How does the study of mars and venus establish the validity of the greenhouse effect

Venus will help us understand what happens when the greenhouse effect is really extreme but it's not a really good example of what will happen to Earth because of human activities. Life on Earth would completely die due to the extreme temperatures way before reaching even half of the concentrations of carbon dioxide on Venus. Mars doesn’t really have any greenhouse effects. Mars does have some atmospheric carbon dioxide but almost no atmosphere but the atmosphere it does have is so thin that it cannot keep energy from the Sun. That’s why there are extreme temperature contrasts between day and night and sun or shade. However, most scientists say that Mars used to be warmer in and even had oceans, which means that the atmosphere was also very different. About 3600 million years ago, something happened and the planet evolved towards its current state. studying the planet could tell us what could have caused such a huge change

Except common history