During prehistoric times, millions of bison roamed North America. But the bison population took a major hit in the 1800s. Hunting, ranching, and western expansion all contributed to the number of bison dwindling to below 1,000 total, which meant the bison was tinkering on the brink of extinction. President Teddy Roosevelt helped spark the bison conservation movement in 1905 by creating the American Bison Society. Thanks to that effort (and some others), the bison population has bounced back to nearly half a million. On May 9, 2016, President Obama signed the National Bison Legacy Act into law, making the bison the U.S.'s first national mammal.