Now again, before we really dive into evolution and natural selection, we need to spend a little bit of time just sort of thinking about how we think about things. And in this video, we're going to look at ladder versus tree thinking. This is just sort of this idea that historically, when we thought about organisms, people would create this hierarchy of complexity. Where today, we really try to focus on evolutionary relationships.
So let's look at these two ways of thinking. First, we'll talk about ladder thinking, and ladder thinking focuses on this hierarchy of species. Now this really comes from Aristotle, the Greek philosopher, and he said that species were unchanging. They had different levels of complexity, and that's really what divided the species, their complexity levels. He created what became known as the Scala Naturae or the great chain of being.
This ordered species from low complexity. At the lowest complexity, Aristotle put plants. And it went up to high complexity, which I bet you can imagine what he put on top, put humans. This way of thinking really dominated Western thought up through and past the Middle Ages. So if we look at this sort of ladder here, here's our hierarchy.
You see plants at the bottom and as you go up, you get invertebrates, fish, mammals, humans, and often people would even just put gods at the top there. Well, that's well and good. But if you really think about it, are humans more complex? Certainly, if you think about the central nervous system, yeah. We probably are more complex than other organisms.
But if you think about other traits, like camouflage. An octopus, well, it's an invertebrate, but it can change its shape, its skin texture, and its skin color to hide. If we think about plants, they can do photosynthesis. They can process sunlight to produce usable energy in the cell.
We can't do that at all. So today, we really want to think about tree thinking. In tree thinking, we're going to say that species are related through common descent. Thinking about relatedness gives us this evolutionary tree view of the world, and that allows us to put our focus on species relationships. Right?
So when I look at this tree, an important thing is there is no hierarchy. There's no thing on top. These things are all at the top of the tree because they're all alive today. And that gives us sort of this little bit of insight that I really want you to know that all populations have been evolving for the same amount of time. So what differentiates species isn't necessarily complexity.
Lots of species are complex in specific ways and not complex in other ways. What differentiates them is how well they fit in their environment. Species have been changing through time to fit in their environment where they reproduce and survive the best. Now when we think of variation, this means that these shared evolutionary histories can explain a lot of species variation. So now when we look at a tree like this, we can ask a question like, well, why do these organisms have bones?
The organisms on this tree that have bones, we can follow their descent with modification. We can follow their ancestry back, and we see they share a common ancestor. So answering questions like that, understanding why things have specific adaptations, those relationships provide us a lot of insights.
Now, again, you may think, well, that seems obvious. I wouldn’t use ladder thinking, but we do need to be careful with our thought. Because a lot of times, words saying things like, bacteria are a lower organism, slip into our language. And while you may know what you mean, it can undermine our thinking in subtle but important ways.
So we're going to practice with tree thinking some more coming up. You should check it out.