Topic: shellfish and migration
Artefacts from the African Middle Stone Age (MSA; ∼200 to ∼50 ka), provide us with the first glimpses of modern human art and culture. Approximately 50 ka, one or more subgroups of modern humans expanded from Africa to populate the rest of the world.
Significant behavioural change accompanied this expansion, and archaeologists commonly seek its roots during this period. Recognizable art objects and “jewellery” become common only in sites that postdate the MSA in Africa and Eurasia, but some MSA sites contain possible precursors, including abstractly incised fragments of ochre and perforated mollusc shells interpreted as beads.
Was population growth the driver of change?
Researchers had previously theorised that it was an increase in population that drove behavioural innovations which in turn led to the creation of these artefacts and eventually, the expansion out of Africa. However, by examining mollusc shells from Stone Age sites, Richard Klein of Stanford University and Teresa Steele of University of California, Davis, have determined that a significant population increase did not occur until the Later Stone Age (LSA), after the out of Africa migration had already begun. Their research appears in the June 2013 edition of the Proceedings of the National Academy of Sciences.
Archaeologists have found precursors of modern human artwork and jewellery, including fragments of ochre with abstract incisions and shells with perforations, in MSA sites and it is therefore concluded that the humans who made them, between 85,000 and 65,000 years ago, must have had modern cognitive abilities and behaviours. During the LSA, these abilities and behaviours allowed humans to create objects as recognizable art and spurred the migration to Eurasia.
Population growth has been the popular explanation for the innovations of the MSA. As population increases, the opportunity for innovation increases, while concurrently, the probability that an idea will be lost decreases.
Symbolic thought in the form of decorative art appears in excavations at Blombos Cave on the southern Cape coast from about 75 ka, associated with small perforated shells that retain traces of red ochre. This suggests that they had been collected and strung together as a necklace. In the underlying level dated to about 80 ka, two pieces of ochre were found, engraved with a pattern of lines that formed diamond shapes. The question to be asked is how and when this transformation to modern human behaviour began.
Testing a new hypothesis
To test the hypothesis that a large increase in population drove MSA innovation, Klein and Steele measured the shells of slow-growing molluscs found in MSA and LSA middens on the southern and western coasts of South Africa. They reasoned that selection pressure, caused by an increase in human population, would decrease median shell size. Frequent foraging by large numbers of humans would have prevented many shellfish from reaching their full size.
Pinnacle Point Cave 13B, South Africa and Bajondillo Cave, Spain show that human shellfishing began at least 160–150 ka, during the MSA in Africa and the coeval Middle Palaeolithic (also known as Mousterian) of Europe.
The researchers found that the median size of MSA shells was larger than that of LSA shells. This showed that selection pressure, and therefore human population, was greater in the LSA rather than the MSA. In addition, shellfish from smaller species were more common in LSA than in newer Stone Age sites. As selection pressure increased with population size, humans would be less likely to overlook smaller shellfish as a source of food.
Need for more data
Klein and Steele claim that because the population increase did not occur until after the migration out of Africa had already begun, there must be another explanation for the cultural advancements of the MSA. They hope that other sites around the African coasts, especially northwest Africa, can be used to investigate the possibility that MSA molluscs that were collected as a foodsource were generally larger than those in later sites and thus add weight to the evidence that MSA population growth did not underlie innovation.
Alternative explanations, particularly for the birth of cognitive and behavioural innovation at the MSA/LSA interface, include the pressure of late Pleistocene climatic fluctuations and perhaps even changes in the human genome that ancient DNA analyses promises to reveal.
June 22, 2013