TERTIARY TO PRESENT/Pliocene 491 north with significant levels of dispersal already occurring prior to the final emergence of the isthmus In addition, while more than 90% of the angiosperm species of lowland rainforests in central America are estimated to be of South American origins, most montane plant species in South America have North American roots Plants The Bering Strait between Siberia and Alaska initially opened during or just before the Early Pliocene This new connection between the Arctic-Atlantic and the North-Pacific basins resulted in biotic movements referred to as the ‘marine trans-Arctic interchange’ Data from invertebrates and algae suggest that, overall, and in line with the present prevailing currents, the trans-Atlantic interchange heavily favoured invasion from the Pacific to the Atlantic However, data from before 4.8 Ma reveal the presence of Atlantic bivalves in the Pacific, whereas after 3.6 Ma North-Pacific molluscs suddenly became widespread in the North Atlantic This confirms that, in accordance with existing models, currents through the Bering Straits flowed from north to south during the Early Pliocene, but were reversed some time between 4.8 and 3.6 Ma, probably as a consequence of the gradual closure of the central American seaway The Pliocene’s climatic trends were reflected in the nature and distribution of plants worldwide During periods of cooling, many areas saw a transition from broadleaved to coniferous woods or from closed to more open, grassy vegetations Climate change evidently influenced the patterns of distribution of plants, but changes in the nature of the vegetation may in turn have influenced Pliocene climates Around the Miocene to Pliocene transition, the biomass of plants using C4 as opposed to C3 photosynthetic pathways increased in Africa, Asia, North and South America, probably as a result of decreasing concentrations of atmospheric CO2 The increase took place during the Late Miocene in the lower latitudes and during the Early Pliocene in the higher latitudes However, some local environments remained C3-dominated and no increase in the biomass of C4-plants appears to have taken place in western Europe The continued spread of C4-plants during the Pliocene may have had a direct effect on global climates Due to their shallower roots and their increased efficiency at CO2 fixation, C4-plants return less water from the soil to the atmosphere than C3-plants They thereby affect the hydrologic cycle and generally promote drier conditions downwind Other Biotic Movements Hominin Diversification The emergence of the Panama land bridge and the opening of the Bering Strait resulted in a general intensification of marine currents From a biotic perspective these also resulted in interchange between northern and southern temperate biota in the eastern Pacific and between temperate biota of the eastern and western Atlantic In Africa, the Pliocene epoch saw the evolutionary diversification of the human lineage, the Homininae (see Fossil Vertebrates: Hominids) Only three species of hominin are known from the Late Miocene In contrast, up to 12 additional species (almost all from the eastern rift of the African rift valley and from South African cave sites) have been The Trans-Arctic Interchange Table Hominin diversity during the Pliocene Species Age Localities Ardipithecus ramidus White et al., 1994 Australopithecus anamensis Leakey et al., 1995 Australopithecus afarensis Johanson et al., 1978 5.8 4.4 Ma 4.2 3.9 Ma 3.6 2.9 Ma Australopithecus bahrelghazali Brunet et al., 1996 Kenyanthropus platyops Leakey et al., 2001 Australopithecus africanus Dart, 1925 Australopithecus garhi Asfaw et al., 1999 Paranthropus aethiopicus Arambourg & Coppens, 1968 Paranthropus boisei Leakey, 1995 3.5 3.0 Ma 3.5 3.3 Ma 3.0 2.4 Ma 2.5 Ma 2.7 2.3 Ma 2.3 1.4 Ma Paranthropus robustus Broom, 1938 Homo habilis Leakey et al., 1964 1.9 1.4 Ma 2.3 1.6 Ma Homo rudolfensis Alexeev, 1986 1.9 Ma Middle Awash (Ethiopia) Kanapoi, Allia Bay (Kenya) Laetoli (Tanzania); Koobi Fora, West Turkana (Kenya); Omo, Middle Awash, Hadar (Ethiopia) Bahr El Ghazal (Chad) Lomekwi (Kenya) Taung, Makapanskat, Sterkfontein (South Africa) Bouri (Ethiopia) West Turkana (Kenya); Omo (Ethiopia) Olduvai, Peninj (Tanzania); Chesowanja, Koobi Fora, West Turkana (Kenya); Omo, Konso Gardula (Ethiopia) Kromdraai, Swartkrans, Drimolen, Gondolin (South Africa) Omo, Hadar (Ethiopia); Olduvai (Tanzania); East Lake Turkana (Kenya); Sterkfontein (South Africa) East Lake Turkana (Kenya)