Today, biologists no longer think that the development of life on Earth was an improbable event, one unlikely to have occurred twice in the galaxy or even the entire universe. Some now think that carbon-based life like our own is bound to arise on any Earth-like planet, and that Earth-type bacteria can even spread through space via asteroidal and cometary impact ejecta to colonize accommodating habitats anywhere. Hence, the universe could be teeming with life, even if it is likely to be mostly microbial in size (as it is on Earth itself).

© Wim van Egmond
(used with permission)



Photo from Ciliates.



Earth-type lifeforms found
elsewhere will likely be most
commonly represented by
microbes (as they are on
Earth), such as these two
single-celled protoctists,
Euplotes (left) and Stylonychia,
that move with hairlike cilia.

A key test of this hypothesis would be to ascertain whether carbon-based life ever arose in Earth-like habitats that may have once existed, or still persist, elsewhere in the Solar System -- on Mars, on Venus, and in other planetary bodies (including sub-ice oceans on Europa and other large moons of the outer gas giants) -- as well as around other stars. In 2007, some interesting papers were also published about the likely visual wavelengths or colors of light that may be used for photosynthesis by "alien plants" living under stars bluer or redder than our own Sun. Elsewhere, however, life may take on forms and rely on biochemical processes that are not yet familiar.

For more information, see:

• Life on Earth


• Life on Mars?


• Life on Venus?


• Life on Europa?


• Life Elsewhere?


• Plants under Alien Suns