Editing Drake equation (section)

====Fraction of the above that develops intelligent life, {{math|''f''<sub>i</sub>}}====
This value remains particularly controversial. Those who favor a low value, such as the biologist [[Ernst Mayr]], point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for {{math|''f''<sub>i</sub>}}.<ref name="Ernst Mayr on SETI">
{{cite web
 |title       = Ernst Mayr on SETI
 |url         = http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html
 |publisher   = [[The Planetary Society]]
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20101206171624/http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html
 |archive-date = 6 December 2010
 |df          = dmy-all
}}</ref> Likewise, the Rare Earth hypothesis, notwithstanding their low value for {{math|''n''<sub>e</sub>}} above, also think a low value for {{math|''f''<sub>i</sub>}} dominates the analysis.<ref>Rare Earth, p. xviii.: "We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake or Sagan envisioned. However, ''complex'' life—animals and higher plants—is likely to be far more rare than commonly assumed."</ref> Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable,<ref name="acampbell.ukfsn.org">
{{cite web
 |last        = Campbell
 |first       = A.
 |date        = 13 March 2005
 |title       = Review of ''Life's Solution'' by Simon Conway Morris
 |url         = http://www.acampbell.ukfsn.org/bookreviews/r/morris.html
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20110716063324/http://www.acampbell.ukfsn.org/bookreviews/r/morris.html
 |archive-date = 16 July 2011
 |df          = dmy-all
}}</ref><ref>
{{cite book
 |last=Bonner |first=J. T.
 |year=1988
 |title=The evolution of complexity by means of natural selection
 |url=https://archive.org/details/evolutionofcompl0000bonn |url-access=registration |publisher=[[Princeton University Press]]
 |isbn=0-691-08494-7
}}</ref> implying an {{math|''f''<sub>i</sub>}} approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See [[#Criticism|Criticism]]).

In addition, while it appears that life developed soon after the formation of Earth, the [[Cambrian explosion]], in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the [[snowball Earth]] or research into [[extinction events]] have raised the possibility that life on Earth is relatively fragile. Research on any past [[life on Mars]] is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of {{math|''f''<sub>l</sub>}} but would indicate that in half the known cases, intelligent life did not develop.

Estimates of {{math|''f''<sub>i</sub>}} have been affected by discoveries that the Solar System's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for tens of millions of years (evading radiation from [[nova]]e). Also, Earth's large moon may aid the evolution of life by [[Rare Earth hypothesis#A large moon|stabilizing the planet's axis of rotation]].

There has been quantitative work to begin to define <math>f_\mathrm{l} \cdot f_\mathrm{i}</math>. One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.<ref name="Kipping2020">
{{cite journal
 |last1=Kipping
 |first1=David
 |date=18 May 2020
 |title=An objective Bayesian analysis of life's early start and our late arrival
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=117
 |issue=22
 |pages=11995–12003
 |doi=10.1073/pnas.1921655117|pmid=32424083
 |pmc=7275750
 |arxiv=2005.09008
 |bibcode=2020PNAS..11711995K
 |doi-access=free
 }}</ref><ref name="ColumbiaPR">
{{cite web
 |author1=Columbia University
 |title=New study estimates the odds of life and intelligence emerging beyond our planet
 |url=https://phys.org/news/2020-05-odds-life-intelligence-emerging-planet.html
 |website=Phys.org
 |access-date=23 May 2020}}
</ref>

Planetary scientist [[Pascal Lee]] of the [[SETI Institute]] proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since ''[[Homo erectus]]'' evolved, compared to 4.6 billion years since Earth formed).<ref>{{Cite web|last=Lee|first=Pascal|title=N~1: Alone in the Milky Way, Mt Tam|website=[[YouTube]]|date=24 October 2020 |url=https://www.youtube.com/watch?v=cuJDkIUuDBg| archive-url=https://ghostarchive.org/varchive/youtube/20211211/cuJDkIUuDBg| archive-date=2021-12-11|url-status=live}}{{cbignore}}</ref><ref>{{Cite web |last=Lee |first=Pascal |title=N~1: Alone in the Milky Way – Kalamazoo Astronomical Society |url=https://www.youtube.com/watch?v=wj5nmgoQr50 |url-status=live |archive-url=https://web.archive.org/web/20210315085249/https://www.youtube.com/watch?v=wj5nmgoQr50 |archive-date=2021-03-15 |website=[[YouTube]]|date=6 March 2021 }}</ref>