Pearl millet demographic modelling: 3rd millennium BC and importance of flowering time

A new article in Molecular Biology and Evolution by Clotault et al "Evolutionary History of Pearl Millet (Pennisetum glaucum [L.] R. Br.) and Selection on Flowering Genes since Its Domestication" has some exciting and intriguing results. They have done demographic modelling, using a number of different scenarios of gene flow, based on 20 random genes, and they have also looked for selective sweeps, finding evidence for strong selection on flowering-time related genes. Although they have only modelled single domestication scenarios (which is by no means a done deal in Pearl millet), they have nevertheless taken into explicit account the notion of protracted domestication process (sensu Allaby et al 2008 or Allaby 2010), with gradual fixation of domestication traits, with exponential rather than instantaneous population growth after the bottleneck. 

They find a bottleneck strength and reduction of genetic diversity that fall amongst those estimated for other crops. Their estimated time for domestication returned an intriguingly plausible 4800 years ago, just a few centuries before the earliest archaeological evidence for domestication pearl millet in the Tilemsi valley (see Manning et al, blogged previously). The evidence they have found for selection on flowering related genes makes a whole lot of sense, because the dispersal of pearl millet from a Sahelian zone southwards crossed many different ecological zones, for which adjustments in seasonality would indeed have been important.

Sourcing the 'lost Saraswati' river: new geological evidence

Recently published on-line in Geology  is a paper which might not appear on the surface to be very archaeobotanical, but which is important for thinking about the past agriculture of the Indus valley. This is by Clift et al (2012) on "U-Pb zircon dating evidnece for a Pleistocene Sarasvati River and capture of the Yamuna River". This paper provides sources for the headwater sediments in the various rivers of the Indus system based on zircon finger-printed (geological source dating in the 1000s of millions of years). These dated source profiles in turn are stratified in the Pleistocene and Holocene river sequences which have been dated by OSL. These river systems include the now extinct Ghaggar-Hakra river, often equated with the 'lost Saraswati" of Indian epic. The paper shows that while the Ghaggar-Hakra used to be much larger in the Pleistocene, drawing on the headwaters that now feed the  Yamuna, tha Yamuna had begun to flow  east into the Ganges before the End of the Pleistocene, and therefore well before the start of Harappan urban societies. Throughout the Holocene, including the Harappan period this river was fed only by seasonal monsoon rain in the east. This rain-fed Ghaggar-Hakra was  active until after 4.5 ka and was then covered by dunes before 1.4 ka. What this means is that the Ghaggar-Hakra, unlike any of the major Indus tributaries, was not fed by snow melt, which begins in Spring and may be unpredictable, but was entirely reliant on swelling its banks from the summer monsoon. This means it would have been an ideal river for winter crop agriculture, along the lines of the Nile flood regime which is keyed to the Blue Nile's monsoon source, with sowing of wheat and barley in Oct.-Nov. as the monsoon flood began to recede to leave behind a rich floodplain. These could then be left to mature until harvests in March or April, without fear of early snowmelt floods ruining crops. It really should come as no surprise then that so many Harappan Bronze Age sites concentrated in this valley. Nevertheless as monsoons gradually weakened (already underway during the Harappan period) with the flood water source retreating eastwards, and the Thar desert expanding, the valley became gradually drier and eventually choked with desert sands. This, however happened in Iron Age or post-Iorn Age times, so thus there is no basis for correlating any catastrophic shift in the Ghaggar-Hakra with the end of the Harappan civilization-- a notion which has often appealed to archaeologists.
[edited for typos 9.2.2102 DF]


For further discussion of the sedimentological results by geologist blogger: see here