Intelligent Species Have Only Risen Once on Planet Earth

Now that's one argument, and we have to be humble in the face of uncertainty. Here, no one knows. But I want to give an argument that rarely gets any air time. The argument is that we are alone, and the argument has nothing to do with astronomy; it has everything to do with biology.

The argument goes like this: look at planet Earth and look at the number of species, not only that exist right now—millions of them—but the number of species that have ever existed on planet Earth—hundreds of millions. When life arose, something like three and a half billion years ago, for about two and a half billion years, for most of the history of life on Earth, there was nothing but bacteria. So life apparently doesn't have much impetus to evolve quickly beyond bacteria; it just remains as simple as possible.

A lot of people have this misconceived idea that Darwin really did away with the idea that evolution has a direction in mind. You see these pictures of evolution that appear in high school textbooks of the monkey that's hobbling around on all fours, and he's hunched over, and then eventually he's standing up and holding a briefcase, as if this is what evolution had in mind. It only seems to be what evolution had in mind in retrospect.

By looking backwards, this American academic, his name is Charlie Lineweaver, calls this the "Planet of the Apes" hypothesis, as if if you removed the humans from a planet, the apes would naturally evolve to fill the intelligence niche. He said you could imagine another situation where you're an elephant that is able to think about themselves, and they reflect on the length of their trunk, and they look back through biological evolution, and they see that trunks get ever shorter.

So what they conclude is, "Ah, evolution has been geared towards making ever longer trunks; that's what evolution is all about." Of course, we can see that that's ridiculous. It just happens to be the case that this creature called the elephant has evolved, and it's got this long trunk, but length of trunk doesn't appear to be a convergent feature of evolution.

So we say a convergent feature of evolution is a feature that exists within biological entities which has arisen again and again independently. Wings is my favorite example. Fish have wings of a certain kind; there's flying fish. Butterflies have wings, so we've got them in insects. They arose in mammals as well, with flying foxes and certain kinds of possums. And of course, birds and dinosaurs had wings as well.

So independently, in all these species, the wings keep on arising. So do eyes; so do organs for sound. But now let's think about the capacity to do mathematics or to build radio telescopes—in other words, to be an intelligent, creative species.

How many times has that arisen in geological history of the Earth? In one species and one species alone. Can we conclude on that basis that therefore it's inevitable that intelligent species will arise? If you were to repeat the experiment by sprinkling a little bit of bacteria around all the bio-friendly planets that exist throughout the universe, would you be guaranteed to get an entity like us?