494 TERTIARY TO PRESENT/Pleistocene and The Ice Age derived from characteristic sediments and landforms whereas evidence for the warmer interglacial stages came mainly from organic sediments and the fossils they contained The stratigraphic relationship between glacial and interglacial sequences was often obscure and the means of correlating between isolated terrestrial sequences was poorly defined The Alpine model of Ice Age events became to a very large extent a self-perpetuating system Although a number of lines of evidence began to emerge suggesting much greater overall complexity, the lack of clarity in the original stage definitions and the virtual impossibility to establish secure interregional correlations led to such evidence being subsumed into the model Against the background of purely geological interpretations, there was also a growing body of archaeological evidence charting human prehistory through the Pleistocene in various parts of the world It was not until the 1860s that it became widely accepted that the human race had any great antiquity, with stone tools that were clearly made by early people being found in undisturbed contexts along with the remains of extinct animals Such a likelihood was first noted by the French antiquarian Jacques Boucher de Perthes, who from 1846 to 1857 published his findings of stone tools from the highlevel (and therefore very ancient) terrace deposits of the River Somme near Abbeville There was considerable opposition to the idea that there were human populations of great age represented in the fossil record, primarily from religious groups and individuals, and to some extent this opposition still survives today For most people, the clinching discovery was that of prehistoric cave art in France and Spain, where people living in the last glaciation had painted pictures of contemporary woolly mammoths, woolly rhinos, reindeer, bison, and horses in the caves they also used for shelter – direct evidence with a ancient human signature It quickly became clear that the record of human antiquity was longest in Africa, Europe, and southern Asia and relatively short in northern Asia, Australia, and the Americas There is probably no other period of Earth history that has been so intensively studied yet so badly misinterpreted than the Pleistocene, and all because the construction of theoretical models has tended to run ahead of the collection and interpretation of hard evidence Far too little attention has been paid to testing models and far too much to reinforcing them The Alpine Model of Pleistocene climatic events eventually fell victim to new evidence from the oceans Cesare Emiliani and colleagues, working in the late 1950s and early 1960s on cores taken from deep-ocean sediments, found evidence from the shells of buried foraminifera that their oxygen isotope content varied through time Emiliani had already worked on the palaeoclimate signals recovered from microfossils in older deposits, and he recognized his new data as being a direct record of changing global ice volume The two significant isotopes of oxygen, 16O and 18O, behave differently during evaporation, leaving the oceans enriched with the heavier isotope When water evaporates from the oceans and falls as rain or snow on the land, it is isotopically ‘light’, and if sufficient frozen water remains on land in large, stable ice-sheets, then the isotopic composition of the oceans will change to a measurable degree When the ice melts, eventually the meltwater will ultimately return to the oceans, thus recentring the isotopic signal Foraminifera living in the sea will deposit shells that reflect the chemistry of the seawater during their lifetime, and when they die, their shells fall to the seafloor and become incorporated in the sediments being deposited there, creating a retrievable record of changing global ice volume through time Emiliani found evidence for many more episodes of global ice accumulation than were allowed for in the Alpine Model When plotted onto a graph, the marine isotope record bore a very strong resemblance to the theoretical curve of variations in the predicted amount of solar energy reaching a particular point on the planetary surface; this is determined by known variations in Earth’s orbit This curve had been calculated by the Serbian astrophysicist Milutin Milankovitch in the early twentieth century to explain long-term climatic variability, but, in the absence of any evidence to back his theory, Milankovitch’s work remained largely ignored Emiliani’s recognition of the resemblance between the Milankovitch solar radiation curve and the oceanic oxygen isotope record has led to a completely new understanding of the driving forces behind global climatic change, now extending far back beyond the Pleistocene Alongside plate tectonics, this must number as one of the greatest breakthroughs in geological science Milankovitch theory examines the combined effects of three known variables in Earth’s orbit: the eccentricity of Earth’s orbit around the Sun, the obliquity of Earth’s axis relative to the Sun, and the precession of the equinoxes that changes the season at which Earth’s axis is most tilted towards the Sun Each of these has respective cycles of approximately 100 000, 41 000 and 23 000 years In the past 700 000 years, the 100 000-year cycle has become dominant, giving major global cold stages at about this interval Prior to this, there was considerable climatic instability during the earlier part of the Pleistocene, but not on anything like this dramatic scale Amazingly detailed climatic signals for the last major climatic cycle