  It is generally considered that animal life was triggered by the rise of
oxygen levels. Based on experiments evaluating the minimum range of oxygen
levels at which sponges can survive, Mills and coauthors
(doi:10.1073/pnas.1400547111) defend the opposite view. However, the authors do
not demonstrate that "animal life was not triggered by the oxygen rise" is the
only possible and unique conclusion from their observation. In this reply, it
is suggested that a mechanism to explain the ability of sponges to survive at
low oxygen biota is Ocean Anoxic Events. These lead to oxygen depletion and a
series of them would selectively favor animals able to survive at low oxygen
levels. Thus, the origin of the ability of marine animals to survive in low
oxygen biota remains to be clarified.
