492 TERTIARY TO PRESENT/Pliocene Figure One of a series of footprints left behind in volcanic ash at the Early hominin site of Laetoli (Tanzania) some 3.6 Ma Scale is cm Photograph by Peter Schmid variations and the cycles changed to 41 Ka, paralleling the periodicity of the Earth’s orbital obliquity variation and the Late Pliocene cycles of northern hemisphere glacials and interglacials From 2.8 Ma, periodically cooler and drier conditions and the transition towards overall lower and more seasonal precipitation strongly favoured open savannah vegetation in east Africa It is clear that the appearance and spread of open savannah cannot be considered to have contributed to shape the earliest evolution and diversification of the hominin lineage Instead, it is more likely, that the gradual reduction of areas of humid and wooded habitat towards the Middle to Late Pliocene transition contributed to the disappearance of many taxa In contrast, the emergence of the genus Homo coincided with substantial aridification of much of the African continent See Also described from the Pliocene to date (Table 2) Around 4.0 to 3.5 Ma a shin bone, probably of Australopithecus anamensis, and a series of fossilized footprints (Figure 2) provide the first clear evidence of bipedal walking The most famous hominin fossil (‘Lucy’, an Australopithecus afarensis) lived just over 3.0 Ma and the oldest stone artifacts are known from the Middle to Late Pliocene transition, $2.5 Ma The Pliocene diversification of early hominins coincided with climatic fluctuations and a general trend towards more arid and less wooded habitats in eastern and southern Africa The traditional view, however, that typical hominin features such as bipedality evolved as a direct response to the increased aridity and opening up of the East-African landscape is no longer considered valid Recent palaeoecological reconstructions of early hominin environments generally point at wooded and wellwatered habitats Only with the advent of the genus Homo, around 2.5 Ma, can hominins be considered to have been fully adapted for life in open and arid environments This is confirmed by data from marine sediments Marine records show that the African climate saw an increase in aridity after 2.8 Ma, coinciding with the onset of large-scale glaciation in the northern hemisphere Before 2.8 Ma, variations in the intensity of the African monsoon were mainly the result of variations in low-latitude insolation due to Earth orbital precession and caused 19 Ka–23 Ka cyclical alternations between dry and wet conditions After 2.8 Ma the African climate became sensitive to the increased amplitude of high northern latitude climate Atmosphere Evolution Carbon Cycle Earth: Orbital Variation (Including Milankovitch Cycles) Famous Geologists: Lyell Fossil Vertebrates: Hominids Tectonics: Mountain Building and Orogeny; Rift Valleys Tertiary To Present: Miocene; Pleistocene and The Ice Age Further Reading Castradori D, Rio D, Hilgen FJ, and Lourens LJ (1998) The Global Standard Stratotype section and Point (GSSP) of the Piacenzian Stage (Middle Pliocene) Episodes 21: 88 93 Dowsett HJ, Barron JA, Poore RZ, Thompson RS, Cronin TM, Ishman SE, and Willard DA (1999) Middle Pliocene Paleoenvironmental Reconstruction: PRISM2 U.S Geo logical Survey Open File Report, pp 99 535, http//pubs usgs.gov Driscoll NW and Haug GH (1998) A short circuit in ther mohaline circulation: a cause for northern hemisphere glaciation? Science 282: 436 438 Haug GH and Tiedemann R (1998) Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermoha line circulation Nature 393: 673 676 Hay WH, Soeding E, DeConto RM, and Wold CN (2002) The Late Cenozoic uplift climate change paradox International Journal of Earth Sciences 91: 746 774 Jackson JBC, Budd AF, and Coates AG (eds.) (1996) Evolu tion and Environment in Tropical America Chicago: University of Chicago Press Poore RZ and Sloan LC (eds.) (1996) Climates and Climate Variability of the Pliocene Marine Micropa leontology 27 Reed KE (1997) Early hominid evolution and ecological change through the African Plio Pleistocene Journal of Human Evolution 32: 289 322