One of the most important tools used to study and understand evolution are evolutionary trees, also known as phylogenies. Phylogeny is a strange name for a complex field, yet phylogenies are something all of us are familiar with, just by another name. The “Tree of Life” is the phylogeny of all living things. As a metaphor, the tree of life summarizes the evolutionary relationships among all species on earth.
I’ll explain the metaphor in a little more detail. If you are already familiar with phylogenies, this may get boring. Sorry. For the rest of you, just close your eyes and picture this:
I’ll start with the leaves and work my back down to the trunk. The green leaves on the tree of life are all the species alive on earth today. Humans are a leaf, chimps are a leaf, dogs and cats are each a leaf. Now, imagine the twigs that these leaves are growing out of. Humans and chimps are growing out of the same twig, since they each evolved from a common ancestor fairly recently. Where once there was one twig, it split, giving rise to two, humans and chimps. Dogs and cats rest on a twig just behind humans and chimps, since the common ancestor of all four species lived longer ago. Make sense?
Now move down the tree a little farther. Imagine the branch that all of the twigs and their leaves are growing out of. That branch is the evolutionary bloodline connecting all the twigs and leaves together. Put another way, that branch is the family name you had to memorize in highschool. Humans and chimps and dogs and cats are apart of the same branch, or evolutionary lineage, because they all all mammals. Moving even farther down this branch, you’ll notice that there are some twigs far from the tips of the tree that have no leaves at all. These are the twigs of extinct species — mammals that lived once but are no longer alive today. Think of mammoths, a group of mammals which have been extinct for over 4,000 years. We have plenty of evidence that they once existed, so we know that a leaf used to be on that branch, but Mammoths are no longer around: their leaf is dead. It is worth remembering that the tree of life is so much bigger than just the leaves we see — many different species once lived on Earth but are extinct now. At different points in time, the tree of life has been thicker or thinner, greener or browner, depending on the number of species alive.
Look even farther down the branch to the fork where the branch splits. If you follow that split up towards its leaves, you’ll find a whole new evolutionary lineage (say, reptiles) with its own set of twigs (for example, dinosaurs) and species (T. rex). Where ever there is a fork like the one connecting mammals and reptiles, there once was a living species, a leaf from when the tree of life was younger. But like the twig connecting humans and chimps, this early twig split, forming two twigs. As the tree of life grew over time, this splitting happened over and over again to produce the new branches and forks we see now.
Now look even further down the tree, all the way to the trunk. Just like the split in the branch between mammals and reptiles, the trunk is filled with its own branches, each a different linage of living organims — algae and plants, insects, birds (a group of reptiles), and mammals. So what does the trunk represent? Obviously, the common ancestor that gave rise to all living species on earth. This is perhaps the most challenging (and exciting) implication of evolutionary biology. If there are leaves which share a common twig, then there must be a branch connecting them. And if there is a branch connecting them, then that branch must connect to others, and so on, until all branches meet at the trunk. Therefore, if both chimps and humans exist today where before there was only one species, then the two lineages of mammals and reptiles must similarly have formed from one, and the kingdoms of plants and animals must have formed from one, and so on and so on to the base of the tree.
Darwin made this exhilarating inference over 150 years ago when he wrote, “I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.” He made this statement years before we knew anything about inheritance, the structure of DNA, or what a sequenced genome looked like . And yet, the discovery and study of each of these has repeatedly supported his conclusion that all life on earth has a common origin, what is called common descent. To me, this is the most important and useful concept behind phylogenies. Indeed, Darwin must have thought so as well, for throughout his whole book, The Origin of Species, there is only one figure: a phylogeny.
Below is a phylogeny of ~3,000 living species based on ribosomal RNA sequences that David Hillis‘ lab produced in 2003. Although clearly not representative of all living species now or ever, it is a remarkable example of how phylogenies illustrate the drama of evolution, the tree of life.