The Tangled Tree: A Radical New History of Life

by David Quammen

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!