This change in environment presumably caused prehumans to take up walking . Near 7 Myr ago, a major vegetation shift occurred. Experiments indicate that C4 plants thrive when CO 2 values are less than 500 ppm, while C3 plants outcompete C4 plants when CO 2 values are more than 600 ppm. Thus, the shift in vegetation may record a decrease in atmospheric CO 2 . In addition to long-term climate changes, shorter climate changes may also have played an important role in human evolution.
African climate shifts to the growth of the Northern Hemisphere ice sheets.
At this time, many browsing animals became extinct, and new grazing animals appeared both by evolution and by immigration from Asia. Originally, scientists didn't think that changes in the Northern Hemisphere ice sheets affected African climate.