Did oxygen boost fuel rise of large mammals?
A sharp increase in the Earth’s atmospheric oxygen levels about 50 million years ago was responsible for the rise of the large mammals, a new study claims. "We argue that the rise in oxygen content allowed mammals to become very, very large—mammals like 12-foot-tall sloths and huge sabre-toothed cats," says Paul Falkowski at Rutgers University in New Jersey, US, who led the study. Higher oxygen levels means animals can grow larger and still maintain the supply of oxygen to their muscles.