The Fossil Record The fossil record provides physical evidence about history of life on Earth. It shows how different groups of organisms, including species have changed over time. Fossils of insects, mammoths, and footprints are just a few examples of the different remains paleontologists have found.

WHAT ARE FOSSILS? Fossils are preserved remains or trace remains of ancient life forms that once lived on Earth. Scientist use fossils to infer what the past life form and environment were like.

The Fossil Records Here we see the fossil record of modern horses. As you can see the forefeet and teeth of the horse have changed over time.

Geographic Distribution of Organisms American Beaver: Lives in North America Coypu: Lives in South America Capybara: Lives in South America Muskrat: Lives in the North and Central America Geographic Distribution of Organisms Darwin noticed that organisms in different regions of the world had similarities in physical appearance and structure. As you can see these 4 different species have similar characteristics such as teeth, limbs, hair, nose, diet and habitat. Scientists hypothesize that a common ancestor of these species settled in different regions and evolved to suit the different habitats. The result being the 4 different species seen here.

Geographic Distribution of Organisms Even though these species are separated by thousands of miles of land and water you can see similarities in characteristics that have allowed them to survive. Perhaps an ancient ancestral group of organisms with similar traits evolved into these 4 species changing to suit their different environments.

Similarities in Embryology Embryology is the study of development of an embryo, or fertilized egg. As the embryo grows it follows a pattern of changes. Scientists compare this pattern of growth between different species. Since the fish, turtle, salamander and human have similar first stage of development we can see they may have had a common ancestor at some point in the distant past. But, if we compare the salamander and fish we see more similarities, leading us to think they shared a common ancestor more recently.

Comparing Patterns of Embryology in different species Here we can see more patterns of development. Scientists call similarities in patterns of development homologies. You can see homologies between all 4 species at the first stage of development. But when you look at the 2nd stage you only see homologies within the reptile group (lizard and tortoise) and mammal group (pig and human).

Homologous Structures in Different Species Structures that have different mature forms but develop from the same embryonic tissues are called homologous structures. Here we can see the homologous arm structures of 4 different species. Since they possess homologous structures they may have a common ancestor. Each arm has adapted in ways that enable organisms to survive in different environments. Therefore changing into different species.

Homologous Structures in Different Species Here is another excellent example of homologous structures as evidence for evolution. The similarities are obvious but it’s the differences that really support evolutionary theory. Each species has adapted to suit its particular environment. The Tarsier leaps and clings to rocks. Its large finger pads helps it do this. The gibbon swings from tree to tree and has elongated strong fingers to do so. The chimpanzee may be able to manipulate crude tools with its stubby thumb, but the human thumb faces the forefinger. This provides the dexterity for countless purposes.

DNA Sequences Scientists look for similarities and differences in the sequences of nitrogen bases from samples of DNA in different species. Based on similarities they draw conclusions about common ancestry. Here we can see the Pax6 gene in several different species. The pax6 gene is the master gene for the early development of the eye. In all 4 species there is a large similarity in gene structure leading us to believe the gene was also in their common ancestor.

DNA Sequences This diagram shows the number of amino acid differences between the hemoglobin proteins in different species. The Macaque and Human have only 8 amino acids different making them more closely related compared to the Dog. But, the Dog is more related than a bird because the Dog has only 32 amino acids different compared to 45 for the bird. Finally the Lamprey is least related because it has 125 different amino acids, more than any of the other organisms.