Evidence for Evolution (PDF Version for printing is here)

Fossils and Evolution. Fossils are the remains or traces of an organism from prehistoric times (older than 4000 BC; Fig 1). Most organisms do not fossilize and those that do are usually destroyed by geological processes or they never surface for examination. Fossils are usually formed when an organism is covered by sediments that then harden into sandstone, slate, mudstone or flint. Organisms also fossilize when they are buried in volcanic ash or entombed in tar or tree sap.

A mold fossil forms when the material surrounding the organism hardens followed by removal of the organic matter. This leaves behind an impression (or mold) of the organism. Petrifaction fossils are formed through two main processes: permineralization and replacement. Permineralized fossils are created when ground water percolates through the remains of the organism and leaves behind minerals in the cellular spaces. Petrified wood is an example of a permineralized fossil. Replacement fossils are formed when ground water first dissolves out the tissue and then leaves minerals in their place. Both types of petrifaction fossils are generally composed of either SiO2 or CaCO3.

Comparative Anatomy. The discipline of comparative anatomy is important in interpreting relationships among organisms. Structures are said to be homologous if they have similar embryonic origins (Fig 2) and analogous if they are similar only in function. The wings of birds and flies are examples of analogous structures. They serve the same function, but obviously have different embryonic origins (one is made of bone and flesh, the other is composed largely of non-living chitin). On the other hand, the wings of birds and the foreleg of a frog are homologous structures (although these limbs have different functions, their embryonic origins are similar; Fig 3). To comparative anatomists, homologous structures are important because they imply an evolutionary linkage between two species. Also important are vestigial organs; structures that have no discernable function that are thought to be evolutionary "left-overs". Just as a cave fish will often lose their eyes over long periods of time, vestigial organs are the remains of structures that once had a purpose in our evolutionary past, but are no longer necessary. An example of a vestigial organ in humans is our appendix. The appendix serves no useful purpose today, but in herbivores, a larger appendix-like pouch contains bacteria that allow them to digest cellulose found in woody plants (Fig 4). Additional examples of vestigial organs in humans include the semilunar fold at the corner of the eye (Fig 5). This pink structure is all that remains of a secondary clear eyelid found in many vertebrates known as the nictitating membrane. This structure allows an animal to blink without going temporarily blind. An even more extraordinary example of a vestigial organ are the remains of a pelvic girdle in whales. These structures clearly show the terrestrial origins of whales. See also examples of convergent evolution where similar environments result in similar morphologies, even though the organisms are not evolutionarilly related to one another.Good examples of convergent evolution can be seen in the marsupial animals of New Zealand and Australia (HERE). Also, read this.

Embryology. A study of an organism's embryonic development provides further clues to its evolutionary past. At one time it was asserted that "ontogeny recapitulates phylogeny". Simply put, this hypothesis suggests that, as an organism goes through its embryonic development (ontogeny) it repeats (recapitulates) the stages in its evolutionary history (phylogeny). At one point during your development, for example, you possessed gill slits and thus seemingly passed through the aquatic phase of our evolutionary history (fig 7). This notion of embryology as an "instant re-play" of evolution has been called the biogenetic law. Although the relationship between evolution and embryonic development is more complex than once thought, related organisms do show similarities in their embryonic development (Fig 8). These similarities can be traced to the conservative nature of embryology: small changes in early in development can have drastic consequences in later stages (through a "domino effect). Thus, although gill slits are not needed in adult humans, removal of this stage from our development could change the arrangement of tissues around the slits and affect development of unrelated structures. Thus, structures such as our gill slits are retained as "cellular scaffolding". From an evolutionary perspective they are valuable because they reveal our kinship with other members of our phylum.

Comparative Serology. As you are aware, people vary in their blood types, and these differences can be ascertained through immunological testing. Techniques similar to those used for blood typing can be used to test and compare the blood sera of different species. When so tested, closely related species show greater similarities in blood constituents than do more distantly related species.

Genetic Similarities. A fragment of aligned DNA sequences that codes for a protein common to each primate species and the chromosome banding patterns for the first chromosomes are depicted in figure 1. Changes in the DNA sequences are double underlined. For the chromosomes, sold indicates similar sequences, hatching, a possible shift in the banding patterns as compared to humans. Changed patterns are stippled (Fig 9, 10).