Two years ago a New Scientist headline announced the “world’s first baby born with new ‘3 parent’ technique.” Whereas an embryo is usually produced by one sperm and one egg, this technique uses genetic material from three separate people. First performed by a New York fertility clinic in Mexico to evade US legal restrictions, the procedure has now been replicated several times. A clinic in Ukraine started offering a similar procedure in late 2016 (albeit on shaky ethical and medical grounds), while regulatory authorities in the United Kingdom approved its use for two women earlier this year.

THE TANGLED TREE: A RADICAL NEW HISTORY OF LIFE by David Quammen Simon & Schuster, 480 pp., $30.00

The cases in the UK and Mexico each involve a woman who carries a rare disease of her mitochondria, the cellular structures that produce energy in our cells. Mitochondria have their own DNA and can harbor their own genetic diseases. These are passed on solely through the maternal line—mitochondria are transmitted down by eggs but not sperm. One approach to blocking transmission of these illnesses involves inserting the DNA-filled nucleus from the egg of the woman into a donor egg full of healthy mitochondria but stripped of its own nucleus. Fertilize that hybrid egg with a sperm, and presto! A child could be born nine months later with DNA from three people and without a catastrophic mitochondrial disorder.

This result could be a blessing—but the science behind it sounds confusing. Isn’t DNA supposed to be our sacred blueprint, wound up in chromosomes and locked up safely in the nucleus of our cells? What are these mitochondrial creatures doing with their own DNA, replicating within our cells and bequeathing diseases to our offspring? If they seem like invaders, it’s because they once were. Mitochondria, it turns out, were originally bacteria; their free-wheeling existence came to an end one day deep in evolutionary history when they entered another single-celled organism and started a new life inside. That receiving cell was the ancestor of all animals, plants, fungi, and protists. The origin of the mitochondria is reflected in the basic shape of its DNA: It is looped in a simple circle, just like a bacteria’s, unlike the linear chromosomes found in our nuclei.

Children conceived with a third person’s mitochondria are, it follows, the offspring of three parents, but the third parent is, in at least some sense, the descendant of an ancient bacteria. After those bacteria took up residence in our common ancestor, they proceeded to swap genetic material with the nucleus, further blurring the boundaries between host and invader, self and other over the eons.

This is not what we think of as Darwinian evolution, the transmission of genes and traits down the family line. DNA, it turns out, can also be passed laterally, between individuals, including those of different species. This discovery represented a tectonic shift in our understanding of nature, a story that David Quammen tells wonderfully in his exhaustively researched book, Tangled Tree: A Radical New History of Life. Crisscrossing the country to interview scientists and visit labs, Quammen provides a vivid portrait of the scientific process, and of the quarrelsome, quirky, (in one instance) evil, and brilliant scientists behind it. We may like to think of DNA as the neat bequest of our parents, the fusion of two unique, circumscribed human lineages. Yet it is—and we are—something more: short strands within a vast interwoven genetic web, stretching back to the earth’s earliest days, linking all living things.