What if Earth, not Mars, had been a barren wasteland at the end of the last Ice Age?

The last Ice Age ended 10,000 years ago, and the current era, the Holocene, began. As the climate warmed, plants sprouted, and they attracted herbivores, which then attracted carnivores, and soon the entire food chain began to flourish. However, imagine a world where this didn’t happen on Earth, but on Mars instead. 10,000 years ago, Earth was a barren wasteland, devoid of any life except the most hardy microorganisms. Mars, on the other hand, gradually warmed up and became habitable. Liquid water began to flow on its surface, and the air became thick enough to sustain life. The first forms of life on Mars would have been microorganisms like bacteria and archaea. These microbes would have thrived in the Martian environment, feeding on chemicals and minerals in the soil and water. Over time, they would have evolved into more complex organisms, such as plants and animals. <br><br>Eventually, Mars would have supported a diverse range of flora and fauna, from towering trees and lush forests to vast herds of herbivores and carnivores. Meanwhile, on Earth, life would have struggled to survive. The harsh climate and lack of resources would have made it difficult for organisms to thrive. Perhaps small pockets of life existed in areas like near volcanic regions, where geothermal heat could sustain microbial life. <br><br>This world would likely have faced numerous challenges in developing civilizations. Colonization would likely have been difficult, as the planet would lack a magnetic field to protect against harsh radiation from space. Mars also has only about one-third of Earth’s gravity, which would have put evolutionary pressures on Martian animals and likely animals would have grown larger. Many of today’s Earth animals wouldn’t be able to survive on a planet with one-third of its gravity. Additional consequences of a lower gravity would be that the planet has trouble retaining an atmosphere, which already happened a long time ago in the planet’s history. <br><br>Martian colonies would need to adapt to these challenges to create self-sustaining ecosystems capable of supporting human life. By terraforming the planet - making it more Earth-like, they aimed to create a world where humans could live comfortably and thrive. They would have had to pump greenhouse gases into the atmosphere to improve air quality and increase temperatures. This could have been achieved by mining subterranean deposits of dry ice (frozen carbon dioxide), releasing methane and ammonia ices trapped in the ground, or importing from other planets and moons. <br><br>How would this have affected the world? Would humans ever have colonized Mars? Would the universe be similar to what we have today, and if so why? Would single-celled organisms be able to survive on Earth, or would the planet be a complete barren wasteland? <br><br>This was a post I originally posted on r/AskTeenGirls under my throwRA account u/CherryPitStains.