Common Descent

We are descendants of fish.

Many other animals are also descended from fish.
Which is to say, we have a common ancestor with many animals.

The process of determining common descent consists of examining differences in amino acid sequences . For example, we examined a section of Cytochrome C protein's amino acid code. We compared the sequences of different animals, and counted the number of differences. If there are many discrepancies, then the two animals are less related. Horses and donkeys only have 1 difference, whereas horses and yeast have 44 differences. Horses and yeast have 44 differences as well, but yeast and wheat have 47 differences.
Based on our data, we can construct a cladogram.
The closer a branch is to the original animal on the cladogram, the more closely the two are related.