Aliens may be living among us, but we wouldn't know it because they'd be microbes that do not have the standard biochemistry of Earth-dwelling organisms.
As well as the many forms of life based on DNA that are known to science, the Earth may be home to "shadow life," a second creation of organisms that make up an unnoticed realm of "life as we don't know it," according to Paul Davies of Arizona State University, a cosmologist and theorist of extraterrestrial life.
Such "weird life" would never have been identified by scientists because the techniques we use for studying microbes are based on the familiar biological processes that drive the living things we understand, Davies told the American Association for the Advancement of Science conference in Chicago on Saturday.
The identification of such life on Earth could aid efforts to find life on Mars or elsewhere in the solar system.
A second terrestrial creation would also indicate that life arises easily when the conditions are right, suggesting that it is common throughout the universe.
The search for aliens should thus begin at home in a "mission to Earth," Davies said.