DNA has been known to jump from one species to another in a phenomenon called horizontal transfer. Sometimes, the DNA of parasites will jump into the genomes of their hosts, and evidence of this host-parasite interaction can be preserved for millions of years. Researchers studying the genomes of birds have discovered the "DNA fossil" of a parasitic roundworm that, at least nowadays, infects only mammals. The findings were published in Nature Communications last week. 

Small stretches of DNA that can jump around from one place to another are called transposable elements. Bird flu and HIV/AIDS, for example, are known to have jumped into our species from their original animal hosts. Lymphatic filariasis (better known as elephantiasis) and loiasis (or African eye worm) are tropical human diseases that affect 170 million people. We know they're caused by the nematode worms Brugia, Wuchereria, and Loa, which are spread by mosquitoes, but their ancestral host range is still unknown. 

A couple years ago, a team led by Uppsala University’s Alexander Suh discovered a new type of transposable element that occurs in certain bird genomes but not others. They called it AviRTE. Curiously, the phylogeny of AviRTE sequences didn’t match the bird evolutionary family tree – suggesting that AviRTE likely originated in a non-avian genome and had jumped to birds multiple times. 

"We made a list of all the possible parasites of birds that might be in a position to transfer their DNA to the germ line cells of birds. We started trying to amplify AviRTE from parasite samples, without any luck," study co-author Christopher Witt from the University of New Mexico told IFLScience. "Then finally, Alex struck gold by finding AviRTE in a published nematode genome." 

Based on their DNA database search, the only other animals that share this newly discovered transposable element are nematode worms that parasitize mammals today, including humans. The team then analyzed DNA sequence divergence among all AviRTE copies from nematodes and birds. 

"DNA fossils" of the worms were found in seven different groups of bird spanning all of the tropics: trogons, mesites, parrots, hummingbirds, hornbills, manakins, and tinamous (pictured above). Furthermore, the horizontal transfer between worms and birds in the tropics occurred in two episodes: around 25 to 22 million years ago involving the Brugia and Wuchereria lineages and around 20 to 17 million years ago with the Loa lineage. 

But AviRTE is not found in mammalian genomes. "The fact that there are no AviRTEs in mammals suggests that these worms did not parasitize mammals at that time when it was actively 'jumping,'" Witt added. "Now they do infect mammals, and they’re a major public health problem for tropical humans."