Evolution  (Page 8/9)

The anatomy of modern organisms also reflects this common ancestry. The limbs of all tetrapods contain a similar number and arrangement of bones, even though the size and shape of the bones can vary greatly in different organisms. For example, the two bones in your forearm, the radius and the ulna, have counterparts in other mammals (figure 2.4), in reptiles, in birds, and even in fossil dinosaurs and pleisiosaurs . If all of these structures were specifically created for moving around in a different environment (e.g. water for the plesiosaur and air for the bird or bat), simple engineering principles would dictate that different structures would be more efficient in those different situations. Yet the same structures, endlessly modified, are found in all of them. The simplest explanation for this is that the organisms share a common ancestor where that structure originated, and evolutionary mechanisms resulted in the modifications in size and shape that we see today. This phenomenon is known as homology ; structures are said to be homologous structures if they occupy similar positions and arise from a common ancestral structure.

Even vertebrates who have lost these limbs in the course of evolution (e.g. snakes) had similar structures prior to that evolutionary change. Figure shows the fossilizedremains of a creature ( Tetrapodophis amplectus ) that lived in what is now Brazil 120 million years ago. It had a snake-like body and may be the ancestor to all snakes, butit also had four small limbs. The forelimb shown in that figure clearly has the humerus, radius, ulna and hand bones that are found in modern vertebrates.

Homologous forelimb bones in a fossil snake

Embryologists also made predictions based on this evolutionary explanation. They predicted that homologous bones would arise from similar structures during the development of the embryo. For example, the forearm bone that we call the radius, which looks radically different in the forearms of a bat or a human or a mouse or a bird, would come from similar structures in the embryos of bats, humans, mice or birds. Those predictions also were found to be correct. So homology argues strongly for an explanation that invokes descent with modification.