Human evolution is the subject here, specifically the question of whether shifting climates forced our ancestors to adapt or die. This first installment of a three-part NOVA series brings in geologists, paleoclimatologists, and archaeologists to read the fossil record alongside ancient climate data, looking for the moments when temperature swings, drought, or changing landscapes lined up with major shifts in early hominin biology and behavior. The film treats climate not as backdrop but as a possible engine, asking what pressure would have to look like to push a species toward bigger brains, new tools, or life outside Africa. Interviews with researchers working directly with fossil evidence and climate reconstructions carry the argument, rather than dramatization or narration alone. It sets up the two later episodes in the series, which follow the same climate-and-evolution question through later stages of the human story.