Sahara at least 7 million years old

A concise 4-paragraph article by Mathieu Schuster and colleagues reports on dune deposits that show the Sahara formed during the Late Miocene.

After the mid-Holocene humid period (6000 years ago), arid conditions developed throughout North Africa, culminating in the formation of the Sahara, which is the largest warm-climate desert on Earth (9,000,000 km2). However, earlier desert recurrences in the region are also documented. Direct evidence for eolian deposition is given by thermoluminescence dating for the Late Pleistocene; e.g., in Mauritania [25 to 15 thousand years ago (ka)] (1) or in Tunisia (86 ka) (2). The latter is currently considered as the oldest terrestrial record for desert conditions in the Sahara (2), even if firm evidence exists for a pre-Quaternary Great Western Sand Sea in Algeria (3). Some earlier arid episodes (Miocene-Pliocene) were also suggested by marine records off West Africa (4); but until now, no contemporary in situ eolian deposits were known in the Sahara region. In the northern Chad Basin, we recently identified and dated widespread outcrops of eolian dune deposits that are distributed over an area more than 2000 km2. Our results testify that the onset of recurrent desert conditions in the Sahara started at least 7 million years ago (5-7) (Schuster et al. 2006:821).

The desert comes and goes, expanding and contracting -- and those vacillations are recorded by this earliest evidence, also:

In the Toros Menalla region, these eolian sandstones are conformably overlain by a horizon bearing abundant vertebrates fossils, including Sahelanthropus tchadensis, the earliest known Hominid [sic] (5, 7). In this horizon, named the Anthracotheriid Unit, biostratigraphic correlation of the mammalian fauna indicates an age of 7 Ma (57).

Now, this isn't news (which I'm sure Science didn't bother to check) since Vignaud and colleagues (2002) published the same evidence, complete with the wind direction chart:

The lower part of the section (at least 4 m thick) is composed of fine to very fine white sands, poorly cemented, and is mainly constituted by numerous quartz grains, without matrix. The grains are well sorted, well rounded, matt and frosted, and are strong evidence for aeolian modelling. The foreset laminations (avalanche laminations in front of the aeolian dune) represent a typically aeolian deposit. These sands show cross-beddings that progressively decrease in size from the bottom (1 - 2 m) to the top (20 cm). This facies exhibits typical alternations of grain-fall and grain-flow laminations, characteristic of aeolian dune deposits. Our interpretation is confirmed by frequent wind ripples at the foot of the fossil dunes, whose crests are perpendicular to the direction of dune progradation. These fossil dunes are, to our knowledge, the oldest evidence for desert conditions in the southern Sahara area (Vignaud et al. 2002:152).

I guess this is the science journal equivalent of getting "punk'd" -- "Ha ha! You published what we printed four years ago!"

I opened up the Vignaud paper to double-check the paleoenvironment in the fossil-bearing layer. From the faunal list, they conclude this:

The oldest known East African hominids (Ororrin [sic], Ardipithecus) are contemporary with faunas associated with wooded environments. Younger australopithecines lived in a wider range of habitats. In contrast, the TM 266 vertebrate fauna contemporary of the Toros-Menalla hominid suggests a mosaic of environments from gallery forest at the edge of a lake area to a dominance of large savannah and grassland. Determining the precise habitat of the TM 266 hominid locality among the mosaic of environments available to it constitutes a research challenge to be met by further laboratory and field studies currently in progress (Vignaud et al. 2002:155).

They (Vignaud et al. 2002) interpreted the succession of dune and lacustrine deposits to mean that the hominids lived in a mosaic environment near sandy desert, but locally including marshy/swampy, lake, and gallery forest. An alternative interpretation might be that the desert really receded (or disappeared) during the later time period when the hominids were there. In either case, the paleoenvironment is interesting, because it means that the Sahelanthropus-like primates colonized (and possibly repeatedly recolonized) areas that were periodically dune desert (and therefore probably not habitable by large primates). This may not mean much in terms of locomotion -- the hominid-bearing unit is clearly water-rich, and we can't refute the idea that the surroundings were as woodland-like as those preserved in the Late Miocene Middle Awash localities.

But I think it is a good hypothesis that all of these apes (or hominids) were very cosmopolitan compared to extant chimpanzees and gorillas. The question is whether their actual dispersal abilities were different from chimpanzees. Prehistorically, genetics would seem to indicate that chimpanzees had long-distance dispersal; the only fossil evidence of chimpanzees has been found in a region that historically did not support chimpanzees; and they today successfully utilize relatively open savanna at the eastern end of their range.

So it is by no means obvious that the cosmopolitan nature of these Late Miocene lineages would have required a specialized terrestrial adaptation -- at least not beyond the specialization of knuckle-walking. So why become bipeds?

References:

Schuster M et al. 2006. The age of the Sahara Desert. Science 311:821. Full text (subscription)

Vignaud P et al. 2002. Geology and paleontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Full text (subscription)