Constructive evolution and the emergence of complex cells

Abstract

The octopus is an incredibly complex organism that learns hunting strategies, displays social behaviors and memorizes its local environment. To see how evolution gave rise to octopuses, we have to understand how evolution constructs new functions, structures and behaviors. In many cases, new functions – like the ability to learn – emerge from an ensemble of smaller components – such as neurons – which initially arose for another purpose – say muscle coordination. One of the largest leaps in evolutionary biology was the origin of complex cells, eukaryotes, which involved many innovations at the cellular and genomic level. An important step in the origin of eukaryotes was an endosymbiosis (a merger) between two cells and their genetic material, giving rise to the mitochondrial and nuclear compartments. We designed a computer model to study if and how the merger between two simple cells can drive the evolution of complexity and found that cells in endosymbiosis indeed evolve greater complexity. The reason for this is that selection for fast growth, which precludes high levels of complexity in free-living cells, is weakened in endosymbiosis due to the appearance of new functional constraints. Specifically, cells in endosymbiosis also have to coordinate their growth to be successful, since one cell growing faster than its partner impedes the symbiotic relationship. Using our model and bioinformatic analyses, we show how complexity (could have) emerged before, during and after the origin of eukaryotes. Our results show that evolution is an incredibly creative process, allowing organisms to adapt to many different challenges, and that some of these challenges may in fact be responsible for the evolution of complex organisms, eventually giving rise to such fascinating creatures as birds, octopuses and humans.