G. Wanger / JCVI / G. Southam /

Almost everywhere scientists go on Earth, they find life: the cold, dark depths of the oceans; snuggled up to piping-hot hydrothermal vents; buried under the Antarctic ice; and in South America's parched Atacama Desert. "Life can adapt to really tough conditions and, of course, most of the universe is going to be filled with habitats that are tough," Shostak says. For example, Mars is a harsh environment, but some of the microbes found on Earth, including the one shown here found deep in a mine, could survive beneath the surface of the Red Planet, he notes. These findings of so-called extremophiles have allowed scientists to scale back their list of requirements for extraterrestrial life. "We just say it has to have some liquid water, and maybe that's it," Shostak says.

An emerging line of thought even argues that multicellular life may be exceedingly rare in the universe because of a probable rarity of Earth-like planets. This line of reasoning has been dubbed the Rare Earth hypothesis[?]and relies on that fact that many improbable coincidences converged to make complex life on Earth possible. For example, the Earth just happens to have an axial tilt, period of rotation, and proper mix of light to heavy rock types in its crust to best support a vigorous hydrological cycle[?]. The reason we are so blessed is because of a chance event: the collision between the young Earth with a Mars-sized body 4,450 million years ago. This giant impact sent much of the felsic rich mantle of Earth into orbit, thus forming the moon, setting Earth into a much faster rotation, and creating our characteristic axial tilt. In addition, the removal of such a huge amount of light-rock types (felsic rock) allowed for the formation of the first ocean basins (which are composed of the heavier mafic rock types). Furthermore, the presence of different crustal rock types allows for the existence of plate tectonics, which is a vital component of the carbon cycle. The collision had to occur at a very precise angle; too direct and Earth would have been obliterated, too shallow and the Mars-sized body would have been deflected. This is just part of the Rare Earth hypothesis.