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Evolving the Alien: The Science of Extraterrestrial Life (published in the US, and UK second edition as What Does a Martian Look Like?: The Science of Extraterrestrial Life) is a 2002 popular science book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart.[1][2][3]

Evolving the Alien: The Science of Extraterrestrial Life
Evolving the Alien cover.jpg
Cover, first UK hardback edition
AuthorJack Cohen and Ian Stewart
CountryUnited Kingdom
PublisherEbury Press
Publication date
Media typePrint, e-book
Pages369 pp.
576.8/39 22
LC ClassQB54 .C64 2002b

The concept for the book originated with a lecture that Cohen had revised over many years, which he called POLOOP, for "Possibility of Life on Other Planets".[4]


Cohen and Stewart argue against a conception of extraterrestrial life that assumes life can only evolve in environments similar to Earth (the so-called Rare Earth hypothesis), and that extraterrestrial lifeforms will converge toward characteristics similar to those of life on Earth, a common trope of certain science fiction styles. They suggest that any investigation of extraterrestrial life relying on these assumptions is overly restrictive, and it is possible to make a scientific and rational study of the possibility of life forms that are so different from life on Earth that we may not even recognise them as life in the first instance.

Cohen and Stewart return to two contrasts throughout the book. The first is between exobiology (which considers the possibilities for conventional, Earth-like biology on Earth-like planets) and xenoscience (which embraces a much broader and more speculative range for the forms that life may take).[5] The second contrast drawn is between parochials (features of life that are likely to be unique to Earth) and universals (features that are likely to appear wherever life arises).

Cohen and Stewart supplement the limited hard science available on their subject with numerous references to science fiction of both their own and others' creation, including 28 plot summaries of classic SF stories.[6][5]


In its review of Evolving the Alien, the London Review of Books commented that Cohen and Stewart's speculative approach is useful in exposing errors in conventional thinking, and is "painlessly educative" on the current state of the applicable sciences, but criticized their "naive optimism" with respect to future technological breakthroughs in human space travel.[5]

In his review for Nature, Lawrence M. Krauss admitted to not being completely sure of the book's intended goal and audience, but noted that it may serve as a counterpoint to arguments put forth in Peter Ward and Donald Brownlee's book, Rare Earth.[7]


  1. ^ Jack Cohen and Ian Stewart: Evolving the Alien: The Science of Extraterrestrial Life, Ebury Press, 2002, ISBN 0-09-187927-2
  2. ^ Jack Cohen and Ian Stewart: What Does a Martian Look Like: The Science of Extraterrestrial Life, Wiley, 2002
  3. ^ Jack Cohen and Ian Stewart: What Does a Martian Look Like: The Science of Extraterrestrial Life, Ebury Press, 2004, ISBN 0-09-188616-3
  4. ^ Stepney, Susan (October 30, 2002). "Author: Jack Cohen". Retrieved 28 April 2015.
  5. ^ a b c Hamilton-Paterson, James (March 6, 2003). "Forget the Klingons". London Review of Books (Vol. 25, No. 5). pp. 19–20. Retrieved 28 April 2015.
  6. ^ Cowie, Jonathan. "Non-Fiction Review: Evolving the Alien". Concatenation. Retrieved 28 April 2015.
  7. ^ Krauss, Lawrence M. (January 9, 2003). "Aliens Unlimited". Nature. 421: 114–115. Bibcode:2003Natur.421..114K. doi:10.1038/421114a.