1. Common Ancestry

2. Common Ancestor
Most scientists believe that all forms of life can trace their origins back to a single common ancestor.

3. Common Ancestry
We show the relationships between different species using cladograms and phylogenetic trees. Both of these instruments show these relationships in a very similar way.

4. Common Ancestry
Different species that are more closely related have a much more recent common ancestor.

5. Common Ancestry
Scientists use evidence that they uncover in fossils, studying biogeography and investigating homologous structures to draw conclusions about the relatedness of organisms.

6. Common Ancestry
Trilobite
This fossil is of an ancient arthropod
(insects, centipedes, crustaceans, arachnids)
Fossils can help us study organisms that are now extinct. By studying the fossils, we can learn about what species that are alive today might be related to the extinct species.

7. Fossil of archaeopteryx Artist’s Rendering of archaeopteryx
The study of fossils has led many scientists to theorize that birds can trace themselves back to a common ancestor that was a dinosaur.

8. Homologies are similar structures in different species that were derived from a common ancestor. These homologous structures are studied to determine the relatedness of species.

9. Human
Bird
Porpoise
Elephant
Some animals have similar anatomy in their limbs, but use their limbs for different purposes. However, it is similar structure that shows common ancestry. The different functions of these limbs is due to natural selection.

10. Common Ancestry
Homologies can also appear in the molecular structures of organisms. Similarities in the DNA and proteins of different species can be used to determine the relatedness of species.

11. Common Ancestry
Two species are more related when their molecular structures are more alike. The species which are more alike most likely have a more recent common ancestor.
Cytochrome C is a mitochondrial protein found in most eukaryotic organisms. It is compared across species to determine the relatedness of different species.

12. Common Ancestry
Different species can also display homologies in their development. Human embryos have gill slits and tails, and this suggest that humans have a common ancestor with other animals species that still have these traits.

13. Common Ancestry
Fossils provide evidence that the continents were once joined together in a supercontinent called Pangea.
Biogeography is the study of the distribution of species around the earth. By studying the species that live in an area now (and in the past), scientists can study how species have changed and how they might be related.

14. Biogeography
The field of biogeography has identified several large zoogeographic regions that are separated from each other by geographic boundaries such as mountain ranges, broad deserts or oceans.

15. Biogeography
Species in a region are isolated from other regions and have been able to become adapted to their own region over a long period of time.

16. Charles Darwin, who first proposed the theory of evolution, studied the distribution of finches on the Galapagos Islands. Darwin concluded that the different species came from a common ancestor, and the differences between the species were a result of natural selection.