Humans evolved in Africa, but where and how remain unclear. Here it is proposed that the southern coastal plain (SCP) of South Africa may have served as a geographical point of origin through periodic expansion and contraction (isolation) in response to glacial/interglacial changes in sea level and climate. During Pleistocene interglacial highstands when sea level was above À75 m human populations were isolated for periods of 360e3400 25-yr generations on the SCP by the rugged mountains of the Cape Fold Belt, climate and vegetation barriers. The SCP expands five-fold as sea level falls from À75 to À120 m during glacial maxima to form a continuous, unobstructed coastal plain accessible to the interior. An expanded and wet glacial SCP may have served as a refuge to humans and large migratory herds and resulted in the mixing of previously isolated groups. The expansive glacial SCP habitat abruptly contracts, by as much as one-third in 300 yr, during the rapid rise in sea level associated with glacial terminations. Rapid flooding may have increased population density and competition on the SCP to select for humans who expanded their diet to include marine resources or hunted large animals. Successful adaptations developed on an isolated SCP are predicted to widely disperse during glacial terminations when the SCP rapidly contracts or during the initial opening of the SCP in the transition to glacial maxima. The hypothesis that periodic expansion and contraction of the SCP, as well as the coastal plain of North Africa, contributed to the stepwise origin of our species over the last 800 thousand years (kyr) is evaluated by comparing the archeological, DNA and sea-level records. These records generally support the hypothesis, but more complete and well dated records are required to resolve the extent to which sea-level fluctuations influenced the complex history of human evolution.

A Holocene sea-level curve is constructed from the facies distribution and radiocarbon ages of sediment recovered from the distal, southern salt marsh of Langebaan Lagoon, on the southwest coast of South Africa. Calibrated radiocarbon analyses of an oyster-rich bioclastic gravel indicate that the Flandrian Transgression flooded the lagoon to 0–3 m above present-day levels by 6750 years ago (6.8 ka). Organic matter and shell material dated in distal lagoonal sediments indicate that sea level returned to present-day levels by 4.9 ka and have since remained within 61 m of present-day levels. Bleached shell and a hiatus in sedimentation suggest an approximate 1 m sea-level lowstand between 2.5 and 1.8 ka. Changes in the macrobenthos assemblage since 1.5 ka that include the loss of the oyster Ostrea atherstonei, razor clam Solen cap ensis, brown mussel Arcuatula capensis and periwinkle Oxystele variegata reflect loss of hard substrate, decreased tidal-flow velocities as reworked sands prograded into the southern lagoon and possibly cooler seasurface temperatures. Calibrated radiocarbon ages of bulk organic matter from diatom-rich, Zostera muddy quartzose sands indicate a 0.5 m sea-level highstand at 1.3 ka followed by a 0.5 m lowstand at 0.7 ka. Dating of fining-upward, organic-rich (2 wt% TOC) noncalcareous muds indicates that the present-day salt marsh has grown by aggradation (1mmy-1) and progradation since 0.7 ka.

Phosphorus is a critical element in the biosphere, limiting biological productivity and thus modulating the global carbon cycle and climate. Fluxes of the global phosphorus cycle remain poorly constrained. The prehuman reactive phosphorus flux to the ocean is estimated to range from 0.7-4.8 x 10 12 g/yr. Uncertainty in the reactive phosphorus flux hinges primarily on the uncertain fate of phosphate adsorbed to iron oxyhydroxide particles which are estimated to constitute 50% or more of the chemically weathered-phosphorus river flux. Most reactive phosphorus is initially removed from seawater by burial of organic matter and by scavenging onto iron-manganese oxide particles derived from mid-ocean ridge (MOR) hydrothermal activity. Calculation of the oceanic phosphorus burial flux is complicated by early diagenetic redistribution of both oceanic and terrestrial phosphorus. Increased phosphorus input during periods of warm, humid climate is offset to some degree by increased burial rate as productivity shifts to expanded shallow-water estuary and shelf areas where phosphorus is rapidly decoupled from organic matter to form phosphorite. Phosphorus scavenging is greater if high sea levels are associated with increased MOR hydrothermal activity such as during the Late Cretaceous. Less phosphorus is derived from weathering during cool, dry climatic periods but a more direct transportation of phosphorus to the deep ocean, and a shift of productive upwelling regions to deeper water areas allows more phosphorus to be recycled in the water column. Lowered sea level results in less effective trapping of phosphorus in constricted estuary and shelf areas and in an increase in the phosphorus flux to the deep ocean from sediment resuspension. A decrease in MOR spreading rates and the resulting decrease in phosphorus scavenging by iron-manganese oxide particles would result in more phosphorus for the biosphere. Orogeny and glaciation may accelerate chemical weathering of phosphorus from the continents when the increased particle flux is exposed to warm and humid climate. Large, reworked phosphorite deposits may proxy for short-term organic carbon burial and correspond to periods of increased reactive phosphorus input that cannot be accommodated by longterm organic matter and iron-oxide particulate burial.

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