Bringing trees back into the human evolutionary story: recent evidence from extant great apes.

Hypotheses have historically linked the emergence and evolution of defining human characteristics such as bipedal walking to ground-dwelling, envisioning our earliest ancestors as living in treeless savannahs (i.e. the traditional savannah hypothesis). However, over the last two decades, evidence from the fossil record combined with comparative studies of extant apes have challenged this hypothesis, instead favoring the importance of arboreality during key phases of hominin evolutionary history. Here we review some of these studies, including a recent study of savannah chimpanzees that provides the first model of how bipedalism could have been adaptive as an arboreal locomotor behavior in early hominins, even after the forests receded during the early Miocene-Pliocene transition. We suggest that whilst a shift to exploiting open habitats catalyzed hominin divergence from great apes, adaptations to arboreal living have been key in shaping what defines humans today, in counter to the traditional savannah hypothesis. Future comparative studies within and between great ape species will be instrumental to understanding variation in arboreality in extant apes, and thus the processes shaping human evolution over the last 3-7 million years.