In southern Mexico, the deep and raspy roars of howler monkeys reverberate through the lush jungles enveloping the city of Comalcalco. The sound — akin to the rumble of distant thunder or the grumble ...

Thirteen million years ago, a medium-sized monkey known for guarding its territory among the treetops with a fearsome "howl" started doing something new. This monkey, one of the oldest known ancestors ...