What did hominins like humans and their archaic hominin ancestors evolve?
One important factor was a major wave of global cooling from 7 million years ago that created the Savannah ecosystems that our ancestors evolved to fill.
The time period during which this happened is known as the late Miocene Epoch.Around 7 million years ago, landscapes and ecosystems across the world began changing dramatically. Subtropical regions dried out and the Sahara Desert formed in Africa. Rain forests receded and were replaced by the vast savannas and grasslands that persist today in North and South America, Africa and Asia.Up to now, these events have generally been explained by separate tectonic events -- the uplift of mountain ranges or the alteration of ocean basins -- causing discrete and local changes in climate. But in a new study, a team of researchers has shown that these environmental changes coincided with a previously undocumented period of global cooling, which was likely driven by a sharp reduction in atmospheric carbon dioxide.
The paper spelling all of this out is:
Timothy D. Herbert, Kira T. Lawrence, Alexandrina Tzanova, Laura Cleaveland Peterson, Rocio Caballero-Gill, Christopher S. Kelly. "Late Miocene global cooling and the rise of modern ecosystems." Nature Geoscience (2016)