You Are A Fish

Hi, welcome to MinuteEarth. I’m Jasper and you are a fish. Well… sort of. Of course, you don’t live in water. Nor do you have gills or fins. But there is certainly something fishy about you… and it has less to do with physical appearance, and more to do with evolutionary history and how we classify species.

So what makes a fish a fish? Well, traditionally, animals were grouped together based on shared physical characteristics, but this traditional strategy of grouping animals came long before we knew about evolution. Evolution revealed that all animals are related by ancestry, and the more recently they share a common ancestor, the more closely related they are. Sometimes animals that look very similar are actually very distantly related.

So today, scientists name groups of species using “clades” which take evolutionary relatedness into account. A clade is a group that includes a single most recent common ancestor and every one of that ancestor’s descendants. An easy way to identify a clade is to cut one – and only one – branch off the tree of life. All of the species that fall off will form a clade. And all species within that clade are more closely related to each other than they are to any species outside the clade.

For example, let’s look at the Great Ape section of the Tree of Life. We could cut here and have a clade for Humans and Chimpanzees. This shows us that we’re more closely related to Chimpanzees than Gorillas because Gorillas are left out. But we could also go up a bit and make a cut that includes Gorillas too. This is a larger clade and it shows us that we’re more closely related to both Chimpanzees and Gorillas than Orangutans. So the whole evolutionary tree is actually a Clade of Clades of Clades… all the way down to the tips.

To figure out how we relate to fish, let's pick two animals we currently call fish - say, a goldfish and a shark - and first look at how they're related to each other. Now let's use our Cladistic strategy and see if we can cut out a “fish” clade. To include both of these species, we’d need to cut the tree here, at the Gnathostomata branch. Now let’s add Humans to the tree and see where we are. That’s right!... We’re actually in the same Clade as goldfish and sharks because there’s no way to separate Humans from all fish without making more than one cut.

So if we wanted to group all the animals we currently call “fish” together under a Cladistic naming scheme, humans - and in fact all mammals - would also need to be part of that group. So is there something wrong with saying that Goldfish and Sharks are both fish, but Humans aren’t? No, it just means that category isn’t a clade and in some circumstances that may be more useful. But if you think about it, that traditional association of the word “fish” with “gilled, finned, water-dwelling animal” does blind us to this interesting truth: although goldfish are related to sharks, they’re actually more closely related to us.

So maybe you should go around telling everyone that you’re a fish. And when people give you confused looks, just tell them you swim with the cladistic school of thought.

You might have noticed some of the evolutionary trees in this video were created by an app. Well, we actually built that app so you should definitely check it out! You can use it to explore the Tree of Life and build your own trees! Just visit MinuteLabs.io and look for the Tree of Life Explorer. You can also keep up to date with our latest creations by subscribing to MinuteLabs on YouTube or supporting us on Patreon.