  One of the unique features associated with the Earth is that the fraction of
its surface covered by land is comparable to that spanned by its oceans and
other water bodies. Here, we investigate how extraterrestrial biospheres depend
on the ratio of the surficial land and water fractions. We find that worlds
that are overwhelmingly dominated by landmasses or oceans are likely to have
sparse biospheres. Our analysis suggests that major evolutionary events such as
the buildup of O$_2$ in the atmosphere and the emergence of technological
intelligence might be relatively feasible only on a small subset of worlds with
surface water fractions ranging approximately between $30\%$ and $90\%$. We
also discuss how our predictions can be evaluated by future observations, and
the implications for the prevalence of microbial and technological species in
the Universe.
