Animal evolution is often presented as a march toward
complexity, with different living animal groups each
representing grades of organization that arose through
the progressive acquisition of complex traits. There are
now many reasons to reject this classical hypothesis.
Not only is it incompatible with recent phylogenetic
analyses, but it is also an artifact of ‘hidden biology’,
that is, blind spots to complex traits in non-model species.
A new hypothesis of animal evolution, where many
complex traits have been repeatedly gained and lost, is
emerging. As we discuss here, key details of this new
model hinge on a better understanding of the Porifera
and Ctenophora, which have each been hypothesized to
be sister to all other animals, but are poorly studied and
often misrepresented.