My chimp/chump story from last week is a simplified example of how one ape species split into two. This splitting is called speciation. The apes in the story are members of the species that was the most recent common ancestor of humans and chimpanzees (and bonobos.)
Why five million years ago? We infer that date from several pieces of evidence. We have 4.4 million year old fossils of creatures called australopithecines, who are our ancestors, but not chimp ancestors. The split must have happened before then.
DNA provides the link we have yet to find in the fossil record. We know how quickly DNA mutates on average. We count the differences between human DNA and chimp DNA to calculate the number of years it should take for those differences to accumulate. The answer is five million years.
We feel confident in the five million year figure because this method was also used to calculate the date of our split from orangutans. The fossil record confirms that the orangutan date is correct so we know the method works.
Now that we know when the human-chimpanzee split happened, we might wonder why it happened then. Five million years ago was the end of a harsh cold snap. Africa was colder and drier than it had been so the forests were breaking up and turning into woodlands. We all know about the mass extinctions at the end of the Cretaceous period. Climate changes killed off the dinosaurs and many other animals and gave the mammals a chance. Or when the internet liberated music and all music became free and corporate music died and only the most talented bands… OK, bad example.
The point is that environmental changes can drive evolutionary change. This is what happened five million years ago. It was getting really competitive in the trees. Those who were above average at tree living were more likely to survive to reproduce. Those who could make it elsewhere were also more likely to reproduce. Those who reproduce are likely to pass along whatever advantages they have to their children. The advantages don’t have to be very big to have a large effect. Even small advantages will become more common over a number of generations. Future mutations will increase existing advantages or create new ones. The mutations eventually add up to a huge change. It’s important to remember that this takes place over a very long time. It’s not like in the Ringo Starr movie Caveman in which people go from walking bent-over to walking fully upright in a minute.
If, as in this story, the proto-chimps stay in the trees and the proto-humans live on the ground, the two groups will stop inter-breeding. Eventually, so many differences will accumulate that the two groups can no longer inter-breed. Then we say that speciation has happened.
We will give speciation five Lemmys because we wouldn’t even be here without it.