Answer:
Certainly, here's a thesis statement for your high school history essay on how Mars used to be habitable:
"Through geological and climatological evidence, it is clear that Mars, once thought of as an arid and inhospitable planet, possessed the conditions necessary for habitability in its distant past, offering intriguing insights into the potential for life beyond Earth."
You can expand on this thesis by discussing specific evidence, such as the presence of water channels, ancient lake beds, and the possibility of a thicker atmosphere in Mars' history. Be sure to support your thesis with relevant research and examples in your essay.