Editing Drake equation (section)

====Fraction of the above that actually go on to develop life, {{math|''f''<sub>l</sub>}}====
Geological evidence from the Earth suggests that {{math|''f''<sub>l</sub>}} may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that [[abiogenesis]] may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains [[anthropic bias]], as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical [[hypothesis testing]] standpoint, without assuming that the underlying distribution of {{math|''f''<sub>l</sub>}} is the same for all planets in the Milky Way, there are zero [[degrees of freedom (statistics)|degrees of freedom]], permitting no valid estimates to be made. If life (or evidence of past life) were to be found on [[life on Mars|Mars]], [[Europa (moon)|Europa]], [[Enceladus]] or [[Titan (moon)|Titan]] that developed independently from life on Earth it would imply a value for {{math|''f''<sub>l</sub>}} close to 1. While this would raise the number of degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.

Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for [[bacteria]] that are unrelated to other life on Earth, but none have been found yet.<ref>
{{cite journal
 |last=Davies |first=P.
 |year=2007
 |title=Are Aliens Among Us?
 |journal=[[Scientific American]]
 |volume=297 |issue=6 |pages=62–69
 |doi=10.1038/scientificamerican1207-62
|bibcode = 2007SciAm.297f..62D }}</ref> It is also possible that life arose more than once, but that other branches were out-competed, or died in mass extinctions, or were lost in other ways. Biochemists [[Francis Crick]] and [[Leslie Orgel]] laid special emphasis on this uncertainty: "At the moment we have no means at all of knowing" whether we are "likely to be alone in the galaxy (Universe)" or whether "the galaxy may be pullulating with life of many different forms."<ref>
{{cite journal
 |last1=Crick |first1=F. H. C.
 |last2=Orgel |first2=L. E.
 |year=1973
 |title=Directed Panspermia
 |url=http://profiles.nlm.nih.gov/ps/access/SCBCCP.pdf |archive-url=https://web.archive.org/web/20111029060655/http://profiles.nlm.nih.gov/ps/access/SCBCCP.pdf |archive-date=2011-10-29 |url-status=live
 |journal=[[Icarus (journal)|Icarus]]
 |volume=19 |issue=3 |pages=341–346
 |bibcode=1973Icar...19..341C
 |doi=10.1016/0019-1035(73)90110-3
}}</ref> As an alternative to abiogenesis on Earth, they proposed the hypothesis of [[directed panspermia]], which states that Earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship".

In 2020, a paper by scholars at the [[University of Nottingham]] proposed an "Astrobiological Copernican" principle, based on the [[Principle of Mediocrity]], and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, {{math|''f''<sub>l</sub>}}, {{math|''f''<sub>i</sub>}}, and {{math|''f''<sub>c</sub>}} are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).<ref>{{cite journal |last1=Westby |first1=Tom |last2=Conselice |first2=Christopher J. |title=The Astrobiological Copernican Weak and Strong Limits for Intelligent Life |journal=The Astrophysical Journal |date=15 June 2020 |volume=896 |issue=1 |pages=58 |doi=10.3847/1538-4357/ab8225|arxiv=2004.03968 |bibcode=2020ApJ...896...58W |s2cid=215415788 |doi-access=free }}</ref><ref>{{cite news |last1=Davis |first1=Nicola |title=Scientists say most likely number of contactable alien civilisations is 36 |url=https://www.theguardian.com/science/2020/jun/15/scientists-say-most-likely-number-of-contactable-alien-civilisations-is-36 |access-date=19 June 2020 |work=The Guardian |date=15 June 2020}}</ref>