  Cooperation is a persistent behavioral pattern of entities pooling and
sharing resources. Its ubiquity in nature poses a conundrum. Whenever two
entities cooperate, one must willingly relinquish something of value to the
other. Why is this apparent altruism favored in evolution? Classical solutions
assume a net fitness gain in a cooperative transaction which, through
reciprocity or relatedness, finds its way back from recipient to donor. We seek
the source of this fitness gain. Our analysis rests on the insight that
evolutionary processes are typically multiplicative and noisy. Fluctuations
have a net negative effect on the long-time growth rate of resources but no
effect on the growth rate of their expectation value. This is an example of
non-ergodicity. By reducing the amplitude of fluctuations, pooling and sharing
increases the long-time growth rate for cooperating entities, meaning that
cooperators outgrow similar non-cooperators. We identify this increase in
growth rate as the net fitness gain, consistent with the concept of geometric
mean fitness in the biological literature. This constitutes a fundamental
mechanism for the evolution of cooperation. Its minimal assumptions make it a
candidate explanation of cooperation in settings too simple for other fitness
gains, such as emergent function and specialization, to be probable. One such
example is the transition from single cells to early multicellular life.
