In the early Tertiary period, the ecosystems were vastly different from today, offering unique opportunities for smaller mammals to thrive. During this time, animals the size of a bobcat could dominate their habitats, showcasing the diversity and adaptability of life on Earth. This era was characterized by the evolution of various species that would later fill ecological niches as larger dinosaurs had become extinct.

Bill Bryson's "A Short History of Nearly Everything" highlights how size and adaptability played critical roles in the survival of species during significant evolutionary transitions. The reference to creatures like bobcats illustrates that dominance in nature is not solely reserved for large predators, but that smaller animals were able to leverage their size and lifestyle to become successful in their own right.