The Savanna “Hypothesis”: Tracing an enigmatic idea through time

The savanna hypothesis is a long-lived idea concerning the role of the environment in the origin of hominin species—those that walk on two legs like humans and their ancestors. Scientists in the 19th century referred, vaguely, to apes and humans or human ancestors evolving away from each other and the latter moving to inhabit savannas—or more broadly, out of the forest and into the open. There were even suggestions that humankind emerged on savannas with no connection to forested apes. Definitions of savanna at this time were sparse, except possibly that they were open and grassy.

Raymond Dart, in his 1925 paper on the discovery at Taung, in 1924, of the small-brained, bipedal Australopithecus africanus, noted that open savanna with patches of woodland—such as those found in South Africa near Taung—could have led to selection for a variety of human-like characteristics such as bipedalism and a larger brain. Such characteristics would distinguish early human ancestors from forest dwelling apes; in Dart’s view, A. africanus was thus the base of our lineage. During the following 50 years, others took Dart’s idea that early hominins lived on an open savanna as fact, leading to speculation about whether living in a savanna environment ultimately caused the origin of bipedalism. Was bipedalism important for seeing above tall grasses or, perhaps important, for freeing the hands to carry or hunt?

The discovery in Ethiopia of Lucy in 1974 confirmed, without a doubt, that Australopithecus species were bipedal, widespread, and small-brained. Lucy, however, had curved fingers and toes—what did that mean? Was this evidence of the importance of trees in the way they moved across the landscape or just a holdover from Lucy’s ancestors? The Lucy discovery not only prompted new ideas about the origins of bipedalism, but also prompted researchers to investigate the evidence for, and changes in, the savanna habitat. But savannas did not lose their appeal. In the 1980s, Vrba examined species turnovers on the African continent that she said were caused by climate-induced aridity and grassland expansion. These turnovers occurred at 5.0 million years ago (Ma) and 2.6 Ma, give or take a few hundred thousand years. Each episode of aridity expanded grasslands (now used interchangeably with savannas) and was likely responsible for the evolution of more hominin (and other mammalian) species.

During the past 30 years, paleoecologists have used numerous techniques to refine reconstructions of paleohabitats and paleoclimate at individual African hominin localities from the late Miocene to the Pleistocene. Research ranging from isotope and pollen analyses to community ecology and biomarkers have explored the importance in hominin evolution of habitats across space and through time. Today we know that understanding the interactions between various hominin species and their ecosystems (encompassing climate, vegetation, and other organisms) goes far beyond savanna, requiring a much more nuanced understanding of biomes, paleoclimates, mosaic habitats, and hominin morphology, to answer contextual and adaptational questions about human evolution.