Fraction of the above that develops intelligent life, fi[edit]
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
fi.
[36] Likewise, the Rare Earth hypothesis, notwithstanding their low value for
ne above, also think a low value for
fi dominates the analysis.
[37] Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable,
[38][39] implying an
fi approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See
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 the
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 our estimate of
fl but would indicate that in half the known cases, intelligent life did not develop.
Estimates of
fi 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
novae). Also, Earth's large moon may aid the evolution of life by
stabilizing the planet's axis of rotation.
There has been quantitative work to begin to define {\displaystyle f_{\mathrm {l} }\cdot f_{\mathrm {i} }}
. 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.
[40] [41]
