Skip to main content
    • Aa
    • Aa

Five- or six-step scenario for evolution?

  • Brandon Carter (a1)

The prediction that (due to the limited amount of hydrogen available as fuel in the Sun) the future duration of our favourable terrestrial environment will be short (compared with the present age of the Earth) has been interpreted as evidence for a hard-step scenario. This means that some of the essential steps (such as the development of eukaryotes) in the evolution process leading to the ultimate emergence of intelligent life would have been hard, in the sense of being against the odds in the available time, so that they are unlikely to have been achieved in most of the earth-like planets that may one day be discovered in nearby extrasolar systems. It was originally estimated that only one or two of the essential evolutionary steps had to have been hard in this sense, but it has become apparent that this figure may need upward revision, because recent studies of climatic instability suggest that the possible future duration of our biologically favourable environment may be shorter than had been supposed, being only about 1 Gyr rather than 5 Gyr. On the basis of the statistical requirement of roughly equal spacing between hard steps, it is argued that the best fit with the fossil record is now obtainable by postulating the number of hard steps to be five, if our evolution was exclusively terrestrial, or six, if, as now seems very plausible, the first step occurred on Mars.

Hide All
P.C.W. Davies (2003). Does life's rapid appearance imply a Martian origin? Astrobio. 3, 673679. arXiv:astro-ph/0403049.

A. Leger , J.M. Mariotti , B. Mennesson , M. Ollivier , J.L. Puget , D. Rouan & J. Schneider (1996). Could we search for primitive life on extrasolar planets in the near future? Icarus 123, 249255.

J. Tarter (2001). The search for extraterrestrial intelligence (SETI). Ann. Rev. Astron. Astrophys. 39, 511548.

K. Caldiera & J.F. Kasting (1992). The life span of the biosphere revisited. Nature 360, 721723.

J.W. Schopf (1993). Microfossils of the early Archaean Apex chert: new evidence of the antiquity of life. Science 260, 640646.

D.C. Catling , C.R. Glein , K.J. Zahnle & C.P. McKay (2005). Why O2 is required by complex life on habitable planets, and the concept of planetary oxygenation time. Astrobio. 5, 415438.

L. Arnold , S. Gillet , O. Lardière , P. Riaud & J. Schneider (2002). A test for the search for life on extrasolar planets. Looking for the terrestrial vegetation signature in the Earthshine spectrum. Astron. Astroph. 392, 231237 (arXiv:astro-ph/0206314).

N.Y. Kiang , A. Segura , G. Tinetti , Blankenship R.E. Govindjee , M. Cohen , J. Seifert , D. Crisp & V.S. Medows (2007). Spectral signatures of photosynthesis II. Coevolution with other stars and the atmosphere of extrasolar worlds. Astrobio. 7, 252274 (arXiv:astro-ph/0701391).

J.J. Brocks , G.A. Logan , R. Buick & R.E. Summons (1999). Archean molecular fossils and the early rise of eukaryotes. Science 285, 10331036.

M. Livio & A. Kopelman (1990). Life and the Sun's Lifetime. Nature 343, 25.

M. Livio (1999). How rare are extraterrestrial civilizations, and when did they emerge? Astroph. J. 511, 429431 (arXiv: astro-ph/9808237).

M.A. Line (2002). The enigma of the origin of life and its timing. Microbiology 148, 2127.

C.H. Lineweaver & T.M. Davis (2002). Does the rapid appearance of life on Earth suggest that life is common in the universe? Astrobio. 2, 293304 (arXiv:astro-ph/0205014).

Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

International Journal of Astrobiology
  • ISSN: 1473-5504
  • EISSN: 1475-3006
  • URL: /core/journals/international-journal-of-astrobiology
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Altmetric attention score

Full text views

Total number of HTML views: 1
Total number of PDF views: 14 *
Loading metrics...

Abstract views

Total abstract views: 123 *
Loading metrics...

* Views captured on Cambridge Core between September 2016 - 17th October 2017. This data will be updated every 24 hours.