...in the June 13, 1997, issue of Science [a report] by Frederico J. Gueiros-Filho and Stephen M. Beverley of Harvard describes the "Trans-kingdom Transposition" of a gene-size piece of DNA known as a transposable element (19). The particular transposable element they studied, called mariner, has already been found in planaria, nematodes, centipedes, many insects, and humans (20). Until recently, transposable elements were considered to be functionless, or "junk DNA." But John McDonald, a professor in the department of genetics at the University of Georgia, concludes, "It now appears that at least some transposable elements may be essential to the organisms in which they reside. Even more interesting is the growing likelihood that transposable elements have played an essential role in the evolution of higher organisms, including humans" (21).
... viruses could easily provide a way for new genes never before encountered by a species to become part of its genome. That viruses install new genes into their hosts is not speculative — it is a well known fact. That transferred genes are important in evolution is becoming well established. _Panspermia
Bacteria and parasites also play roles in evolution and speciation. This trans-species gene transfer phenomenon is easiest to see in bacteria :: bacteriophage interations, but has also been seen in vertebrates, with evidence of having occurred in humans.
Such evolutionary interactions between humans and microbes is not likely to have ended. The particularly high infection and infestation rates in tropical parts of the globe suggest that intense evolutionary pressures are being applied. The extremely high STD infection rates among particular population groups suggests that the interaction between genes, behaviour, and evolution is more complex than generally admitted.
Each individual human is actually a vast interacting colony of microbial and microscopic creatures. We have only barely begun to understand all the gene transfers and transformations which are occurring continuously.