For decades, paleoclimatologists posited that the dramatic expansion of C4 grasses during the Late Miocene (approximately 8 to 6 million years ago) was driven primarily by a global decline in atmospheric carbon dioxide levels. Because C4 photosynthesis is structurally more efficient under low carbon dioxide concentrations than the ancestral C3 pathway, researchers reasoned that dropping carbon dioxide levels inevitably conferred a competitive advantage on C4 species. However, recent high-resolution proxies derived from fossilized leaf waxes suggest that atmospheric carbon dioxide concentrations had already stabilized near pre-industrial levels long before the C4 expansion began. Instead, geochemical signatures point to heightened seasonality and widespread aridification marked by monsoon intensification as the crucial catalysts. C4 grasses possess key physiological adaptations—namely, superior water-use efficiency and rapid regeneration capabilities following seasonal fires—that allowed them to outcompete C3 vegetation in regions subject to prolonged dry spells punctuated by torrential rains. Consequently, while atmospheric carbon dioxide composition provided the baseline physiological precondition, regional hydrological shifts and fire dynamics were the proximate forces dictating the terrestrial biome conversion.
Based on the passage, which of the following can be inferred regarding the expansion of C4 grasses during the Late Miocene? Consider each of the choices separately and select all that apply.
- Atmospheric carbon dioxide concentrations alone were insufficient to dictate the specific timing of the Late Miocene C4 grass expansion.Answer
- BThe physiological traits of C4 grasses evolved as a direct evolutionary adaptation to the monsoon intensification occurring during the Late Miocene.
- Early paleoclimatological theories failed to account for the chronological gap between carbon dioxide stabilization and the vegetation shift.Answer