Fossils and bones are one record of ancestry; DNA is another. Every species carries a copy of its ancestors' genes, changed a little by mutations along the way. Comparing sequences, which you can read with the genetic code, confirms the story told by fossils and anatomy.
Key idea
Two species that shared a common ancestor more recently have had less time to collect differences, so their DNA and protein sequences are more alike. Counting the differences in the same gene or protein lets us compare how closely species are related.
Why it works
After two populations split, each collects its own mutations, generation after generation. Like two copies of a book recopied by hand in two places, the longer they have been apart, the more differences pile up. So the number of differences works like a rough clock since the split.
Worked example: Counting differences
Part of a protein in four species:
Species W: Met-Gly-Lys-Val-Ala-His
Species X: Met-Gly-Lys-Ile-Ala-His
Species Y: Met-Ala-Arg-Ile-Ala-His
Which species is most closely related to W?
Species W: Met-Gly-Lys-Val-Ala-His
Species X: Met-Gly-Lys-Ile-Ala-His
Species Y: Met-Ala-Arg-Ile-Ala-His
Which species is most closely related to W?
- W vs X: one difference (Val/Ile).
- W vs Y: three differences (Gly/Ala, Lys/Arg, Val/Ile).
Answer: Species X, with the fewest differences.
Humans and chimpanzees are a good example. In the parts of their genomes that line up, about 98.8% of the DNA bases are the same. That is why they sit next to each other on the tree. Gorillas are a bit less similar, orangutans less again, matching the order of branching that fossils and anatomy suggest.
Worked example: Reading the clock
A gene differs at 6 bases between species A and B, at 6 between A and C, and at 2 between B and C. Which two shared the most recent common ancestor?
- The smallest difference is between B and C.
- A differs from both by the same amount, so A branched off first.
Answer: B and C.
Common mistake
Because human and chimpanzee DNA is so similar, people sometimes say humans evolved from chimpanzees. They did not. Both species descend from a common ancestor that lived millions of years ago, and each line has changed since. Living chimpanzees are our cousins, not our ancestors.