Undeniable by Bill Nye Page B

to another branch and another bifurcation. That’s why that Tree of Life metaphor is so powerful.
    Darwin himself coined the phrase “tree of life,” after he had drawn a sketch depicting the branching points in the natural history of living things. As we study the relationships between species today, we see a great many more species in the present day than in the distant past. This is true even after taking into account the five (or six) major mass extinction events. What all this diversity and creation of new species implies is that you and I, as animals on the Tree of Life, are related to all kinds of other organisms that we might not expect to be related to.
    For a century and a half, scientists of all stripes (biologists, paleontologists, archeologists, pathologists, immunologists, astrobiologists) have been classifying all of life, living and extinct, to fill in all the branches on that Tree of Life. You might think, then, that by this time we’d have it all sketched out. Certainly you’d expect that we’d all agree about the main branches. Well, we don’t. But we’re workin’ on it.
    With the discoveries in the 1970s of organisms that were heretofore unknown to science, we’ve had to rethink which living thing is related to which other living thing. For a long time, scientists generally agreed that there were animals and plants. Wow, Bill. Thanks. Seriously, and they are still often given the organizational designation: Kingdom Animalia and Kingdom Plantae, as we still Latinize biology words. But after thinking about this in terms of evolution—that is, in terms of which organism came into existence before which other organism—scientists realized that animals and plants have much more in common with each other than they (or we) have with almost all of the other living things here on Earth. It’s a stunning fact. Most of the living things on our planet are microscopic, and those microscopic organisms are less like you than you are like a cabbage.
    Scientists have climbed down the Tree of Life; or you might say that they’ve moved right to left on the time line of evolution. Either way, they’ve reevaluated who seems to be related to whom. As I write, we now consider nature to have given rise to three or four foundational types of living things, or domains of life. I’m going with four. We have Bacteria, Archaea (microbes that are fundamentally different from bacteria), Eukarya (that’s us, animals and plants together), and Vira. You could also call that last one Viruses. (I took some Latin in school, and I prefer this style of pluralization for this particular second declension noun, describing this particular domain of living or nearly living things.)
    Not everyone agrees with me that Vira deserve a domain. A traditional argument is that viruses are not really, fully living things. They need a host cell to reproduce; they cannot reproduce on their own. Viruses make no effort to take in their own nutrients. They do not maintain a steady metabolism. They remain intact for extraordinary periods with no interaction with their surroundings. No energy comes in or goes out.

    My view is that viruses would not exist at all if we did not have other domains of living things for them to parasitize. Vira interact with the other forms of life, which makes them more like us than they are different, and certainly more like life than like nonlife. The recent discovery of giant viruses like Mimivirus and Pandoravirus, so large that they blur the line between virus and bacterium, strengthens the case. At the same time, Vira clearly do not belong to any of the other three domains. For me, they should have their own significant branch on the Tree of Life.
    After we’ve sorted out life’s domains, it becomes a bit tangled as to whether we should continue to go with the old, increasingly fine designations. If we did, it would go domain, kingdom, phylum, class,