Shadows of Forgotten Ancestors

Either you adapted to the oxygen, or you hid from it, or you died. Many died. Some reconciled themselves to live underground, or in marine muds, or in other environments where the deadly oxygen could not reach. Today all of the most primitive organisms—that is, the ones least related by genetic sequence to the rest of us—are microscopic and anaerobic; they prefer to live, or are forced to live, where the oxygen isn’t. Most organisms on Earth these days deal well with oxygen. They have elaborate mechanisms to repair the chemical damage done by oxygen, as—gingerly, held at molecular arm’s length it is used to oxidize food, extract energy, and drive the organism at high efficiency.

Human cells, and many others, deal with oxygen through a special, largely self-contained molecular factory called a mitochondrion, which is in charge of dealing with this poison gas. The energy extracted by oxidizing food is stored in special molecules and safely shipped to workstations throughout the cell. Mitochondria have their own kind of DNA—circles, or daisy chains, of As, Cs, Gs, and Ts, rather than double helices, instructions different at a glance from those that run the cell proper. But they’re enough like the DNA of the chloroplasts to make it clear that mitochondria also were once free-living bacteria-like organisms. The central role of cooperation and symbiosis in the early evolution of life is again evident.

Luckily for us, biochemical solutions were found to the oxygen crisis. If not, perhaps the only life on Earth today other than photosynthetic plants would be slithering in ooze and sucking at thermal vents in the abyssal depths. We have risen to the challenge and surmounted it—but only at enormous cost in the deaths of our ancestors and collateral relatives. These events show that there is no inherent foresight or wisdom in life that prevents it from making, in the short term at least, catastrophic mistakes. They also demonstrate that, long before civilization, life was producing toxic wastes on a massive scale, and for that miscalculation paying stiff penalties.

Through some such biochemical oversight, had things gone a little differently, perhaps all life on Earth would have been extinguished. Or perhaps some devastating asteroidal or cometary impact would have killed off all those tentative, fumbling microbes. Then, as we’ve said, organic molecules—both those synthesized on Earth and those falling from the skies—might have led to a new origin of life and an alternative evolutionary future. But the day comes when the gases leaking out of volcanos and fumaroles are no longer hydrogen-rich, no longer easy to make organic molecules from. Part of the reason is the oxygen atmosphere itself, which oxidizes these gases. Also, there gets to be a time when extraterrestrial organic molecules arrive so infrequently that they are an insufficient source of the stuff of life. Both these conditions seem to have been satisfied by around 2 or 3 billion years ago. Thereafter, if every living thing were to be wiped out, no new life could arise. The Earth would remain a desolate wasteland of a world into the remote future—until the Sun dies.

——

Back then, around 2 billion years ago or a little before, the oxygen in the Earth’s atmosphere—steadily increasing, to be sure, over preceding ages of geological time—began quickly to approach its present abundance. (In today’s air, one in every five molecules is O₂)

The first eukaryotic cell evolved a little earlier. Our cells are eukaryotes, which in Greek means, roughly, “good nuclei,” or “true nuclei.” As usual, we chauvinistic humans admire it because we have it. But they’ve been very successful. Bacteria and viruses are not eukaryotes, but flowers, trees, worms, fish, ants, dogs, and people are; all the algae, fungi, and protozoa, all the animals, all the vertebrates, all the mammals, all the primates. One of the key distinctions of the eukaryotic cell is that the governing machinery, the DNA, is encapsulated and set apart in a cell nucleus. As in a medieval castle, two sets of walls protect it from the outside world. Special proteins bond and contort the DNA, enveloping and embracing it, so a double helix that uncoiled would be about a meter long is compressed into a submicroscopic chamber at the heart of the cell. Perhaps the nucleus evolved—in the oxygen-rich vicinities of photosynthetic organisms—in part to protect DNA from oxygen while the mitochondria were busily exploiting it.

Each long DNA double helix is called a chromosome. Humans have 23 pairs of chromosomes. The total number of As, Cs, Gs, and Ts is about 4 billion pairs of letters in our double-stranded hereditary instructions. The information content is roughly that of a thousand different books with the size and fineness of print of the one you’re reading at this moment. While the variation from species to species is large, similar numbers apply to many other “higher” organisms.