The following decade saw an explosive recognition of the bizarre, counterintuitive phenomenon called horizontal gene transfer and the role it has played throughout the history of life. That explosion occurred during the 1990s but had deep precedents, even before Woese’s work opened the door to appreciating its unimaginable prevalence and significance.

The first recognition by science that any such thing as H.G.T. might be possible dates to 1928, when an English medical researcher named Fred Griffith first detected a puzzling transformation among the bacteria that cause pneumococcal pneumonia: one strain changing suddenly into another strain, presto, from harmless to deadly virulent. At the Rockefeller Institute in New York during the 1940s, Oswald Avery and two colleagues identified the “transforming principle” in such instantaneous transmogrifications as naked DNA — that is, genes, moving sideways, from one strain of bacteria into another. To say that seemed odd is an understatement. Genes weren’t supposed to move sideways; they were supposed to move vertically, from parents to offspring — even when the “parents” were bacteria, reproducing by fission. But by 1953, the great Joshua Lederberg, then at the University of Wisconsin, had shown that this sort of transformation, relabeled “infective heredity,” is a routine and important process in bacteria. Still more unexpectedly, as later work would reveal, H.G.T. is not unique to bacteria.

Slowly at first, during the 1980s and early 1990s, H.G.T. became a favored research focus in more than a few labs. Many researchers had followed Woese’s lead, using ribosomal rRNA as the basis for comparing one organism with another, judging relatedness and constructing trees of life. But then, as new tools and methods made gene sequencing easier and faster, and as more powerful computers allowed analysis of the vast troves of genomic data, researchers went far beyond 16S rRNA, comparing other genes and whole genomes. What they found surprised them: that many genes had moved sideways from one lineage of life into another. Such genes might be absent from most living species within a group (say, a family of butterfly species), implying that it was absent too from the common ancestral form, but it might show up unexpectedly in one species of butterfly in that family, matching closely to a gene that exists only in another kind of creature (say, a bacterium), classified to an entirely different part of the tree of life. How could that happen? If the gene was absent from the common ancestor, it couldn’t have gotten into the anomalous butterfly species by vertical descent.

Researchers have identified three primary mechanisms by which H.G.T. occurs, each of which has a formalized label: conjugation, transformation and transduction. Conjugation is sometimes loosely called “bacterial sex.” It occurs when two individual bacteria (they needn’t be of the same species) form a copulation-like connection, and a segment of DNA passes from one to the other. (It’s isn’t really bacterial sex because it involves gene exchange but not reproduction.) Transformation is what Fred Griffith noticed in 1928: uptake of naked DNA, left floating in the environment after the rupture of some living cell, by another living cell (again, not necessarily of the same species). Transduction is a sort of drag-and-drop trick performed by viruses, picking up bits of DNA from cells they infect, then dropping those DNA bits later within other infected cells, where they may become incorporated into the genomes.

Conjugation was known to be widespread and common among bacteria. H.G.T. by transformation and transduction could potentially occur among other creatures too, even eukaryotes — even animals and plants — though that prospect was far more uncertain and startling, into the 1990s and beyond. Then improved genome sequencing and closer scrutiny brought more surprises. A bacterium had sent bits of its DNA into the nuclear genomes of infected plants. How was that possible? A species of sea urchin seemed to have shared one of its genes with a very different species of sea urchin, from which its lineage diverged millions of years earlier. That was a stretch. Still another bacterium, the familiar E. coli, transferred DNA into brewer’s yeast, which is a fungus. Brewer’s yeast is microbial, a relatively simple little creature, but nonetheless eukaryotic. This mixing of fungal host and bacterial genes happened via a smooching process that looked much like bacterial transformation, the researchers reported, and “could be evolutionarily significant in promoting trans-kingdom genetic exchange.” Trans-kingdom is a long way for a gene to go.

New investigations, as time passed and improvements in gene-sequencing technology made more complete genomes available, showed that far more radical leaps were happening, and not infrequently. For instance: There’s a peculiar group of tiny animals known as rotifers, once studied only by invertebrate zoologists but now notable throughout molecular biology for their “massive” uploads of alien genes. Rotifers are homely beyond imagining. They live in water, mainly freshwater, and in moist environments such as soils and mosses, rain gutters and sewage-treatment tanks. Some species favor harsh, changeable environments that sometimes go dry, and their individuals reproduce without sex. Despite the absence of sexual recombination, which shuffles the genetic deck in a population and offers new combinations of genes, these rotifers have managed to find newness by other means. One means is horizontal gene transfer. Three researchers at Harvard and Woods Hole sequenced portions of the genome of a certain rotifer and found all sorts of craziness that shouldn’t have been there. More specifically, they found at least 22 genes from other creatures, most of which, they concluded, must have arrived by H.G.T. Some of those were bacterial genes, some were fungal. One gene had come from a plant. At least a few of those genes were still functional, producing enzymes or other products useful to the rotifer.

Some of these individual cases were later challenged, but the trend of discoveries held. H.G.T. also started showing up among insects. Again this was supposed to be impossible. There were fervent doubters. Alien genes cannot move from one species to another, they insisted. The germ line of animals, meaning the eggs and the sperm and the reproductive cells that give rise to them, is held separate from such influences. It’s sequestered behind what biologists call the Weismann barrier, named for August Weismann, the German biologist of the 19th and early 20th centuries who defined the concept. Bacteria cannot cross that barricade, the Weismann barrier — so said the skeptical view — to insert bits of their own DNA into animal genomes. Impossible. But again it turned out to be possible.