Those who believe that extraterrestrial intelligent life does not exist
argue that the conditions needed for life—or at least complex life—to
evolve are rare or even unique to Earth. This is known as the Rare Earth
hypothesis, which attempts to resolve the Fermi paradox by rejecting the
mediocrity principle, and asserting that Earth is not typical, but unusual
or even unique. While a unique Earth has had historical support on
philosophical or religious grounds, the Rare Earth Hypothesis uses
quantifiable and statistical arguments to argue that multicellular life is
exceedingly rare in the universe because Earth-like planets are themselves
exceedingly rare and/or many improbable coincidences have converged to
make complex life on Earth possible. While some have pointed out that
complex life may evolve through other mechanisms than those found
specifically here on Earth, the fact that in the extremely long history of
life on the Earth only one species has developed a civilization to the
point of being capable of space flight and radio technology seems to lend
more credence to the idea of technologically advanced civilization being a
rare commodity in the universe.

For example, the emergence of intelligence may have been an evolutionary
accident. Geoffrey Miller proposes that human intelligence is the result
of runaway sexual selection, which takes unpredictable directions. Steven
Pinker, in his book How the Mind Works, cautions that the idea that
evolution of life (once it has reached a certain minimum complexity) is
bound to produce intelligent beings, relies on the fallacy of the "ladder
of evolution": As evolution does not strive for a goal but just happens,
it uses the adaptation most useful for a given ecological niche, and the
fact that, on Earth, this led to language-capable sentience only once so
far may suggest that this adaption is only rarely a good choice and hence
by no means a sure endpoint of the evolution of a tree of life.

Another theory along these lines is that even if the conditions needed for
life might be common in the universe, that the formation of life itself, a
complex array of molecules that are capable simultaneously of
reproduction, the creation or extraction of all base components that it
uses to build itself, from the environment, and of obtaining energy in a
form that it can use to maintain the reaction (or the initial abiogenesis
on a potential life-bearing planet), might ultimately be very rare even if
worlds that might have the proper initial conditions for life might be common.

Insofar as the Rare Earth Hypothesis privileges Earth-life and its process
of formation, it is a variant of the anthropic principle. The variant of
the Anthropic Principle states the universe seems uniquely suited towards
developing human intelligence. This philosophical stance opposes not only
mediocrity, but the Copernican principle more generally, which suggests
there is no privileged location in the universe.

Opponents dismiss both Rare Earth and the anthropic principle as
tautological — if a condition must exist in the universe for human life to
arise, then the universe must already meet that condition, as human life
exists — and as an unimaginative argument. According to this analysis, the
Rare Earth hypothesis confuses a description of how life on Earth arose
with a uniform conclusion of how life must arise. While the probability of
the specific conditions on Earth being widely replicated is low, we do not
know what complex life may require in order to evolve.