Either way, Peter Ward has the beginnings of an answer. In a new book, the University of Washington paleontologist puts forth an expanded "tree of life," or biological classification system, to account for a variety of life forms that would not fit in the current system.
Among them are viruses, long considered to be non-living bits of protein and nucleic acid but which Ward argues are as alive as the many parasites that infect humans and other organisms. The revamped classification system also would include life based on RNA instead of DNA, and life found away from Earth that likely would be based on silicon or elements other than the carbon-hydrogen-oxygen-nitrogen mixture that is the backbone of life on Earth.
"To get to DNA life you had to go through non-DNA life, which we no longer have," Ward said. "But just because a type of life goes extinct doesn't mean you don't classify it. Otherwise you wouldn't have dinosaurs on the tree of life. And until now there hasn't been any place to put RNA life."
In the current popular classification system the highest levels are three domains ?bacteria, archaea and eukarya, the last of which includes all animals. Ward's plan places those three domains within a larger dominion, which he calls "terroan" to signify Earth origins. Another dominion he calls "ribosa" because it is based on ribonucleic acid, or RNA. Other dominions could be formed to cover life discovered to have a different base than DNA or RNA.
The dominions would be placed within broader classifications called "arborea," which contain life that does not mix with that of other arborea. The Earth arborea would contain all life forms found on this planet and other arborea would contain life found awa
Source:University of Washington