2012-10-22 – So many different types of dogs. Sheep dogs. Chihuahuas. Pit bulls. Poodles. Collies. It’s amazing that they are all dogs. And yet they can mostly interbreed. With all the variation, they are a single species.

We are amazed by this because we tend to look at the adults of the species. Animals acquire their distinguishing features little by little through development. We don’t see this because developing animals are hidden from us. Developing mammals are hidden in wombs. Developing birds and reptiles, amphibians and fish are hidden in eggs.

Anti-abortion partisans like to make unborn humans visible to us in the hope of engaging our empathy. But they don’t show us unborn animals. Unborn humans are more similar in appearance to unborn dogs than they are to born humans.

And this is not true just of dogs and humans. The similarities run to all sorts of animals. Here is a link to images of unborn elephants, dogs, and dolphins. We all start out pretty much the same.

I raise this because our mental image of adult animals has a profound effect on how we view evolution. Getting from the adult ape to the adult human seems like such an insurmountable obstacle. But adult apes did not evolve into adult humans. The path doesn’t go from adult to adult. It passes through embryonic stages in between. Evolution makes tiny changes in developmental pathways that result in adult forms. But take a look at the embryos. The changes emerge slowly in the development of an animal.

Think of it this way. If I am in Los Angles and want to fly to Chicago, I’m going to start flying in the same direction as if I’m flying to New York. Well, not exactly the same direction, but close. An observer watching both planes will have a hard time telling the difference between the Chicago flight and the New York flight. Not an impossible time, but a difficult time. The differences will become greater as time passes and will become undeniable when the New York-bound flight continues east past Chicago.

The development of different types of animals is similar. The older the embryo, the more it begins to resemble the adult form. Differences are slight at the beginning, but the differences grow.

Evolution happens when a slight variation occurs at the embryologic stage that has an effect on the survival of the animal.

The weird thing about all of this, is that the huge and improbable changes are the ones that occur between conception and the eventual adult form of an animal. Differences between species are minor in comparison, at least when viewed at the embryonic stage. And they don’t occur within a few weeks or months, as the developmental changes do. They take millions of years.