Quaternary - Hominin Evolution, Climate Change, Migration : American paleontologist Elisabeth Vrba and other scientists have suggested that climate changes 2.5 million years ago accelerated the evolution of hominins (members of the human lineage), giving rise to our genus, Homo. The details of this process, and the exact pathways of ancestors and descendants, are highly controversial (see human evolution). Even so, most paleoanthropologists and archaeologists believe that a shift from forests to drier savanna lands in Africa imposed evolutionary pressures that favoured an upright stance and ability to run and walk long distances. This posture freed the hands for grasping and made possible the eventual use of tools.