“We may never find other life away from Earth, but we have already made aliens on this planet and we will continue to do so at an increasing pace,” says Peter Ward, author of Life As We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life (Viking, 2005). “In the last five years we’ve come to realize that we can make microbial life in a lot more ways than Mother Earth did.”

Aliens on this planet? Ward is talking about laboratory work here on Earth that has modified life as we commonly understand it. That includes creating microbes with at least one amino acid beyond the 20 found in the DNA of native Earth life. Genetic modification also constitutes, in Ward’s view, the creation of an alien lifeform, as does modifying a lifeform to reduce its complexity. Ward, a paleontologist who studies these matters within the University of Washington’s astrobiology program, is perhaps best known to Centauri Dreams readers as the co-author (with Donald Brownlee) of Rare Earth: Why Complex Life is Uncommon in the Universe (Springer, 2000), a book arguing that highly developed life is so rare in the cosmos that humans are unlikely ever to encounter it.

There are ample reasons why Rare Earth remains controversial, especially as we study the findings of exoplanetary science, and Ward’s new work will surely raise some eyebrows as well. The paleontologist presents a tree of life that attempts to account for lifeforms that don’t fit into conventional classification systems. A case in point: the virus, normally considered a non-living combination of protein and nucleic acid. Ward argues that viruses are in fact alive. He would also extend his classification system to include life based on RNA instead of DNA:

“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.”

To make the classification work, Ward creates a dominion he calls terroan, to signify Earth origins; the dominion is a broad umbrella that covers the standard top domains of bacteria, archaea and eukarya, the last of which includes all animals. He also creates a ribosa dominion to cover life based on ribonucleic acid, or RNA, and would consider forming other dominions as needed for life found to have a different base than DNA or RNA. Such life might be based on silicon or elements other than the mixture of carbon, hydrogen, oxygen, and nitrogen that is the basis of life on Earth. The entire Earth with all its dominions would then be placed in a higher classification called an arborea; each living world would thus have its own arborea, those forms of life that do not mix with other arborea.

Centauri Dreams tends to agree with Ward’s pessimism about finding complex life anywhere near Earth, but also agrees wholeheartedly that we need to revise our conventional analysis of life to include what we are possibly going to find in extreme environments like Titan or Mars, and perhaps even in the atmospheres of the gas giants. The suspicion here is that life is all but ubiquitous, even if mostly non-sentient, and the notion that all living things must be carbon and water-based may not stand the test of outer Solar System exploration.