Springs, palm groves, and the record of early hominins in Africa

Hominins evolved in Africa during a period of overall regional cooling, drying, and increasingly variable climate. Despite prevailing regional aridity since the mid-Miocene, data show that early hominins Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus ramidus lived in environments made of mosaics of grasslands, mixed grasslands, woodlands, and forests, where wooded habitats were maintained by edaphic rather than regional (climatic) humidity. Groundwater systems (springs, seeps, shallow aquifers) and surface water (rivers, lakes), locally create wetter and more wooded environments in addition to that supported by precipitation alone. However, edaphically sustained woodlands are rare to missing in most published paleoeclogical interpretations of hominin archeological sites. To explore the importance of groundwater to the record of hominins in Africa, we provide newly acquired field data from spring sites in the Awash Valley, Ethiopia, and Lake Eyasi-Lake Manyara region, Tanzania, and re-evaluate published data from the Ardipithecus-bearing Aramis Member, Ethiopia. Results show that (1) in arid Eastern Africa, a wide variety of microhabitats such as groundwater-fed wetlands, Hyphaene palm woodlands, Phoenix reclinata palm woodlands, and structurally complex and species-rich forest patches exist due to local variability of geologic, topographic, and hydrologic conditions. (2) These microhabitats carry some characteristic pollen and phytolith signals, that may be easily masked by the signal of surrounding grass-dominated shrublands and grasslands. (3) The Aramis Member (Awash Valley, Ethiopia), which is to date, the best documented paleo-groundwater ecosystem, is not a riparian habitat. It is one of >50 examples (within 22 geographically distinct areas) in Africa and the Middle East where evidence of groundwater systems co-exist with hominin and/or archeological remains. Springs are commonly localized features of limited area within a landscape, but provide ecological continuity through time and diverse microhabitats, some of which may be densely forested. At the local scale, springs create microclimates, distinctive vegetation, and increase soil nutrients, species richness, structural complexity, and provide habitat for animals. At the landscape scale, they represent hydro-refugia favoring increased connectivity among animals and allowing migrations during dry periods. We conclude that in the East African Rift where low, highly seasonal rainfall and high evaporative demand limit vegetation growth in many areas, groundwater-fed zones create diverse microhabitats and play a major role in ecosystem functioning. It is likely that, within a context of increasing aridity and expansion of grass-dominated open habitats during the Mio-Pliocene, early hominins and many other animals viewed edaphically sustained woodlands as attractive habitats. (C) 2019 Elsevier B.V. All rights reserved.