Hairpins

If I had any super power, it would be the ability to travel back in time, to be invisible, and to observe hairpins.

For any two animals* A and B, there were once two sisters A' and B', one whose descendants led to animal A and the other whose descendants led to animal B. In fact, the sister A' is also the direct ancestor of ALL other species more closely related to A than B. (And B' is also the direct ancestor of ALL other species more closely related to B than A).

Dawkins calls the mother of these two sisters a hairpin (source). If you go back in time looking at ancestors of animal A you would eventually get to A' and then the hairpin. Then you can go forward in time on a different path (B') to eventually lead you to animal B. While this is a basic concept of evolution, this is something lost on a lot of people. The hairpin is the common ancestor of the two animals. It is not the same as a modern living creature. We are not descendants of chimpanzees and there are no crocoducks.

There are as many hairpins as there are species alive today minus one. Think of a tree where the tips of the branches (leaves) are modern living species, each point in the tree that splits (internal node) is a hairpin.

*The sisters idea breaks down when you get to closely related examples: such as me and my aunt, but the hairpin idea still holds.