sequence comparison reveals relationships

Imagine three old letters, copied and re-copied by hand over many generations, that have all drifted from the same original. Even without the original, you can line up the three copies side by side and read their shared history: where two copies agree but the third differs, you can guess which two were copied more recently from each other. Living things carry exactly such hand-copied texts inside them — their DNA and protein sequences — and comparing those sequences lets us read the history of life without any written record.

The trick rests on a simple fact: when a cell copies its DNA to make offspring, the copy is almost perfect but not quite, so over time small changes (mutations that become fixed differences, called substitutions) accumulate. Two species that split from a common ancestor long ago will have piled up many differences; two that split recently will still look nearly identical. So if you align the same gene from a human, a mouse, and a fish — writing one letter (A, C, G, or T) above the matching letter in the others — the number and pattern of mismatches becomes a measurable record of how long ago, and in what order, their lineages parted. The more shared, derived changes two sequences carry, the closer their kinship.

This is the foundation of molecular evolution and phylogenetics. Before sequences, biologists compared bones, shells, and body plans, which can mislead because unrelated animals sometimes look alike. Molecular sequences gave a far larger, more objective dataset: every gene is a separate witness to history, and there are thousands of them. One honest caveat: a single gene can tell a misleading story (it may have been swapped between microbes, or evolve unusually fast), so reliable conclusions come from comparing many genes and reading them together, not from one alignment alone.