Vegetation of Northern Tanzania during the Plio-Pleistocene: A synthesis of the paleobotanical evidences from Laetoli, Olduvai, and Peninj hominin sites

Abstract

More than 40 years of scientific investigations of the hominin bearing Plio-Pleistocene sediments of northern Tanzania have provided a number of paleobotanical data, which, taken as a whole, provide today a way to investigate vegetation changes between 4 and 1 Ma, at a time when our early ancestors emerged. Here, I have integrated the data from all vegetation proxies obtained for the paleontological sites of Laetoli, Olduvai, and Peninj (i.e. macroscopic plant remains, pollen and phytolith assemblages, carbon and oxygen isotopic ratios measured on carbonates, and organic biomarkers). This important, yet discontinuous botanical record suggests some similarities between past and present-day vegetation at the regional scale: Afromontane forests with Olea, Podocarpus, Juniperus, Hagenia abyssinica in the highlands, and wooded grasslands with grasses and drought-adapted Acacia, Commiphora, Capparidaceae, and Chenopodiaceae and/or Amaranthaceae in the lowlands were present in the southern Serengeti–Crater Highlands region since 4 Ma. Grasses of the C4 photosynthetic type made their first appearance in the record at ∼3.7 Ma, i.e. during the mid-Pliocene, ∼700 ky before major pCO2 and temperature decline. C4 grasses became dominant in the vegetation soon after (∼3.66 Ma), probably in response to reduced precipitation. At ∼2.6 Ma, phytolith and isotope indicate grassland with abundant C4 xerophytic grasses that document strong aridity during the Intensification of Northern Hemisphere Glaciation period (∼2.7–2.5 Ma). After 2 Ma, the detailed and diverse record at Olduvai indicates complex vegetation patterns linked to oscillating precipitation, varying lake levels, and the presence of geological faults. Hence, despite low (basin-averaged) reconstructed paleo-precipitation amounts of ∼250–700 mm/y, C4-grasslands, closed woodlands, wetlands, and palm-groves co-occurred on short spatial scales near saline Lake Olduvai. Freshwater wetlands and palm-rich woodlands occurred in highly localized areas on the lake margins, where aquifer barriers and/or outcrops caused by geologic faults allowed groundwater discharge. Botanical evidence of paleo-springs at Laetoli, Olduvai, and Peninj includes the marked presence of Typha (cattail) and Hyphaene (palm tree) in association with Acacia pollen grains, and (at Olduvai) abundant forest indicator phytoliths and organic and isotopic biomarkers. At Olduvai, freshwater wetlands were most developed when lake level and fluvial competence where low, i.e. during dry periods. When wet–dry climate variability was extreme in East Africa (∼1.9–1.7 Ma), freshwater springs may therefore have offered a sustainable habitat (i.e. refuge) for several species, including hominins, and favored hominin and artifact concentration at these specific places, particularly during environmentally stressful times.