The Tangled Tree: A Radical New History of Life

by David Quammen

outright on grounds that God’s acts of

information in DNA become physically animate?

By one estimate, the total mass of bacteria exceeds the total mass of all plants and animals on Earth. They have been around, in one form or another, for at least three and a half billion years, strongly affecting the biochemical conditions in which most other living creatures have evolved. That we don’t see bacteria is simply because our eyes are not calibrated to the appropriate scale. There may be more than a billion bacterial cells in an average ounce of soil, and five million in a teaspoon of fresh water, but we can’t hear their crackle or their fizz. A single kind of marine bacteria known as Prochlorococcus marinus, which drifts free in the world’s tropical oceans and photosynthesizes like a plant, may be the most abundant creature on Earth. One source places its standing population at three octillion individuals, a number that looks like this: 3,000,000,000,000,000,000,000,000,000.

Amazing passage

The radiation room and the darkroom were across the hall, and then storage, three more spaces barely bigger than closets. Woese gave Fox a table in his office, Fox said, with

branches fuse, like when algae and slugs come together.” She was right: the tree of life is

progressive lineages but without committing to specifics. Bronn himself was no evolutionist

so

the vicissitudes of time. Romer shaped a tree of all vertebrates, trees of the vertebrate classes, and trees of some of their subdivisions: fishes, amphibians, reptiles, mammals,

words that went unspoken. “I’m an assistant professor. I need to get tenure, and I need to ultimately become an associate professor

progenote. The progenote was a theoretical construct, as he explained. It was a hypothetical

was done. His twin brother, Jay, is also a medical researcher—one of the three scientists whose labs first isolated the causal

very damn useful.

general.” The fact that resistance genes could move sideways so easily was a clue that genes

All those researchers from Harvard and elsewhere wanted to understand why. The best clues lay in aspects of bdelloid life history that I’ve already mentioned: their tolerance for desiccation, their reproduction without sex. Desiccation can be damaging to membranes and molecules, even when a creature survives the drought, and biologists suspect that such drying-and-rehydrating stresses cause bdelloid DNA to fracture and leave cell membranes leaky. Given that they’re surrounded in their environments by living bacteria and fungi, plus naked DNA remnants from dead microbes, the porous membranes and fracturing could make it easy for alien DNA to enter even the nuclei of bdelloid cells and to get incorporated into bdelloid genomes as they repair themselves. Let me say that again: broken DNA, as a cell fixes it, using ambient materials, may include bits that weren’t part of the original. If that mended DNA happens to be in cells of the germ line, the changes will be heritable. Baby rotifers will get them and, when the babies mature, pass the changes along to their own daughters. Thus a bacterial or fungal gene can become part of the genome of a lineage of animals.

This is a key point!

Human cells are continually exposed to bacteria—the ones that live routinely in our guts and on our skin; the ones that infect us sometimes. That intimate juxtaposition has consequences. One consequence, unsuspected before but suggested by this Hotopp study in 2013, is that bits of naked bacterial DNA, possibly from broken-open bacterial cells, may often get integrated into cells (not necessarily germline cells) of a person’s body. Into cells of the stomach lining, for instance. Or blood cells. By “integrated,” what I mean is, not just absorbed or injected into the human cell but patched into its DNA. The good news about any such horizontal transfer, bacterial DNA into nongermline human cells, is that the change isn’t heritable. It won’t be passed to future generations. The bad news is that it might trigger cancer. How? By disrupting the cell genome in a way that allows runaway cell replication.

Amazing

your hotel. Himself, he would go back now to the lab for more work. We shook hands. Let’s