Genes are normally inherited from parents, but they can also be inserted into a genome by viruses, plasmids, and other foreign agents — a phenomenon called horizontal transfer. Bacteria are promiscuous gene swappers, but horizontal transfer has been documented in only a few multicellular organisms: a handful of plants, insects, and fishes.

That short list just grew notably longer, thanks to a study by geneticists at the University of Texas at Arlington.

John K. Pace II, his graduate advisor Cedric Feschotte, and two colleagues were studying the genome of the bush baby, a nocturnal African primate, when they discovered a group of transposons — long DNA strands that can move around and copy themselves within the genome. Surveying GenBank, a database of gene sequences, the team was surprised to uncover the same transposons in other vertebrate genomes: tenrec (a hedgehog), little brown bat, mouse, rat, opossum, green anole lizard, and African clawed frog all had them. But the other twenty-seven vertebrate genomes in the database did not.

Only horizontal transfer could explain how a small group of distantly related species came to share the same transposons.

Originally, the transposons may have jumped from a parasitic or prey species independently to each of the eight vertebrates, perhaps carried by an infectious virus. Or they may have jumped to one vertebrate and from there to the others. In any case, the transfers didn’t happen yesterday. The amount of genetic variation between the transposons indicates that they jumped between 15 million and 46 million years ago.

The findings were detailed in the Proceedings of the National Academy of Sciences.