If living species share ancestors, the past should leave traces: fossils of in-between forms, and bodies built from the same parts. In Comparing Body Structures you met the idea. Here is the evidence in more detail; natural selection explains how the changes happened.
Key idea
Transitional fossils show mixes of old and new features. Homologous structures are the same parts, inherited from a common ancestor, used for different jobs. Vestigial structures are reduced parts with little or no use left. All three point to common ancestry.
- Tiktaalik, found in 2004 on Ellesmere Island in Arctic Canada, lived about 375 million years ago. It was a fish with gills and scales, but with a flat head, a neck and sturdy fins with wrist-like bones: a step toward four-legged animals.
- Early whales: Pakicetus, about 50 million years old, was a land mammal with an ear like a whale's. Later fossils show legs shrinking as whale ancestors moved into the sea. DNA links whales to hippos and other hoofed mammals.
- The forelimb: a human arm, a whale flipper, a bat wing and a cat leg have the same bones in the same order, built for different jobs.
- Vestigial parts: whales still have small hip bones; some snakes have tiny leg spurs; cave fish have eyes that do not see.
Why it works
Why would a whale have hip bones, or a bat wing have the same bones as your arm? Engineers designing each animal fresh would not need to reuse parts. But evolution can only modify what an ancestor already had, so old parts get reshaped, and parts that lose their job shrink but linger.
Worked example: Homologous or analogous?
Are a bird's wing and a butterfly's wing homologous?
- Both are used for flying.
- But a bird wing has the bones of a vertebrate forelimb; an insect wing is a thin outgrowth of the exoskeleton with no bones.
- They evolved separately for the same job.
Answer: No. They are analogous, not homologous.
Common mistake
Analogous structures, like the wings of birds and insects, or the streamlined bodies of sharks and dolphins, look alike because they do the same job. They are not evidence of close kinship. Only homologous structures, the same parts doing different jobs, show shared ancestry. Check: look at the inner structure, not the outer use.