How do we know that all life on earth is related?

A very reasonable question. Evolutionary biologists like to drop this fact as if it’s obvious. And in a sense it is once you know what you’re looking at. But it can be tricky to explain from first principles. Let’s use my favourite analogy.

Imagine we take a trip to a museum. In my imagination this is always the National Museum of Scotland in Edinburgh. A building I fell in love with as soon as I saw it when I went there as a student. In the museum there are lots of different rooms with different artefacts. And let’s imagine that we walk into a room that’s full of all of different kinds of boats. For the purposes of this story it doesn’t matter whether they’re original boats or reproductions but they’re all ancient, very old boats from thousands of years ago. So we have to imagine we’re walking through this room and we see a hollowed out canoe from somewhere in Central America and a reed boat from the Nile and an animal skin covered canoe from somewhere in North America and a Polynesian log boat and all sorts of other ancient types of boat and we wander through the room looking at the different kinds of boats and then notice a plaque on the wall you know the usual explanatory sign that you get in any room in a museum. And this plaque says that these are all examples of boats from ancient cultures. And it goes on to note that all of these boats are so old that they were all developed before any of these cultures had any contact with each other. In other words, all of the boats were built by people who had never seen any of the other types of boat.

Now imagine we stop and look back at the boats and wonder if this is really true because all of the boats look kind of similar. They’re all long and narrow in one dimension and they’re all hollow fairly there. They’ve all got at least one pointy end. Some have two pointy ends. They’ve all got a fairly smooth surface. And if we look at the other bits, they all seem very similar too. So the boats that have oars, the oars are long and have thin handles and broad-bladed paddles. And the ones that have sails have large sails made out of something lightweight and they’re generally put up on some sort of pole. And the ones that have rudders, to steer the boat, the rudders all look similar. They’re all at the back and they’ve got broad blades similar to the paddles.

And so we may be forgiven for doubting whether the sign is true, for doubting whether all of these boats really were built completely independent of each other. Because if none of the builders of these boats had ever seen any of the other boats, how come they all ended up with the same design?

The answer to this puzzle is not too tricky to figure out. The boats look the same because of physics, because there is one natural best design that’s objectively best for things that have to move through the water. Things that move through the water have to be smooth, so they don’t create a lot of drag, and narrow in one direction for the same reason. And if those things are going to stay on the surface of the water, then they need to be hollow or light. And if they’re going to carry useful things, like people or cargo, then it’s better for them to be hollow inside, so that you can fit cargo in. And the same argument is true for things like the oars. Oars work best if they’re long so they can act as a lever to amplify human muscle power and if they have broad blades at the end so that they can move a lot of water. Sails work best if they’re large to catch a lot of air but light and they catch a lot of air best if they’re relatively high up on a pole that we tend to call a mast. So there are good objective reasons for all of these things to end up looking the same.

When we look at the animal world, we see similar patterns. If we look at fish and dolphins and squid and seals, they all have kind of similar shapes. They’ve all got smooth bodies and at least one pointy end to move through the water efficiently. And if they’ve got fins or flippers, they tend to be shaped a little bit like oars. They’ve got broad, flat, thin material to be able to move a lot of water around. And for this reason, we can’t look at similarities between, for example, sharks and dolphins, and conclude that they must be closely related because they look similar. They look similar not because they’re closely related but because they look the way that their bodies have to look to move efficiently through the water. Famously, this is called convergent evolution by people in the business.

Incidentally, we can tell that sharks and dolphins are not closely related as soon as you look at the tail fins because on sharks they go up and down, and on dolphins they go left to right. And that’s even before you think about the fact that dolphins breathe air.

Now let’s imagine ourselves back in that room in the museum. Same boats, same sign on the world that says all of these boats were developed completely independently. But this time after reading the sign we go back for a closer look at the boats and we notice that they all have names. Never all got names painted or carved into the side or the back. Now that’s not unusual in itself. We could easily imagine that many cultures have independently come up with the idea of naming their boats. There are lots of reasons not to want to do this. But imagine we look closer and we see that all the names are written in the same language. We have the same script. They use the same alphabet. And to take it a step further, let’s imagine that we can read the script. And all of the boats have exactly the same name.

Now, in your imagination, you can imagine whatever name you like for the boat. It doesn’t make any difference. But immediately we can see that boats having the same name written in the same script is something fundamentally different to having similar shapes. We’re not surprised when boats have similar shapes independently, because the shapes are the best for moving through water because of physics. But there’s no sense in which one name for a boat is better than another. And there’s no sense in which one alphabet is better than another. And so if we actually see this feature on all of the boats, we see that they have the same name written in the same script, then we would have to conclude that the sign is false.

We would have to conclude that all of these names came from a common source. It’s just far too unlikely that a bunch of boat builders all independently came up with the same alphabet to represent the sounds of their language, and then all came up independently with the exact same name. So we would have to conclude that these boats had either been copied from one another or copied from some other common ancestor that isn’t still around.

Well, it turns out that when we look at all the different forms of life that we have so far discovered, we see something very similar to the situation with the names of the boats. I’m deliberately not going to go into too much detail here. But it turns out that when we look at the chemical language of all the cells of all different organisms that have ever been discovered, they have certain similarities. I’m talking about things like their genetic code, ribosomes, things that are common to all different types of cells, all different kinds of organisms. And they’re similar in such a way that it couldn’t come about by chance and couldn’t come about because they’ve converged on the one best way. When we look at, for example, the genetic code that’s shared by all life, it’s similar in a way that could really only have come about if it was all copied from a common ancestor.