You may be surprised to learn that goldfish are more closely related to humans than to sharks! That's because sharks do not have bones, they only have "skeletons" of cartilage. Goldfish and humans evolved from a common ancestor that had evolved a proper skeleton of bone so goldfish and humans share the homologous structures of true bones. The large number of similarities between sharks and goldfish is due to their shared lifestyle so those are analogous similarities (caused by converging evolution - or more properly, they were adaptations that did not so much converge as remained the same over millions of years). Granted, it is hard to tell which similarities shared between goldfish and sharks are homologous and which are analogous. It takes an expert with a great deal of training to catch those subtle differences and the fossil record reinforces that knowledge.

Perhaps this graph will help you understand why goldfish are more closely related to humans than sharks. Notice the branching patterns in the chart. Early in evolution, many millions of years ago, a major division occurred. Soon after that split one group (on the left) evolved true bones and passed that trait onto all its descendants. Bones are a homologous similarity shared between goldfish and humans. The other group (on the right) did not evolve bones and could not inherit them because they had broken away from that branch of the "family tree" after bones had evolved.

These two divisions allow us to split animals up into "bony" and "nonbony" animals. That is a split based upon a similarity passed on by ancestry - a homologous similarity. Sharks are nonbony while humans and goldfish are bony. Goldfish have a similar form to sharks because of their similar lifestyle (so in this graph they are more towards the side of the sharks in form) but their ancestry (the path of the blue line) shows that goldfish and humans shared a common ancestor (a bony one) long ago.
So, goldfish are more closely related to humans than they are to sharks!
(Phew!)

Don't let this graph (example) worry you. It is meant to show you that things are not always as simple as they may at first seem. Indeed, most biology textbooks are quick to group sharks and fish together into a single group called "fish". However, from a purely evolutionary standpoint, the "fish" group (called "Pisces") should be properly subdivided into "bony fish" (called "teleosts") and nonbony fish" (called "elasmobranchs"). We humans (and all other vertebrates) are descendents of the "bony fish" line.

Now let's get back to some homologous similarities that are easier to understand and learn more about how they can teach us about evolution.

This drawing shows limbs from several mammals and I have identified the humerus (blue) ulna (yellow), radius (red), carpels (purple) and digits (light blue). The human arm should be familiar to you. Notice the way the bones in the limb have evolved into different shapes in order to accommodate the lifestyles of each animal. Although these changes produce radically different limb structures, their underlying bones are still there - only their size and shape have changed.

Mammals like the whale or horse have five digits as embryos but they eventually give way to the structures you see here. Why should a whale have five digits as an embryo and four digits as an adult? Why does a whale have any digits at all? Goldfish and sharks do not have hands (fins) like those of a whale. Why does the horse have a single digit when born but five digits as an embryo?
Why does the bat have five digits? A bat's wing might be just as good, perhaps better, with more or fewer digits, so why five?

All these peculiar similarities and embryological changes are due to the fact that all mammals evolved from a common ancestor that had five digits and a limb that included a humerus, ulna and radius. Each of these bones has a homologous structure in each mammal because all mammals descended from a common ancestor. That is the logical explanation.

Homologies do not stop with anatomy. They extend to all levels of an organism. What is the smallest, most fundamental "thing" that an organism inherits from its descendants? It is also the foundation upon which the entire organism (including its anatomy) is based?
Once you've come up with something that you think might fit that descriptions, check it out on the next page.