Woody cover and hominin environments in the past 6 million years

Abstract

The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). (13)C/(12)C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than ∼40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years.

Diet of Theropithecus from 4 to 1 Ma in Kenya

“…1,000 y for a single locality), 80% woody cover would correspond to soil δ 13 C contributions from C 3 woody cover, C 3 forbs and herbs, and C 4 grasses or sedges of ca. 80%, 10%, and 10%, respectively (30). Thus, a true forest would have little of the dietary resources used by T. brumpti, and therefore, T. brumpti would have obtained its dietary resources from outside any narrow riparian forest corridor.…”

“…Paleosol evidence from the Koobi Fora and Nachukui Formations from this time interval suggests a habitat that had 40-60% woody cover (30,40). Stable isotope studies of modern soils in East Africa show that closed riparian forests [>80% woody cover; e.g., Tana River (30)] have negligible C 4 biomass in the understory but that C 4 plants are found in riparian woodlands [<80% woody cover (30)]. Paleogeographic reconstructions for this time interval (41) show that the proto-Omo River flowed through the region, and this river was likely accompanied by a narrow (hundreds of meters wide) riparian forest corridor but with grassy woodland (i.e., >40% woody cover) (definition in ref.…”

“…35-40‰ C 4 -based. Paleosol evidence from the Koobi Fora and Nachukui Formations from this time interval suggests a habitat that had 40-60% woody cover (30,40). Stable isotope studies of modern soils in East Africa show that closed riparian forests [>80% woody cover; e.g., Tana River (30)] have negligible C 4 biomass in the understory but that C 4 plants are found in riparian woodlands [<80% woody cover (30)].…”

“…Primary forest resources are almost entirely C 3 ; soil carbon isotopes (30) show that few, if any, C 4 plants are present in forests (>80% canopy cover). Even open grasslands in Africa can have an important abundance of nonwoody C 3 forbs and herbs present (30). C 4 plants are primarily tropical grasses and sedges, both of which are monocots.…”

Tropical grassy biomes: linking ecology, human use and conservation

“…These biomes were the cradle of human evolution [7], and in our contemporary world, they support the livelihoods and wellbeing of over one billion people [8]. With the population of Africa alone set to treble by 2050 [3], the continuing pace of climate change [9], increasing atmospheric CO 2 concentrations [9] and the increasing agricultural development of TGBs [3,10], there is an urgent need to understand the unique ecology of these systems.…”