Last week in the New York Times, renowned science writer and NG Phenomena blogger Carl Zimmer discussed bacteria that live only inside other bacteria that live only inside a certain bug. The smaller bacteria has whittled down its genome to only perform a few specific functions–the rest of life’s processes are handled by the world it lives in (which just so happens to be a living thing of its own).
This led him to wonder “How simple can life get?” and to answer that “the true essence of life is not some handful of genes, but coexistence.”
After uttering a Keanu-Reeves-like “whoooa,” I figured we could get an entire blog post out of that. Carl agreed. Here is an edited version of our correspondence:
If the true essence of life is coexistence or inter-dependency, can things larger than one organism be alive?
CZ: There’s no question that interconnectedness is a feature of all living things, from the smallest microbe to the biggest tree. And obviously, an ant colony or some other group of cooperating organisms is made up of living things. [Explore swarm theory with NG Explorer Iain Couzin.] But I don’t think that a group of interdependent species has to be a distinct organism of its own. That’s more of a matter of how well defined the boundaries are between the things that make it up.
Your heart can’t exist outside your body, and you can’t exist naturally without your heart, and your heart and the rest of your body carry the same DNA. So there’s no boundary separating you and your heart. On the other hand, the E. coli in your gut has different DNA, reproduces on its own schedule, and can live outside your body. So you and your E. coli are not one, I’d say.
And on yet another hand, our hearts are powered by mitochondria in our cells–little factories for building fuel. They evolved from bacteria that became permanent residents in our cells. They still have a little of their original DNA, but a lot of their genes have been moved into our own genome. I think in that case, the line has been effectively blurred.
In your article you talked about the bug’s genome containing DNA from bacteria that no longer even live within it. Blurring another line, should we think of those bacteria as in some way still living on?
CZ: I wouldn’t say that the missing bacteria that donated their genes to mealybugs are still alive, any more than I’d say my great-great-great-grandfather is still alive, because a small fraction of my DNA has a sequence that matches his.
[But] cooperation has been crucial to life since the beginning. Ever since then, some genes have moved from one species to another. Life in general has hybrid origins.
What about going in the other direction: Is one cell within a multi-cellular organism alive in the same way a single-celled organism is?
CZ: This all depends on what you mean by “life,” “living,” and “alive.” A beating heart is surely “living,” although I’d call it “living tissue,” rather than a distinct organism.
But what words should we use when human cells can survive in labs long after their original owner is dead, as in the case of Henrietta Lacks [whose cancerous cells have continued to divide decades after her death]? Some scientists have even suggested her cells have become a separate species.
What about gametes? A fish that spawns in a river sends out sperm or eggs that float about doing their own thing entirely.
CZ: I would agree that gametes are alive. They have genes, can sense their environment, maintain a stable environment inside them, and so on. But so are the cells in your heart.
You’ve also written a lot about viruses, which occupy a strange place between having genes, but not really being “alive” themselves. Where do they fit in?
CZ: A number of scientists are rethinking the “viruses-are-not-alive” mantra. It’s hard to see how something that’s dead can take over a living cell with the sophistication that viruses use. A number of virus species actually sit in their host cell like miniature cells, drawing in molecules and producing new viruses.
Some researchers call viruses “non-cellular life,” and I think there’s a lot to consider in that title. [Explore more research on viruses from NG Explorer Nathan Wolfe.]
Well we’ve talked about things alive and not-quite alive at every scale there is. Are there any things you can think of that we consider to be alive at both the individual and multi scale?
CZ: We are made up of trillions of cells that produce a higher level of life … I’d say that our own bodies are a great example of something alive at different scales.