Shadows of Forgotten Ancestors

It’s a good deal for both parties. They open up a little fast-food concession stand inside your body, at hardly any cost to you. You provide a stable and protected environment for them (so long as you take care not to digest your guests). After many generations have passed, you’ve evolved into quite a different kind of being, with little green photosynthetic power plants inside of you reproducing when you do, clearly part of you, but also clearly different. You’ve become a partnership. This seems to have happened a half dozen times or more in the history of life, each instance leading to a different major group of plants.³

Today every green plant contains such inclusions, called chloroplasts. They are still rather like their free-living one-celled bacterial ancestors. Nearly every bit of green in the natural world is due to chloroplasts. They are the photosynthetic engines of life. We humans pride ourselves on being the dominant lifeform on this planet, but these tiny beings—unobtrusive, the perfect guests—are in a sense running the show. Without them, almost all life on Earth would die.

They’ve made many concessions to their hosts. They’ve achieved a working mutual assistance pact of long duration, called symbiosis. Each partner relies on the other. Still, the chloroplasts are recognizably a latecomer to the cell. The clearest sign of their separate origin is the difference between their nucleic acids and the plant cell’s own nucleic acids, although long ago they had a common ancestor. The signature of their separate, early evolution before joining forces is plain. The original chloroplast seems to have come from a photosynthetic bacterium very much like those living in stromatolite communities today.⁴

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You look at these little one-celled beings under the microscope and you’re struck by their apparent self-assurance. They seem to know with such certainty what they’re about. They swim toward the light or attack prey or struggle to escape from predators. Because they’re transparent, you can see their internal parts, the DNA-driven protoplasmic clockwork, making them go. Their ability to transmute the food they come across into the molecules they need—for energy, for parts, for reproduction—is downright alchemical. The plants among them convert air and water and sunlight into themselves not haphazardly, but according to specific recipes, the mere writing out of which would fill many volumes on organic chemistry and molecular biology. Each of them is only one cell; no organs, no brains, no snappy conversation, no poetry, no higher spiritual values—and yet they can do, without any apparent conscious awareness, far more along these chemical lines than can our vaunted technology.

And there’s something else they can do that we can’t. They can live forever. Or nearly so. These asexual, one-celled organisms reproduce by fission—not nuclear, but biological fission. A little furrow, an indentation, appears and ripples down the middle of the organism. The internal parts are divided more or less evenhandedly, and suddenly we have before us not one organism but two. It has split in half. We now see two smaller beings, each nearly identical to its single parent and genetically the same, identical twins. Quickly, each grows to adult size. Later, the process continues. Except for the odd mutation, remote descendants are perfect facsimiles of their ancestors. In a real sense, the ancestors never died. At no point along the way are there corpses of aged parents. If there are no accidents, no drop of poison released by other microbes, no extremes of temperature, no running out of food, no encounters with a big, bad amoeba, then they continue to live on, the natural slow falling to pieces of their organic body parts mitigated or reversed by their frequent reproduction.

These ubiquitous, invisible, and most humble organisms are immortal—at least by human standards. There are enough natural vicissitudes that they cannot go for too long without encountering one disaster or another. But at least some of them live for more lifetimes than the most extravagant and credulous disciple of reincarnation or “multiple life regression” ever imagined. The current official record is held by a laboratory stock of the one-celled organism called Paramecium, familiar to high-school biology students. Eleven thousand successive generations of paramecia have been carefully nurtured in the test tube, with no senescence or aging apparent.⁵ (In humans, eleven thousand generations would take us all the way back to the dawn of our species.) Except for the slow buildup of mutations, the paramecia at the end of this train of generations were genetically identical to those at the beginning. In a way, the longing for immortality, so characteristic of Western civilization, is a longing for the ultimate regression into the past—to our single-celled ancestors in the seething primeval ocean.

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