Biologists have been constructing trees since before we knew why tree-shapes made such convenient organizing structures for living things. Since Charles Darwin (and Alfred Russel Wallace) first made the case that diverse groups of living species arise from common ancestors, we understand that tree-like relationships reflect this common descent, so that if we can infer the specific structure of those relationship-trees, or phylogenies, we can begin to draw conclusions about how individual species evolved to be what they are today.
Back in the day, we had to estimate phylogenies using directly observable characteristics—measurements of particular parts of particular bones (for mammals), or the shape of the antennae (for butterflies), or the capacity to synthesize lysine (for paramecia). If species that are more recently related tend, on average, to look more similar than each other, this kind of morphological data can be useful. But! When you’re setting out to reconstruct a phylogeny from a whole pile of morphological measurements—dietary preferences, tooth counts, fur color, wing length—what do you do when different traits support different relationship structures? Different traits will naturally change at different rates over evolutionary time, and it’s rarely obvious what those rates are.
Starting in about the 1970s, though, it became increasingly straightforward to directly compare the genetic codes of differnt species. That’s appealing for several reasons: first, because DNA is as direct a marker of inheritance as you can find, it provides a record that’s independent of potentially misleading, morphological similiarities. Second, because DNA sequences have only four character states—the good old “bases” adenine, guainine, thiamine, and cytosine—it’s more tractable to estimate how they change over time.