Shadows of Forgotten Ancestors

At some point, probably extremely early on, organisms had to be more than a single molecule, no matter how talented that molecule might be. For very precise instructions to be followed to the letter, for reproduction to occur with high fidelity, other molecules were needed—to scour building blocks from the adjacent waters and bend them to your purpose; or, like DNA polymerase, to be midwife in the replication process; or to proofread a newly minted set of genetic instructions. But it did you no good if such accessory molecules kept drifting out to sea. What you needed was a kind of trap to keep useful molecules captive. If only you could surround yourself with a membrane that, like a one-way valve, lets in the molecules you need and doesn’t let them out … There are molecules that do that—that, for example, are attracted to water on one side of them, but are repelled, absolutely revolted by water on the other. They’re common in Nature. They tend to make little spheres. And they’re the basis of cell membranes today.

The earliest cells, although able simultaneously to multiply and divide, could not possibly have been conscious in anything like the sense that humans are. Still, they had certain behavioral repertoires. They knew how to copy themselves, of course; how to convert molecules from the outside, different from them, into molecules on the inside that were them. They were preoccupied with improvements in the precision of replication and the efficiency of metabolism. Some could even distinguish sunlight from darkness.

Breaking down molecules taken in from the outside, that is, digesting food, can be done safely only in a step-by-step fashion, each step controlled by a given enzyme, and each enzyme controlled by its own ACGT sequence, or gene. The genes then must work together in exquisite harmony; otherwise none of them will propagate into the future. In digesting a molecule of sugar, for example, the meticulously choreographed action of dozens of enzymes is required, each picking up where the last one left off, each enzyme manufactured by a particular gene. The defection of a single gene from the common enterprise can be fatal to all of them. An enzyme chain is only as strong as its weakest link. On this level, genes are single-mindedly dedicated to the general welfare of their tribe.

Early enzymes had to be discriminating; they had to take care not to decompose the very similar molecules that constituted the lifeform they were part of. If you digest yourself—the sugars that are part of your DNA, say—you don’t leave many descendants. If you don’t digest others—convenient repositories of organic raw materials and finished molecular goods—you may not leave many descendants either. Cells of 3.5 billion years ago must have possessed some knowledge of the difference between “me” and “you.” And “you” was more expendable than “me.” A dog-eat-dog or, at least, a microbe-eat-microbe world. But wait …

A time came—perhaps 2 or 3 billion years ago—when one being could incorporate another whole. One would nuzzle up to the other, the cell walls or membranes would pucker, and the littler fellow would find itself inside the bigger. Attempts at digestion, with varying success, doubtless ensued. Suppose you are a largish one-celled organism in the primitive oceans who in this way gobbles up some photosynthetic bacteria, tiny specialists who know how to use sunlight, carbon dioxide, and water to manufacture sugars and other carbohydrates. You’ll leave more descendants if you’re better than your competitors in acquiring sugar (a key building block needed to replicate your genetic instructions and to power all you do).

But suppose also that these ingested bacteria—the latest, sturdy, rustproof models—do not succumb to your digestive enzymes. For all they know, they’ve found their way into a molecular Garden of Eden. You protect them from many of their enemies; because you’re transparent, sunlight shines into you for them; and there’s plenty of water and carbon dioxide around. So inside you, the bacteria continue to do their photosynthetic magic. Some sugars leak out of them, for which you are grateful. Some of them die and their interior molecules spill out, available for your use. Others of them flourish and multiply. When the time comes for you to reproduce, some of them wind up inside your offspring. Not yet de jure (because nothing of this arrangement is yet encoded in the nucleic acids), but certainly de facto, an accommodation has been reached between your descendants and theirs.²