Libby, E.,Ratcliff, W. C.

Introduction: Multicellularity is one of the major transitions that allowed the evolution of large, complex organisms, fundamentally reshaping Earth's ecology (1). Early steps in this process remain poorly resolved, because known transitions occurred hundreds of millions of years ago (2) and few transitional forms persist. It is generally accepted that the first steps toward multicellularity were the formation of cellular clusters, followed by the success or failure of those clusters depending on their traits. As clusters of cells adapted, cells lost their evolutionary autonomy, becoming mutually reliant parts in a new higher-level whole (1, 3). This transition may be facilitated by a “ratcheting” process in which cells adopt traits that entrench them in a group lifestyle, stabilizing the group and paving the way for the evolution of multicellular complexity.