"The retreat of forests in the late Miocene-Pliocene era around five million years ago and the more open savanna habitats were, in fact, not a catalyst for the evolution of bipedalism," study co-author Alex Piel, a biological anthropologist at University College London in England, says in a statement. "Instead, trees probably remained essential to its evolution?"with the search for food-producing trees likely a driver of this trait." Scientists reached this conclusion after studying wild chimpanzees?"which are humans' closest living relatives?"in the Issa Valley of western Tanzania. The chimpanzees' environment is what's known as a savanna-mosaic, which features both open landscapes with few trees and densely forested areas with lots of trees. Scientists were keen to study these chimpanzees, because this environment closely resembles that of early human ancestors.