Evolution

Darwin recognized that some of the best evidence of evolution remains hidden within the bodies of living creatures. He reasoned that, if organisms have a history, then their history can be deciphered from their remnants. In fact, virtually all living creatures possess vestigial or rudimentary features that were once functional traits of their ancestors. Fetal whales, still in their mothers' womb, produce teeth like all vertebrates, only to reabsorb them in preparation for a life filtering plankton from their ocean habitat. Snakes, whose vertebrate ancestors ceased walking on four legs millions of years ago, still possess vestigial hind limbs, with reduced hip and thigh bones.

In some cases the same structures may be adapted for new uses. The cat's paw, the dolphin's flipper, the bat's wing, and a human hand all contain counterparts of the same five bones forming the digits (fingers in humans). There is no known environmental or functional reason why there should be five digits. In theory, there could just as easily be four, or seven. The point is that the ancestor to all tetrapods (vertebrates with four legs) had five digits, and thus all living tetrapods have that number, although in a modified form. Such traits, when they reflect shared ancestry, are known as homologous structures or characters. Traits that have functional but not truly ancestral similarity, such as the wings of insects and the wings of birds,are known as analogous characteristics.

Further evidence of evolution involves the most basic homology of all, the genetic code, or the molecular "lan guage" that is shared by all living things. Although the genes of each kind of organism may be represented by different "words," coding for each species unique structures, the molecular alphabet forming these words is the same for all living things on Earth. This is interpreted as evidence that the genetic code arose in a way that a single organism was the ancestor to all organisms that exist today.

Finally, there is the evidence from the fossil records that the remains of invertebrates, plants, and animals appear in the rocky layers of the earth's crust in the same order that their anatomical complexity suggests they should, with the more primitive organisms in the older layers and the more complex organisms in the more recent deposits. No one has ever found a flowering plant or a mammal in deposits from the Devonian age, for instance, because those organisms did not appear on Earth until much later.