwhelming evidence for evolution. Why then, if it is so obvious, is the theme of evolution still in contention in some quarters? Why do we have school boards meddling in technical questions of modern biological science and voting on the teaching of evolution? Can one teach music without reference to the physics of sound? Can one teach visual arts without reference to color? Can one teach religion and deny access to holy scripture? Of course not, unless one makes a mess of it. Neither can one teach biology and reject evolution, which is the basis for understanding of Life. Evolution explains why we - all the living things - are still around despite the great changes that have taken place in the environment of Earth, through the eons. There is good reason for taking a deep and abiding interest in matters surrounding the evolution of Life. Life is the crowning achievement of the history of the Solar System. It is the most deeply puzzling and intriguing part of that history. Also, because the study of Life throws light on our own origin, as humans, such study impinges on our belief systems, on how we think about ourselves, our place in the scheme of things. This question, where we belong in the scheme of things, is one that for thousands of years has been answered by oral religious traditions, and, since the invention of writing, by written religious traditions. Confusion arises when scientific results are interpreted as being in conflict with such traditional teachings. They are not, where religious teachings concentrate on ethical behavior and do not presume to provide textbooks for astronomy, geology, or biology. Figure 3.7.3 A cast of the fossil remains of Archeopteryx, believed to be the link between dinosaurs and modern birds. It is considered to be one of the most important fossils ever discovered. The fossil record, despite its gaps, richly supports the concept of evolution. The earliest period of the existence of Life is forever out of reach: there are no rocks from that time with preserved information. Shortly after the end of the "heavy bombardment" period we find chemical fossils - carbon with a strange isotopic composition. Half a billion years later we find the first unicellular organisms: including their small reef-like mounds, the stromatolites. A billion years later we find clear signs of photosynthetic activity. The oxygen freed in the process stimulated the rise of eukarya, from collaborating bacterial precursors. By a billion years ago, eukarya are in full swing, and half a billion years later they have succeeded in generating pretty much all the familiar animals, plus many that are not familiar. Land plants start shortly after. Three-hundred million years ago we see the first reptiles. From there to birds and mammals is but a small step. Five million years ago the human ancestors split off from the ancestor of chimps, and since then we have learned to climb less, run more, and use our brain, which has since greatly increased in size. Schematic of the evolution of life since its origin more than 4 billion years ago. On the right side is a blow-up of geologic history of the last 500 million years, one ninth of the total age. This period contains rocks with fossils of large multicellular organisms, including corals, trilobites, cephalopods, fishes and reptiles. Using the brain for modeling the solar system makes us unique among all the life forms. Although bees model the path of the Sun across the sky (to tell time and direction) they do so, presumably, without the faintest idea of the nature of the Sun.