Ecologically diverse clades came to dominate the modern oceans because they were better buffered against the successive mass extinctions events which reshaped marine animals over evolutionary time - not because of their higher rates of speciation, according to a new study. The findings overturn a long-held understanding of the role of ecology in the evolution of animal life. In the modern ocean, ecological differentiation and taxonomic diversity are closely correlated. However, the origin and nature of the relationship between the two remains unresolved. For decades it's been broadly assumed that the divergence of a species into different ecological niches (ecological differentiation) is required to fuel species origination, leading to greater taxonomic diversity within ecosystems. Thus, for many clades, ecological differentiation is often thought to be the cause and/or consequence of high speciation rate. However, this hypothesis has been difficult to test, particularly across paleontological timescales and the extinction events that punctuate the history of animal life on Earth. Matthew Knope and colleagues used a comprehensive data set featuring both living (30,074) and fossil (19,992) marine animals to evaluate the relationship between ecological and taxonomic diversity and how it evolved from the late Cambrian to the present day. According to Knope et al., the relationship is more complex than expected and the findings suggest that the cumulative effect of mass extinctions played a crucial role in creating the strong correlation between ecological diversity and genus richness observed in modern oceans. Contrary to the prevailing hypothesis, the authors found that ecological differentiation in marine animal classes was instead associated with lower rates of species origination and that ecologically diverse classes are the most genus-rich today because they have been buffered against extinction. The results show that the observed strong association between ecological differentiation and taxonomic diversity is a relatively recent development, though shaped over 500 million years of evolutionary history.