The video discusses the early climate of Mars, describing it as a warm planet with a thick atmosphere that retained heat from the sun, creating a greenhouse effect. This made the surface conducive to life as we know it. The evidence for this warmer past is found in the topography of Mars, specifically in the remnants of ancient river valleys that indicate a time when water flowed freely on the planet. This evidence suggests that Mars was once very different from the icy cold desert it is today.
source