Evolution  (Page 9/9)

In contrast, the wings of insects and the wings of bats or birds do not have similar structures, although they have similar functions (to propel the organism through the air). These structures are said to be analogous rather than homologous; they share a function but do not arise from a structure that is found in a common ancestor. Indeed, if organisms predominantly had analogous structures, which would be different engineering solutions to a common problem, that evidence would be more consistent with the explanation of independent creation of those organisms. But homologous structures seem to be the much more common observation, making descent with modification a much more scientifically satisfying explanation.

Comparative biochemistry

One of the biggest surprises of modern biology came from the field of science known as biochemistry. Once biochemists started to unravel the mysteries of metabolism, the unity of life on this planet became quite obvious. Creatures with incredibly different morphologies, habitats, and lifestyles all seem to have incredibly similar metabolic pathways. Bacteria, bonobos, bats and bananas all use a molecule known as ATP (adenosine triphosphate) to store and provide energy within their cells, for example. The metabolic pathway known as glycolysis, which you will learn about in subsequent chapters, is found in all the organisms on the planet, and the enzymes that are used in that pathway are quite similar in these diverse organisms. Again, this argues strongly for common ancestry, which is a strong prediction that arises from a “descent with modification” explanation. Once an ancient cell developed these metabolic pathways, there was no need to re-invent that wheel. It is somewhat ironic that some of the best evidence for a particular explanation for the diversity of life comes from the discovery of the unity of life at the molecular level.

Genetics and genomics

Besides ATP, another molecule common to all life forms on the planet is DNA (deoxyribonucleic acid). This molecule stores genetic information, so it is the molecule of heredity. Its role in heredity also means that it can be modified under some circumstances, thus giving rise to the variations described above. Darwin knew nothing about DNA when he proposed his theory in 1859; his ideas about mechanisms of heredity were, in fact, spectacularly wrong. But the discovery of the mechanisms of heredity, starting with Mendel in 1866, and extended by many others in the early part of the 20 ^th century, made it possible to finally propose mechanisms by which heritable variations arise and are transmitted between generations. In fact, the first 4 decades of the 20 ^th century were the years when the two seemingly unrelated fields of genetics and evolution were united. This Neo-Darwinian synthesis, starring Theodosius Dobzhansky, Ernst Mayr, and George Gaylord Simpson, resulted in modern evolutionary theory, and allowed scientists from both genetics and evolutionary backgrounds to work together to make and test predictions.

The elucidation of the structure of DNA by Watson and Crick in 1953, followed soon by the breaking of the genetic code, provided even more evidence for descent with modification. DNA, as you will learn later, functions as a repository of information. In order for the information to be used to build a cell or an organism, it must be read and translated into different molecules. The processes, and the enzymes, that do this work of reading and translating are virtually identical in all living creatures on the planet. The genetic code was, perhaps prematurely, called the “universal” genetic code for precisely that reason; it is translated identically in almost all organisms that have been discovered to date. Once again, this is a strong argument for common ancestry and descent with modification.

But the really impressive outcome of this fusion of molecular knowledge and organismal knowledge comes from the study of the structure of genes, and genomes, at a detailed level. Incredibly, scientists have discovered molecular fossils of a sort – stretches of DNA which are not used in modern organisms, but which remain in the genome as a record of functions in the past. For example, chickens don’t have teeth, but they have genes for tooth proteins, turned off long ago, still lurking in their genomes. Those genes can be turned on under the right conditions, producing toothy structures, which were last seen in dinosaurs, the extinct ancestors of modern chickens. There is no good explanation for these observations, other than descent with modification. Similarly, detailed analysis of the DNA of organisms, including now some long-dead organisms like mammoths and Neanderthals, allows scientists to test predictions about common ancestry, and gain insights into the course of evolutionary change in all organisms. In fact, evidence from analysis of DNA, and other molecules, has allowed us to fine-tune our hypotheses about ancestry and relationships throughout the biological world, as explained in the next chapter on Taxonomy and Phylogeny.