1. Common Ancestry and Evolution
Trilobite
(extinct)
Horseshoe crab
Common Ancestry and Evolution
Charles Darwin

2. Common Ancestry
Scientists believe that all forms of life can trace their origins back to a single common ancestor. Closely related species can also be traced back to a more recent common ancestor.

3. Common Ancestry
Cytochrome C
Evidence of this common ancestry can be found in the homologous structures of different species. These homologies can be anatomical, molecular, or developmental.

4. Common Ancestry
Comparing the DNA sequences of different species provides some of the best evidence for common ancestry. Because proteins are made from genetic information, comparing amino acid sequences also provides great evidence.

5. Common Ancestry
Studying similar structures in the anatomy of different species can also give great insight into how different species are related, and when they might have had a common ancestor in the past.

6. Common Ancestry
And studying organisms as they develop, such as the embryos shown above, provides evidence of homologous structures that often cannot be observed in the fully developed organisms.

7. Common Ancestry
Human
Bird
Porpoise
Elephant
It is crucial that you understand that homologies are similar structures that descended from a common ancestor. The function of these structures may have become quite different.

8. The functions of structures can change as populations are separated and forced to adapt to different environments. Eventually, these populations can undergo speciation and the number of species increases over time.

9. Some species are not able to adapt to new environmental changes and become extinct.
Other species continue to adapt and change becoming the animals we are familiar with today.

10. Evolution
The idea that species slowly change over time as a result of natural selection was proposed by Charles Darwin. Evolving means to slowly change, so this idea is called the theory of evolution.
Charles Darwin

11. Common Ancestor
It has been theorized life began as a single cell that reproduced and slowly changed into all of the known species as populations adapted to different environments over very long periods of time.

12. Trilobite
(extinct)
Horseshoe crab
It is homologous structures that provide the best evidence for evolution. Using this evidence, scientists construct diagrams to show evolutionary relationships between species.

13. Phylogenetic Tree Cladogram
Species with a more recent common ancestor are more closely related to each other, and this would be evidenced by a greater number of homologous structures.

14. Analogous Structures
Photo by Christian Mehlführer
Analogous structures are structures that serve the same function, but do not have similar structures. The wings of a bird and the wings of an insect are analogous structures.

15. Analogous Traits
Analogous traits are the results of species adapting to similar environments. They are NOT based on similar structures, so they do NOT provide evidence for common ancestry.
Bats and dolphins both use echolocation to hunt and navigate in dark environments, but these traits evolved independently in both species.