There are other products of evolution that occurred independently, and are not the product of common ancestry. This is more likely to happen if a feature is useful in certain environments, but wasn't present in the common ancestor. For example, although a shark is a fish, and a dolphin is a mammal, both evolved sleek fins for gliding through the water (one of the many clues that they do not share a common ancestor is that the shark skeleton, like other fish, is structured so that a shark tail swims left to right, but a dolphin's skeleton is structured like that of other mammals, and as a result the dolphin tail swims up and down).
|Left:Right - Great White Shark by Pterantula (Terry Goss)|
|Up:Down - Dolphin (Turisops truncatus) by Arnaud 25|
This is what we call convergent evolution.
Convergent evolution:Just as we can take advantage of shared evolutionary history to learn more about ourselves, we can also study cases of convergent evolution to learn more about, well, ourselves!
The independent evolution of similar features in species of different lineages
- Reece et al., Campbell Biology (9th Edition)
For example, eyes evolved multiple times throughout the evolutionary tree. Specifically, the eyes of squids evolved independently of ours, but function in very similar ways. Because of the similar function, squid eyes can be affected by disorders, like myopia (being near-sighted/short-sighted), that affect human eyes. By studying how the squid eye evolved, and how it develops now, scientists hope to make advances in understanding and treating eye disorders in humans. Emma Goodman put together a wonderful summary of this, "Eyevolution":
'Eyevolution' NESCent 2013 from Emma Goodman on Vimeo.
Evolution is awesome.