Among the many celestial bodies that the ancient Greeks told stories about, they knew of two stars that were quite interesting. They said one star (Phosphorus) brought the light of day, because it appeared in the morning. The other star was Hesperus, the evening star, who was son of Eos. In the present day, we have a name for both of these astronomical bodies. We call them both Venus.

The Greeks didn’t realize that the “morning star” and “evening star” were actually the same object. This eventually became apparent to them, but they retained the same names for each twilight version of Venus.

 

Pictured here, are two things that we still call by different names.

A wing and an arm. In an evolutionary sense, these are the same thing. These are both vertebrate fore-limbs.

In both the case of the two stars and the limbs we have a confusion that stems from separation by time. Venus is seen at dawn and at dusk but cannot be seen in the middle of the night. There is an observational gap between when we can and cannot see Venus. The same is true for the bird wing and the gorilla arm. We can see two ends of an evolutionary transition but not the time that connects the two.

This may not seem to be true at first, but it absolutely is. To understand it better, you have to consider two different ways of thinking about time itself.

The time separating the morning star from the evening star is linear. That means at one point in time you can see the evening version and then later on you can see the morning version.

In contrast, we typically cannot see evolution occur in linear time. In order to see evolution, we usually view it in U-shaped time. Evolution typically occurs in a fashion we call “tree-like”. That means that single lineages branch into multiple lineages. The result is that we can see only the two ends of the process, but not the common beginning.

 

 

To make it easier, imagine that those branches aren’t rigid, but made of spaghetti. Pull one end and you’ll see that the arm and the wing are at two ends of the same timeline, and their common ancestor is the middle of that timeline.

The morning star and evening star are the same thing, just as an arm and a wing are the same thing. In both cases, they appear different to us because we are observing them at different points along their timeline.

This type of evolutionary relationship is called “homology”. In biology, things are homologous if share the same evolutionary origin. The vertebrate limbs one example, but homology isn’t always this simple. But this is a topic for another day, and another blog.

 

Dominic Evangelista has a PhD in biology from Rutgers University. Learn more about his research, or follow him on twitter (@roach_brain) and ask him a question about evolution!