Mars lost its primitive atmosphere in a catastrophic event.
We are wasting time. We should focus all human efforts on our survival before the next massive impact destroys the Earth. Could we divert a truly massive planet killing asteroid or comet? No. Terraforming Mars is a start, but just as there are planet killing events there are also solar system killing events. Imagine a collision course with a cool supergiant star 2,600 times the size of the Sun. We’ve got a long way to go! It would be great if some version of humanity survives the extinction of our sun in 6 billion years, but we have WAY less time than that, perhaps no more than 600 million years.
During the next four billion years, the luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will cause a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years, the level of CO2 will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at CO2 concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The die off of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.