Climatic Cycles and Behavioural Revolutions: The Emergence of Modern Humans and the Beginning of Farming

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Publication of a new volume on the beginnings of Old World farming (Harris 1996) has provided a compendium of current views on this critical inflection-point in human inhabitance of the world. Was it driven by climatic change, as Gordon Childe suggested? And what of the earlier emergence of modern human behaviour: were these two chapters in the same story?

Punctuation is back in fashion; not sophistication in the use of the semi-colon, but the idea of rapid, revolutionary change. After years of being described (in feeble imitation of Lord Acton's description of the Holy Roman Empire) as `neither Neolithic, nor a revolution'(1), the beginnings of farming are increasingly being attributed to a short, sharp shock at the end of the Pleistocene, known as the Younger Dryas(2) event (Moore & Hillman 1992; Bar-Yosef & Belfer-Cohen 1992). At an earlier date, the emergence of modern humans is also coming to be seen, not as a protracted evolutionary process, but as a Human Revolution (Mellars & Stringer 1989). In fact, the everyday (and etymologically justified) usage of the term `evolutionary' -- a gradual unfolding, by contrast with `revolutionary' -- is now at odds with biological usage, where punctuated evolution has been a respectable, if not universally espoused, concept for 25 years (Eldredge & Gould 1972; Gould & Eldredge 1977). These two features, evolutionary punctuation and climatic instability, seem to be closely related; and they form part of a new picture of rapidly changing Pleistocene environments.

Chronological resolution and the speed of change

Punctuality requires precision. With the old-fashioned kind of evolution, chronological questions could be fudged: if farming and modern humanity were (as J.L. Myres described the Greeks) `ever in process of becoming', there was little need for a precise timetable. With a punctuated model, accurate dating is essential. Fortunately, the relevant parts of the timescale are improving both in accuracy (allowing the correlation of different forms of evidence) and precision (aiding the identification of rapid changes). First, accuracy. The tree-ring calibrations of radiocarbon currently reach back only to c. 9500 BC, before which the less precise marine carbonates must be used (Bard et al. 1993; Edwards et al. 1993; Stuiver & Reimer 1993; Bard & Kromer 1995; Kromer et al. 1995). Even these, however, have radically shaken up our expectations: whereas calibration has added roughly a millennium to `raw' radiocarbon dates in the earlier part of the Holocene, the marine carbonate curve suggests that approximately two millennia need to be added to radiocarbon determinations before 10,000 b.p., and that before 12,000 b.p. the difference is perhaps nearer 3000 years -- beyond which the curve may diverge still further.(3) Since this puts the peak of the last glaciation at some time around 20-21,000 years ago (instead of 18,000 b.p.), this has important implications for climatic modellers, who increasingly rely on the Milankovitch curves (Imbrie & Imbrie 1986) to calculate variations in solar budget of different parts of the world, and so reconstruct parameters of climatic change (Kutzbach in Wright et al. 1994), providing a long-term environmental chronology.(4) Because these calculations are naturally done in 'real' (solar) years, it is imperative that the geological community -- hitherto the most reluctant to use anything other than unaltered radiocarbon estimates (and perversely calling them `BP' rather than `b.p.') -- should now routinely think in terms of calibrated dates; otherwise their empirically determined warm and cold phases in the late Pleistocene will be offset from the model's predictions by 10-15% of absolute age. Accurately calibrated radiocarbon dates are essential in correlating radiocarbon-dated phenomena with Milankovitch models, cyclothems, and with direct radiometric methods such as TL or U/ Th disequilibrium dating, which can be used to link culture to climate. …