Question

In cognitive archaeology, researchers have long debated the origins of human symbolic thought. Proponents of the "cognitive leap" model contend that the sudden proliferation of symbolic artifacts—such as engraved ochre and shell beads—in Southern Africa approximately 75,000 years ago points to a rapid genetic mutation that reorganized the human brain for complex recursive language. They argue that the virtual absence of earlier symbolic material culture constitutes definitive evidence that earlier hominins lacked symbolic cognitive capacity. However, critics of this model point to isolated discoveries of pigment processing and long-distance raw material transport dating back nearly 300,000 years, arguing instead for a gradual, mosaic accumulation of behavioral modernity. These critics maintain that the apparent absence of widespread symbolic artifacts prior to 75,000 years ago is an artifact of taphonomic decay—environmental conditions such as humidity decomposing fragile organic goods—and low demographic density. Because small, fragmented populations frequently innovate practices but lack the interconnected social networks needed to transmit them across generations, cultural innovations often vanish locally. Thus, critics conclude that the apparent "explosion" of symbolic culture 75,000 years ago reflects rising population density and favorable preservation conditions rather than a sudden biological restructuring of the human mind.

Which of the following is an unstated assumption upon which the critics' argument against the "cognitive leap" model depends?