The most interesting times in human prehistory were those when African beings set off from their home continent for new habitats. The earliest seems to have been the migration of Homo ergaster some 1.8 Ma ago, and the most riveting, of course, was that of modern humans who set out to colonise the entire habitable planet sometime around 80 to 60 ka ago. It is pretty certain that the population movements were driven by environmental changes that provided a driving pressure to seek survivable conditions beyond Africa, such as episodes of drying in East Africa, and passable exits from the continent, such as sea-level falls to produce land bridges like that of the Straits of Bab el Mandab. One of the glaring gaps in knowledge about those circumstances is evidence for climate change from Africa itself. The problem has been that many of the Great Lakes did not fill until the last 12 ka or so, so provide no sediment cores and proxy climate records for the crucial period in human history. Lake Tanganyika is an exception, being so enormously deep that it survived much of the last glacial episode when Africa was probably a lot drier than now. Cores of Lake Tanganyika sediment reach back at least to 60 ka (more might be had if coring was done using drilling rather than piston coring) and a surprising record has emerged from that time (Tierney, J.E. et al. 2008. Northern Hemisphere controls on tropical southeast African climate during the last 60,000 years. Science, v. 322, p. 252-255).
Deuterium and organic geochemical data from the cores are proxies for water temperature and precipitation in the lake’s catchment, and show fluctuations that clearly match the familiar patterns of climate change from Greenland ice cores, and the intensity of the Asian monsoon recorded in Chinese cave deposits. This match shows clearly that the East African climate followed closely the orbitally-induced changes in solar input at high northern latitudes. But the cause of the linkage is not clear. One candidate is the varying position of the Intertropical Convergence Zone (ITCZ). Yet it seems that known shifts in the ITCZ are not linked to East African fluctuations. So the connection with the Asian monsoon hints at controls by the changes in Indian Ocean sea-surface temperature. The ‘teleconnection’ is characterised by very abrupt shifts from humidity to aridity, and profound aridity around 57, 47.5 and 38 ka. These may have resulted in extreme ecological shifts in the southern East African Rift System, resulting in considerable stresses on human groups. Sadly, data from the most probable first period of migration out of Africa by modern humans (70-80 ka) have not been reached by the piston coring method – maybe they will eventually be accessed by rotary drilling. However, the close linkage with the Greenland record does suggest that cool/arid conditions occurred in the modern human heartland around 70 and 74 ka, when sea level was beginning to fall to 80 m below that at present.
Younger Dryas and the Bat Cave
It seems that bats have a remarkable loyalty to their chosen cave, whatever the weather. Thick guano deposits coat the floors of most caves that are now popular with bats. While the deposits are bioturbated by a narrow range of unwholesome insects, sufficient stratigraphy remains intact for more intrepid scientists to chance their hand at proxy records of climate in the caves’ vicinity; but data are, unsurprisingly, rather scanty. Arid conditions enhance preservation of such cave-floor deposits, and Bat Cave in the Grand Canyon of Arizona has attracted attention (Wurtster, C.M. et al. 2008. Stable carbon and hydrogen isotopes from bat guano in the Grand Canyon, USA, reveal Younger Dryas and 8.2 ka events. Geology, v. 36, p. 683-688). The team from Scotland, Canada, the USA and New Zealand show that both the Younger Dryas and a lesser global cold spell at 8.2 ka are discernible in the guano core from Bat Cave, but the signals arise from a rather circuitous cause. Bat guano is largely made up of the chitinous remains of the insects eaten by the bats, and it is the isotopic variation in the insects’ diet that the chitin preserves. That in turn stems from local vegetation, in some cases pollen or nectar consumed by the bugs, or even the blood of mammals or birds taken by bloodsucking insects – itself several metabolic steps from the local vegetation. These complexities may account for the rather ‘noisy’ guano data, yet it seems likely that other caves will be probed in arid areas where speleothem (from stalactites) has not developed continually through the caves’ lifetimes.