We know the fossil record underestimates diversity at least a little, and we know that forested environments in Africa tend to be underrepresented. Given this, the diversity of Miocene apes may have been rather impressive, because there is a fairly high diversity in what we can assume is a biased record.
But I'd like to make the argument from another angle, that of modern ecological analogues. Let us assume that the greater apparent diversity of apes in the middle and late Miocene compared today can be accurately translated as a modern reduction in ape diversity. Not counting the relatively diverse lesser apes, there are five species (2 chimps, gorilla, human, orang) which can be further divided into 10 subspecies, across the entire old world.
Now look at the size range of all of the living apes. Gibbons are the smallest and gorillas the largest. When a family or subfamily of land mammal is diverse in a particular region (a biome or something larger than a biome) we tend to see that diversity played out along a spectrum of size, and against size we can find additional diversity derived from dietary or subhabitat differences and geography. It seems to me that there is room in the size spectrum between gibbons and chimps, and orangs and gorillas, and there is certainly room above the gorilla size as indicated by the existence in the fossil record of very large Asian forms.
We know that some of the later Miocene apes were bipedal, and it is starting to look like bipedalism or something like bipedalism is showing up among other apes in the Miocene as well. So perhaps there is a spectrum of locomotory pattern along which diversity may be spread.
This gives us a the following size classes: gibbon, siamang, [something in between], chimp, orang, [something in between], goriilla, [something bigger], or at total (a minimum?) of eight size classes across which apes might exist in a world in which apes are divers. Like the Miocene. If we add to this a more arboral form and a more bipedal form, perhaps we double the number, or perhaps we add about five new classes (I'm guessing that a Mighty Joe Young size ape would not have been bipedal!). This gives us about a dozen, conservatively estimated, niches when we divvy up size and so-called positional behavior.
To this we can add geography. It is probably reasonable to assume that a wetter, more forested middle and late Miocene Africa could be divided into at least four or regions, between the West/Central divide that modern biogeogrpahy tells us was effective at least in the Late Miocene, the Congo River divide, North/Central Africa, East Africa and Southern Africa. Let's conservatively assume four, and let's assume that only half (six) of the hypothesized ape species are divided among these areas. That means that 24 species are endemic to varoius regions, and six additional species are more widely spread for a conservative estimate of 30 species.
Among these species there may have been several bipedal forms, but only one of them (plus or minus a little hybridization hanky panky here and there) would have been the human ancestor. Of course, no one at the time suspected that .... (Or they probably would have done something about it.)
This is not an outrageous suggestion. The idea that if you went back in time to a more ape-rich time (and we know it was more ape-rich) and got a current copy of the Guide to the Mammals of Africa, the ape section would have a few dozen species, just like the monkey section or the antelope section today has a few dozen species.
This is really very interesting stuff. I'd like to pass it on to the primatologists of my acquaintance.
Do it quick! I can use the feedback before tonight's talk when I say this stuff out loud!!!
It's my understanding that large areas of sub-equatorial Africa were covered up by Kalahari Sands sometime before the late Pleistocene. (I couldn't find any links in a quick search.) AFAIK much of the covered landscape had a great deal of relief, and was drained by surface water (streams, etc.) rather than the current case where most rainfall sinks into hundreds of meters of sand and is drained by a few deeply cut rivers.
If this landscape contained mostly open woodland it could have contained a large diversity of apes including Australopithicenes. Most of the fossils would have been buried.
Given an inability to find good references to African landscapes and their history, I have no way to guess how plausible this is. Can you comment?
I also wonder whether a similar situation might exist in the Sahel (with Sahara sands rather than Kalahari).
Good question. I'll give you a very rough overview. The entire continent of Africa is divisible into major basins, and all of them have been interior drainages at one point or another in time with major influx of sediment. The sand is everywhere, with the Kalahari deposit (which is many different deposits) being the largest ever, anywhere. There were probably inland seas assocaited with each basin, all of which are now opened, with the one associated with the Kalahari mainly drained by the Limpopo and Orange/Gariep Rivers in the southern part.
I think the time scale for this major basin having been closed way predates the period I'm talking about here, but yes, during somewhat wetter times there may have been ape-friendly (counting the presumed woodland apes like hominid) habitats from somewhere south of the equator all the way to the southern coast.
Apes everywhere, under certain conditions.
I'd gotten the impression that a large amount of windborn sand was deposited over previous hilly landscape sometime around either the Pliocene/pleistocene boundary or a little later, with much of the modern brushland on top of it. My sources are all non-scholarly, however, which is why I'm a little tentative about it.
I actually found some scholarly references for my previous point:
Both are actually on other topics, but the abstracts (I can't get to the actual articles) contain references to large areas being covered by windblown sand in the form of linear desert dunes.
From the latter reference:
"The Kalahari sand dune systems of central and southern Africa are built up from the surface sediments of the Kalahari Beds, extensive continental deposits which accumulated in a great internal basin (De Swart & Bennett 1974). The dunes are thought to have been formed during the last glacial maximum, 18000-13000 B.P. (Heine 1982), under the influence of easterly winds (Lancaster 1979), during a period of less than 150 mm rainfall (the maximum threshold for sand movement) and inn a vegetation-free environment. [...] In Zimbabwe they cover the southwest part of the country with the ridges being most prominent in the far south-west (Thomas 1984)."
I would assume that similar processes took place during earlier glacial maxima, starting in the early pleistocene.
The process is definitely older than these references are suggesting, and it is very much ongoing now, across about the southern 20 percent of the sand field (but not the very very southern extent).
The question becomes, then, how much Pliocene (and early Pleistocene) land that was rough, surface-drained open woodland was later covered up by Kalahari sands and converted into savannas where most of the rainfall soaks down to below the deepest roots and drains horizontally into deep-cut rivers?
This could be important, because in addition to actual climate differences, differences in soil type and water retention could have made for a much friendlier environment for apes. Rough land with a mixture of open woodland and brush could have created a sort of giant "jungle Jim" combining vertical aspects of the land, downwood, and trees, with brush and grass in a very tight mosaic.
Movement through such landscape would have combined some brachiation with scrambling and some running and vertical climbing. Often there would be handholds at waist level or so, allowing a four-handed movement with a vertical body and short arms.
Such a landscape might have been ideal for an Australopithecene.