SETI, the search for extraterrestrial life that has gone on now for more than fifty years, has yielded nothing thus far other than the sound of some very earth-based crickets chirping. There’s a reason for that and Alfred Russel Wallace could have told the folks at SETI not to get their hopes up — and in fact did tell as much in Man’s Place in the Universe, first published in 1903.
A recent article in Nature hailed Wallace as the pioneer of astrobiology, noting that “He reviewed the physical conditions required for organic life in terrestrial ecosystems and concluded that Earth is the only habitable planet in the Solar System.” For Wallace the conditions required for life (much less the complex diversity of life found on earth) were so many that they make the existence of extraterrestrial life highly improbable. As he wrote:

It will be well here just to enumerate these conditions, which are all essential within more or less narrow limits:
Distance of planet from sun.
Mass of planet.
Obliquity of its ecliptic.
Amount of water as compared with land.
Surface distribution of land and water.
Permanence of this distribution, dependent probably on the unique origin of our moon.
An atmosphere of sufficient density, and of suitable component gases.
An adequate amount of dust in the atmosphere.
Atmospheric electricity.
Many of these act and react to each other, and lead to results of great complexity (p. 315).

Today modern researchers know much more about our galaxy, solar system, moon, earth, plate tectonics, magnetic fields, etc. than was known in Wallace’s day. Interestingly, however, the conclusion is the same: we are unique. In The Privileged Planet Guillermo Gonzalez and Jay Richards confirm Wallace’s conclusion of more than a century ago and add quite a few other conditions that Wallace never knew. For more on the life and career of this prescient scientist go to http://www.alfredwallace.org.
Professor Flannery is the author of Alfred Russel Wallace: A Rediscovered Life (Discovery Institute Press).