geology

A paper in the December issue of Geology, by Ted Maxwell and colleagues [1], describes evidence for a "Lake Erie-sized" paleolake in southwestern Egypt. The existence of a large ancient lake has been suspected for many years based on the presence of fish fossils in Middle Pleistocene contexts far from any current body of water. The new paper uses range-sensing imagery to assess the likely extent of the paleolake from elevation data, one known occurrence of fish fossils, and landscape features that appear to substantiate an ancient lake terrace:

We believe that the middle and late Pleistocene drainage was influenced by repeated Nile flooding, following on the working hypothesis of Haynes (1985), who suggested a large Pleistocene lake that drained into the Nile from what he termed the Kiseiba-Dungul depression. Using the elevation of the fossil (Middle Paleolithic) Nilotic fish found at Bir Tarfawi (Van Neer, 1993) as a base level, the SRTM data indicate that a paleolake at that level (247 m) would have flooded the entire Kiseiba-Tushka depression (Fig. 3), and is the same elevation at which the Selima paleochannels and other channel remnants to the west blend into the terrain (Fig. 2). We interpret the combination of topographic coincidence and ages of Middle Paleolithic occupations at Selima and Tarfawi as evidence of at least one lake level at that elevation, forming a local base level, reducing the competence of inflowing streams, and inhibiting channel incision below ∼247 m. Such a lake would have covered an area of 68,200 km², and would have extended from the Sudan border (22°N) north to the Kharga and Dakhla Oases, until dammed by the limestone plateau at 26°N.

They believe that the lake would have been filled by Nile outflow. The paper does not commit to any chronology, except to point out that a few late Acheulean sites are present in the basin near a presumed lower lake level of 190 m, which may represent a relatively stable size, flooded once or multiple times to the higher level of 247 m. Wired has a nice short description of the paper, which includes some dates that are not actually discussed in the paper.

A better understanding of the Nile corridor is of course very important to the issue of human movement into and out of Africa during the Late Pleistocene. More recent Late Pleistocene and Holocene paleolakes are known up and down the Nile valley, from the Fayum to Darfur.

I wonder if a Nile corridor that was ostensibly more habitable may have actually excluded gene flow back into Africa. A denser and more stable human population in this area would have been a relative population source much of the time, sending migrants out into adjacent regions. These regions would have been much less habitable at some times, but displacement of the large Nile valley population may have been impossible. Furthermore, a larger Nile corridor population would have been a reservoir for endemic parasites and diseases that would have posed challenges for migrants into the region.

The problem on an evolutionary timescale is not getting people out of Africa, but explaining the level of population structure between regions that constantly shared an overland and shoreline connection.

References

OK, so maybe it wasn't volcanoes.

Jean-Pierre Valet and Hélène Valladas in a brand new paper [1] propose that geomagnetic excursions at around 40,000 and around 32,000 years ago would have weakened the ozone layer, thereby irradiating the Neandertals with extra ultraviolet:

No special attention has been given to the geomagnetic excursions of Laschamp and Mono Lake which are synchroneous with the extinction and were the most dramatic events encountered by the Neanderthals over the past 250 thousand years of their existence. During this period the geomagnetic field strength was considerably reduced and the shielding efficiency of the magnetosphere lowered, leaving energetic particles reach latitudes as low as 30°. The enhanced flux of high-energy protons (linked to solar activity) into the atmosphere yielded significant ozone depletion down to latitudes of 40–45°. A direct consequence was an increase of the UV-B radiations at the surface which might have reached at least 15–20% in Europe with significant impacts on health of human populations. We suggest that these conditions, added to some other factors, contributed to the demise of Neanderthal population.

The paper gives all the information anyone could want about the geochronology of these events, evidenced worldwide in lava flows.

By the way, I was wondering during the volcano post why it is "lava flow" but "ice floe". I guess it's because an ice floe floats. When the ice isn't floating, as in a glacier, then it flows, too. And I don't think "floe" is a verb. The only remaining mystery: Why does a Google search for "ice floeing" bring me ads for "ice flooring"?

The paper's discussion expresses why the geomagnetic anomalies may be relevant to Neandertal extinction.

The most widespread measure of UV intensity from the public health perspective is the UV index. This is a value on a scale from zero to 11, which is a linear (not logarithmic) function of UV radiation intensity, weighted in a specific way by wavelength. The damaging UV-B waves contribute disproportionately to the UV index value. I mention this because the average UV index is readily obtained for most cities in the U.S. and Europe, and is a standard part of weather forecasts during the summer.

A 20% increase in UV-B radiation sounds dire, but it's roughly what you get by driving from Massachusetts to Virginia. It is true that melanoma rates and other complications from excess UV radiation are higher in populations who have migrated from higher to lower latitudes, increasing their exposure to UV. But these rates do not rise to a level that threatens to drive those populations to extinction.

We do worry about ozone depletion as a risk to threatened endemic populations, as they may be critically affected by new stressors. So the question is, how threatened and endemic were the Neandertals? Would this possible stressor have been enough to have tipped them over the edge to extinction?

I think we can answer these questions pretty easily. The Neandertals were a cosmopolitan population that occupied most of Western Eurasia spread across at least 20 degrees of latitude. The latest Neandertals persisted in the areas of Europe with the highest insolation. UV radiation may have been a stressor but it cannot have been decisive in their decline.

References