The ancient struggle for existence between humans and giant clams

Giant clams are in the news today, helping to drive the expansion of modern humans out of Africa. Can we believe it?

  • The paper (Richter et al.2008) describes a new species of giant clam, distinct from others in reproductive cycle, habitat preference and size.
  • This new species is mainly found in shallow water reefs.
  • Today, the species makes up a very small proportion of the total Red Sea giant clam count.
  • Before the last interglacial, this species made up as much as 80 percent of the giant clam count, as assessed by shells from reef terraces. This proportion decreased around the last interglacial, and again in historic times.

This sounds like the classic megafaunal exploitation story, as it is being reported. Shells become an important debris of humans in Northeastern Africa by 125,000 years ago (Walter et al.2000), and were important elements of the MSA along the coasts of North and South Africa (McBrearty and Brooks2000). So it would not be surprising if these people recovered giant clams, particularly if those clams were readily available in shallow water. Giant clams are similar to large tortoises in terms of their recovery and exploitation, and there is already good evidence that tortoise size decreased with overhunting as Late Pleistocene human populations grew. By the Upper Paleolithic, people in some parts of the Mediterranean began to harvest small shellfish to an extent that put pressure on their populations. The giant clams would be an early example of the same phenomenon, made more precarious by the shallow-water habits of this particular clam species.

Since refuting the Neandertal inferiority complex is a theme this week, I should point out that Neandertals who lived on the coast also exploited shellfish, an observation that I discussed here. The exploitation of coastal resources is not specifically“modern”. Coastal populations of terrestrial predators typically eat marine species, for example, coastal brown bears in Alaska systematically harvest soft-shelled and razor clams (Smith and Partridge2004).

So the clams shouldn’t be surprising. Are they interesting? I think it is another piece of evidence that human populations in Africa during the last interglacial were already large and growing. Archaeological sites from the African Late Pleistocene have been proliferating during the last few decades, but are still underrepresented compared to the density of sites in other regions, especially Europe and the Near East. So you might not get the idea from archaeological sites that the African population was especially large. Yet, across the MSA, we see increasing breadth of faunal exploitation and some systematic recovery of small resources such as shellfish and tortoises. We also see a greater intensity of raw material exploitation and movement, and

Most important, we now have clear genetic evidence for a large and diverse African population during the Late Pleistocene. That includes the mtDNA genealogy, which now supports the interpretation of an effective population size that had perhaps doubled or more by the last interglacial (I discussed that research here). Put that together with the evidence for structure within this ancient population — either regional differentiation or ecological adaptation — and we have some very interesting demographic knowledge about Africa 100,000 years ago.

References

   McBrearty S, Brooks AS. 2000. The revolution that wasn’t: a new interpretation of the origin of modern human behavior. J Hum Evol 39:453–563.

   Richter C, Roa-Quiaoit H, Jantzen C, Al-Zibdah M, Kochzius M. 2008. Collapse of a new living species of giant clam in the Red Sea. Curr Biol 18:1–6. doi:10.1016/j.cub.2008.07.060.

   Smith TS, Partridge ST. 2004. Dynamics of intertidal foraging by coastal brown bears in southwestern Alaska. Journal of Wildlife Management 68:233–240. 0.CO;2]doi:10.2193/0022-541X(2004)068[0233:DOIFBC]2.0.CO;2.

   Walter RC, et al. 2000. Early human occupation of the Red Sea coast of Eritrea during the last interglacial. Nature 405:65–69.