Undeniable by Bill Nye

though, that just because we intuitively think of fish breeding as specific to one species and one species alone, it may be that nature allows a spectrum, with some able to interbreed and others not. They may look separate, like different breeds of dogs, while still being a part of the same species. The confusion between outward appearance and inner, genetic nature is also apparent—in a much more pernicious way—in the way people commonly talk about human races. (More on this in chapter 32.) Humans may have more trouble understanding nature than nature does is all I’m saying.

 
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    THE TREE OF LIFE—OR IS IT A BUSH?
    People routinely use their “family tree” to describe a diagram of all of their relatives. It’s such a common expression that you probably don’t even think about the underlying metaphor when you say it. Tracing our ancestry lends itself to depicting one path of lineage. You and any siblings you may have all branch off from your mother and father. If you have children they branch off from you and your partner, and so on. It’s a slightly confusing convention, since descendants should intuitively move down (descendants should descend, right?), but a tree grows upward (it ascends). Nevertheless, it’s a very useful metaphor, not just for family relationships but for the far broader ones between different kinds of life. So let’s climb on that metaphor and keep taking it further, step by step … or rather, branch by branch.
    As you climb down from the uppermost tips of your family tree, where you are, to the limb below, and then to the limb below that, you are going to meet up with people who came before you—perhaps ancestors from your grandmother’s spouse’s side of the family. Farther down, you’ll begin to cross paths with strangers; you’ll meet people you never met, if you get my drift—ancestors so distant you’ve never even heard of them. Even farther down the family tree we will come upon the branch where Homo sapiens parted ways with other humanlike species. Now the tree begins to include big-picture evolution. If we keep climbing down toward the roots, we come across chimpanzees and orangutans. Below that we would find apes and our ancient common ancestor. Descending farther, we move past the primates and start running into other mammals, including lions, tigers, bears, narwhals, springboks, osprey, and bats.
    If we continue our downward climb, we are going to come across branches leading to other organisms that, at first, might not seem that closely related to us (even my old boss…). I’m talking about lizards, fish, and marigolds. By logical extension, we realize that we must all be descendants of a single type of primordial organism at the very base of the tree. I know, I know; it’s a little hard to believe at first. But at the microbiological level, we are all much more alike than we are different.
    Every organism we can find here on Earth has DNA or its chemical partner, RNA. The acronym RNA is a contraction for ribonucleic acid, and it generally has just one strand compared to the more complex double-stranded structure that is DNA—deoxyribonucleic acid. (“Ribo” derives from an old word for sugar, which was made from a sweet compound called gum arabic. The adjectival form would be arabinose. From there, we end up with “ribo.” Go figure.)
    To me, the common code of life raises a powerful question, one that hits me personally. Through a surprising sequence of events in my life, I have become the chief executive officer of the Planetary Society, an organization cofounded by Carl Sagan, who was one of my professors in school. I took the job, because I cannot help but wonder if Earth is special or routine. And so I ask: Is the way life evolved here similar to the way life comes to be on other worlds, or are there drastically different evolutionary paths life can take? Put another