What is a tree of life?

A phylogeny? An evolutionary tree? A cladogram? We see the branching lines of these diagrams in many museum displays and science articles, but what do they tell us and why are they helpful?

Duncan Murdock, research fellow, explains. 

You are a fish.

Starfish, jellyfish and cuttlefish are not fish.

Actually, no, there’s no such thing as a fish. Let’s take a step back…

It all comes down to common ancestry. All life is related, and we can think of it in terms of a family tree (or ‘phylogeny’): Jackie, Tito, Jermaine, Marlon and Michael were all Jacksons. United not only by a collective inability to control their feet, but also by common descent – they are all their parent’s children*.

By tracing further and further back in MJs family tree we could define ever larger groups united by common ancestors, first cousins (grandparents), second cousins (great-grandparents), all the way to every human, every mammal, every animal, and eventually all life – we are family (ok, that was Sister Sledge, but you get the point).

In the case of the tree of life, species are at the tips of branches and their common ancestors are where branches meet. A true biological group consists of a common ancestor and all its descendants, and we can use characteristics common between two species to imply common descent. Siblings look a lot like each other because they have inherited much of their appearance via common ancestry (i.e. their parents). In a similar way, two closely related species will share lots of inherited characteristics.

However, things are not quite that simple. Wings of bats, birds and insects are not inherited from a common ancestor but independently evolved for the same purpose, in this case flight. To complicate things further, as species evolve they may lose features inherited from their ancestors that other descendants retain. Snakes have lost their limbs, but still sit in the same group as lizards. These problems can be overcome by looking at many characteristics at once, using genetic information to test predicted relationships, and adding fossils to the tree to track change or loss through time (as in snakes).

So, what about fish? ‘Fish’ is used to refer to pretty much anything that swims in water, but this lifestyle in animals like starfish (a relative of crinoids and sea urchins), jellyfish (a relative of corals) and cuttlefish (a relative of squid and octopus) evolved independently from more familiar fish like cod and carp. So, they’re not really ‘fish’ at all. With that in mind, how can we be fish? Well, the last common ancestor of, say, hagfish, salmon, shark and lungfish, is also the common ancestor of frogs, lizards, cats and us! All four-limbed animals with backbones descend from a fish-like ancestor. To complicate things further some have adapted to life back into the water and look much more like a ‘fish’ again, like dolphins, seals and the extinct ichthyosaur. Without a tree of life, we could not begin to unravel the evolutionary path that lead to all the diversity of life we see today.

You are closer to a chimp than a monkey, closer to a starfish than a snail, and closer to a mushroom than a tree. And, of course, there’s no such thing as a fish, but they still go well with chips.

*Joseph Jackson and Katherine Scruse had ten children, including the members of the Jackson 5, twenty-six grandchildren and several great-grandchildren.