Tool-making Human Ancestors Inhabited Grassland Environments Two Million Years Ago

Scientists as far back as Charles Darwin have thought that adaptation to grassland environments profoundly influenced the course of human evolution. This idea has remained well-entrenched, even with recent recognition that hominin origins took place in a woodland environment and that the adaptive landscape in Africa fluctuated dramatically in response to short-term climatic shifts.

During the critical time period between 3 and 1.5 million years ago, the origin of lithic technology and archeological sites, the evolution of Homo and Paranthropus, selection for endurance running, and novel thermoregulatory adaptations to hot, dry environments in H. erectus have all been linked to increasingly open environments in Africa.

However, ecosystems in which grassland prevails have not been documented in the geological record of Pliocene hominin evolution, so it has been unclear whether open habitats were even available to hominins, and, if so, whether hominins utilized them. In their new study, Plummer and colleagues provide the first documentation of both at the 2-million-year-old Oldowan archeological site of Kanjera South, Kenya, which has yielded both Oldowan artifacts and well-preserved faunal remains, allowing researchers to reconstruct past ecosystems.

The researchers report chemical analyses of ancient soils and mammalian teeth, as well as other faunal data, from the ~2.0-million-year-old archeological sites at Kanjera South, located in western Kenya. The principal collaborating institutions of the Kanjera project are QueensCollege of the City University of New York, the Smithsonian Institution's Human Origins Program, and the NationalMuseums of Kenya. The findings demonstrate that the recently excavated archeological sites that preserve Oldowan tools, the oldest-known type of stone technology, were located in a grassland-dominated ecosystem during the crucial time period.

The study documents what was previously speculated based on indirect evidence -- that grassland-dominated ecosystems did, in fact, exist during the Plio-Pleistocene (ca. 2.5-1.5 million years ago) and that early human tool-makers were active in open settings. Other recent research shows that the Kanjera hominins obtained meat and bone marrow from a variety of animals and that they carried stone raw materials over surprisingly long distances in this grassland setting. A comparison with other Oldowan sites shows that by 2.0 million years ago, hominins, almost certainly of the genus Homo, lived in a wide range of habitats in East Africa, from open grassland to woodland and dry forest.

Plummer and colleagues conclude that early Homo was flexible in its habitat use and that the ability to find resources in both open and wooded habitats was a key part of its adaptation. This strongly contrasts with the habitat usage of older species of Australopithecus and appears to signify an important shift in early humans' use of the landscape.

Funding from the L. S. B. Leakey Foundation, the Leverhulme Trust, the National Geographic Society, the National Science Foundation, the Professional Staff Congress-City University of New York Research Award Program, and the Wenner-Gren Foundation for Kanjera field and laboratory research is gratefully acknowledged. Logistical support was provided by the Human Origins Program of the Smithsonian Institution. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.