Phase 1 - exploration - humans can explore the barren Red planet only within protective pressure suits / rovers / habitats:

Phase 2 - planetary ecosynthesis - the air pressure and temperature has risen to the level where only warm clothing and oxygen mask is needed; simple organisms as lichens can be introduced:

Phase 3 - spreading life - humidity and temperature has risen to the level where only oxygen mask is needed; more complex organisms can be introduced:

Annotated images with three phases of exploring and terraforming Mars by Swedish game artist. The images were made with the guidance of planetary scientist Chris McKay from NASA's Ames Research Center.