Enlarge / You don’t need a DeLorean to understand the past. (credit: Wikimedia Commons)

How do we know what today’s lifeforms were like when they first evolved? For years, biologists could make inferences about how recent species shared common ancestors based on an approach called cladistics, which quantified how many similar features they shared. This approach worked with fossils as well as living species, allowing us to group them in the sort of branching hierarchies produced by common descent. But these days, rather than things like bone shape and tooth number, we have DNA.

So how do you build a tree out of that? As it turns out, the general approach of cladistics also works with genetic information.

Cladistics

Let’s say you want to understand the origin of mammals. To do that, it helps to have a separate but closely related group—for the mammals, the reptiles would work well. Reptiles and mammals share a number of features, such as having four limbs (they’re all tetrapods—even snakes and whales, which can have vestigial limbs). Others are distinct to mammals, like fur or the presence of specific bones in the inner ear. You can also have some things that are partly shared (like the egg laying of a platypus) or present in only a subset of mammals (like flight in the bats).