How mammals came to rule the Earth

It was not always as such. The world used to belong to reptilian species, the dinosaurs. They grew to a grand stature, dominating all others by leaps and bounds. In the end, they were undone by a cataclysmic event that dramatically shifted the Earth's climate. Now on top of the world's pecking order are mammals, of which humans are a part. How did these once furry little creatures beat out the competition, and why didn't reptiles return to their state of dominance? A new study out of Vanderbilt University sheds some light on how this occurred in North America. Mammals were able to thrive due to their adaptability and their diversity. They were able to change faster than others as the environment changed. They could dramatically alter their diet, size, shift their range with temperature changes, and regulate their internal body heat. This glimpse into mammal adaptability in the past will help scientists predict the future of mammals in our new era of climate change. The study examined how mammals in North America adapted to climate change in "deep time", from 56 million years ago, the beginning of the Eocene Era to 12,000 years ago, the end of the Pleistocene Era, when the mammoths, saber-toothed tigers, giant sloths, and other megafauna went extinct. It was directed by Larisa R. G. DeSantis, assistant professor of earth and environmental studies at Vanderbilt. "Before we can predict how mammals will respond to climate change in the future, we need to understand how they responded to climate change in the past," said DeSantis. "It is particularly important to establish a baseline that shows how they adapted before humans came on the scene to complicate the picture." Much of the study focused on the examination of fossil remains. However, it is extremely difficult to differentiate closely related species from their fossils. It is even difficult to tell members of different genera apart. What the researchers did was analyze the fossils at the family level, covering a total of 35 different families. For example, they looked at Bovidae (bison, sheep, antelopes), Cricetidae (rats, mice, hampsters), Equidae (horses, donkeys), Ursidae (bears), Mammutidae (mammoths), Leporidae (rabbits, hares), and many others. The researchers concluded that the relative range and distribution of the mammalian families remained consistent throughout major climate changes over the past 56 million years. They were also able to correlate a family's diversity and its range. Families with greater diversities were more stable and had larger ranges. Today, mammals around the world are facing increasingly stressed conditions, and they are going extinct in record numbers. Studies of mammalian adaptability in the past will aid in this modern age of extinction and climate change, sometimes dubbed the Anthropocene. It is perhaps the mammals' wide ranges and striking diversity that will help them survive on land and sea during this chaotic period of the Earth's history.