Necessity is the mother of invention, and changes in climate often lead to changes in the bodies or behavior of species. So it would not be surprising if many human innovations are the result of having to adapt to a changing environment. However, a study at two sites in southern Africa indicates that stone age innovations don't coincide with climatic changes.
The Middle Stone Age sounds like it should be populated by Tolkien's dwarves, but is the name given to the period 280,000-50,000 years ago. At this point, humans living in southern Africa developed new technologies such as shell beads and sophisticated methods for attaching tips to spears and axes. Curiously, some of these advances were abandoned after less than 5,000 years. Paleontologists speculated they were inspired by the need to tackle a changing climate, and changes in the food supply that came with it.
However, University of Oxford PhD student Patrick Roberts studied remains left in Blombos Cave, South Africa between 98,000 and 73,000 years ago, and nearby Klipdrift Shelter 72,000-59,000 years before now. He tracked the climate using ratios of carbon and oxygen isotopes from ostrich eggshells deposited at the sites, and his findings contradict the theory.
Stratigraphic layers from cave sites in South Africa, including some of the specimens found there. Roberts et al/PLOS One
Ostrich eggs make a mighty omelet, and have been a favorite food for hunter-gatherers in southern Africa, with the egg shells left at many sites. In PLOS One, Roberts and his co-authors write: “Ostriches are opportunistic mixed-feeders and are one of very few animals known to eat C3, C4, and CAM vegetation,” referring to the photosynthesis pathways of plants. “They also show no particular preference for any of these groups,” the paper continues, so what they eat changes with climatic conditions.
Ostrich eggshells provide an indication of the mother's diet. When rain falls all year round C4 plants flourish, and leave with them a large concentration of carbon-13 in the eggshells of birds that consume them, compared to the eras dominated by winter rainfall and C3 plants. Meanwhile, oxygen-18 concentrations are inversely related to the humidity during the ostrich breeding season. Roberts took the average of 7-12 eggs in each stratigraphic layer to get a more accurate picture than past studies, which used smaller samples.
From this data, Parker built up a picture of the climatic changes in the region during the caves' occupation. Crucially, any shifts can be compared to changes in the technology evident at the site, rather than having to rely on dating of doubtful precision.
The work confirms the sites' inhabitants adapted their diets to reflect climatic changes, with periods where the rainfall was concentrated in the winter months associated with more seafood consumption and hunting large mammals.
What climate changes don't seem to be associated with, however, are notable increases in technological sophistication. “While technological changes may have a more complex, indirect relationship with these environmental changes, we find no evidence that climate directly drove the technological or cultural innovations of the Still Bay and Howiesons Poort,” the authors conclude, while acknowledging that other proposed explanations have their own problems.