Episode Two of Cosmos, called One Voice in the Cosmic Fugue, is where Carl Sagan loses some folks. This episode is all about evolution, natural selection, and just how it is that we humans are cousins to trees.


Trees? Really? We don’t look like trees. We don’t act like trees. Isn’t it kind of insulting to compare me to something that seems so barely alive as a tree?

For me, Sagan lays out a convincing argument. We and trees are made of the same stuff. Our cells use the same systems, the same chemistry, the same code book. Any tree could read my dna sequence. When you look into the deep past, you find trilobite fossils, but no trees, and no people either. We’ve both evolved since those times. We had to come from somewhere. It only makes sense that we came from the life that was here before us. As we go back in time, those life forms we find in the fossil record converge to single-celled organisms. It makes sense that both we and trees, organisms made of many, many cells, would have evolved from single-celled organisms. And on and on.

But for some people the argument doesn’t work. Why? I wish I knew.

We are alive. Trees are alive. That itself is amazing, because there are so many more ways of being dead than alive. The fact that we share so much in common with trees, the fact that we’re both alive, is a big clue that we’re cousins. But what if “alive” were just a condition, like “acid” or “alkaline.” Couldn’t “alive” have happened again and again, and just look the same, just like all rust looks the same because it’s all the same chemical, or all protons in the universe have the same properties? What makes “alive” so different from “rust” or “proton?”

The answer is that life, our life on Earth, made certain choices. There are signatures, in places like the dna codebook, the particular proteins used for particular tasks, and on and on that show that all life on Earth is descended from a single instance, one particular day when something extraordinary happened.

In certain circles, it is considered important to argue about when life begins. If you learn the story that science tells, you discover that life, our life, had a single beginning on this planet, around four billion years ago. That realization renders the question meaningless. The cells in your body right now, wiggling about, taking in nutrients, burning oxygen, sending signals, are descendents of that first day, that single instance, that still unknown, but common, ancestor.

There’s an unbroken chain between then and now. There was never, never, a time when the cells that connect us to that time weren’t alive. Even the sperm cell and the egg cell that came together to form you were alive, before and after they joined. We are all, people and trees, turtles and butterflies, connected by a four billion year unbroken chain of life.

And that is a story worth knowing!