Brown algae are a large group of algae . In many ways , the evolution of the brown algae parallels that of the green algae and red algae , as all three groups possess complex multicellular species with an alternation of generations . Analysis of 5S rRNA sequences reveals much smaller evolutionary distances among genera of the brown algae than among genera of red or green algae , which suggests that the brown algae have diversified much more recently than the other two groups .