An ancient global warming event shrunk the earliest horses down to the size of scrawny housecats, according to new research that could have implications for what mammals might look like in a future warming world. During what's known as the Paleocene-Eocene Thermal Maximum, or PETM, about 56 million years ago, a massive release of carbon into the atmosphere and oceans boosted average global temperatures by about 10 degrees Fahrenheit (5.5 degrees Celsius) over 175,000 years. Mammals responded to this climate change by shrinking, with about one-third of species getting smaller. Now, new research reveals that these changes occurred in lockstep...