Common ancestors and phylogenetic trees

A lizard looks more like a frog than a mouse, yet on the family tree of vertebrates the lizard and the mouse are the closer relatives.

Same bones, different jobs

The arm of a person, the front leg of a dog, the wing of a bird and the flipper of a whale do very different jobs, yet they are built from the same set of bones in the same order. Biologists call parts like these homologous structures: they are similar because the animals inherited them from a common ancestor, and evolution has since changed their shape and size. Some structures seem to have no use at all, like the wings of birds that cannot fly or the small hind leg bones inside whales. These vestigial structures are leftovers from ancestors in which they did work.

Similar does not always mean related. Insects and bats both fly with wings, but the wings grew from completely different starting parts. Features like these, which evolve separately in unrelated groups because they face similar problems, are analogous structures, and the process is called convergent evolution. The white winter coat of the arctic fox and the white winter plumage of the ptarmigan are another case: both are hidden in snow, but not because they share a recent ancestor.

Other evidence points the same way. Vertebrate embryos, human embryos included, have pharyngeal arches and a tail at an early stage. In humans these arches contribute to head and neck structures, not functional gills. All living things use DNA with nearly the same genetic code, which is what you would expect if all life shares one ancestor, and the more closely two groups are related, the more alike their DNA sequences are. The places where animals live fit too: groups that evolved before the supercontinent Pangaea broke up, about 200 million years ago, are found worldwide, while Australia, isolated for a long time, is full of marsupials found nowhere else.

How to read a tree

A phylogenetic tree is a diagram of who is related to whom. Darwin sketched one in a notebook in 1837, and a branching diagram is the only illustration in On the Origin of Species. In a rooted tree the root, at the left in the lab below, stands for the ancestor of everything on the tree. Each branch point is a lineage splitting in two. The names at the tips are living groups. Two groups that come from the same branch point are sister taxa, and a group that branches off near the root and stays on its own is a basal taxon. A branch point with more than two branches, a polytomy, means the order of splits is not yet known.

To find how closely two groups are related, trace both back towards the root until the paths meet. That meeting point is their most recent common ancestor. The more recent it is, the closer the relatives. Three habits trip people up. The order of names along the tips means nothing, because a branch point can be rotated like a mobile without changing who is related to whom. Unless the tree says so, the length of a branch does not show time. And no living group on a tree is the ancestor of another; sister groups share an ancestor, and neither gave rise to the other.

In the tree above, the lizard and the mouse meet at the branch point marked by the amniotic egg, an egg with membranes that let it develop on dry land. The frog branches off earlier, so it is a more distant relative of both, even though frogs and lizards look alike to us. Crocodiles and birds meet at a branch point of their own, which makes them each other's closest living relatives. Trees are built from fossils, anatomy and, more and more, DNA, and they are hypotheses that get revised as new data arrive. One revision surprised many people: the hippopotamus looks more like a pig, but it may be the closest living relative of whales.