Rubisco, an ancient photosynthetic enzyme of Archean origin, became increasingly inefficient with reduction of atmospheric CO₂ levels in the later geological history. The remarkable expansion of C₄ biomass in late Miocene was an adaptive evolution in photosynthesis, as C₄ photosynthesis was more adaptive to the lower concentration of CO₂, as well as to monsoonal climate with warmer temperature and seasonal aridity. Given the significant difference in carbon isotope between C₃ and C₄ plants, the δ¹³C of paleosols and mammalian tooth enamels was widely used as an indicator of proportions of C₃ to C₄ plants in vegetation. Since the discovery of the dramatic shift of δ¹³C in the late Miocene paleosol from Pakistan over 10 years ago, the expansion of C₄ biomass then has become a matter of hot debate within the scientific community: whether it was caused by monsoon climate, decreasing atmospheric CO₂ level, or aridification？Though it remains an enigma up to now, there are useful lessons to be drawn from a brief review of the story as presented in this paper.