What is one reason mammals didn't exist until 200 million years ago, even though fossil evidence supports that life existed on Earth up to 3.8 billion years ago?

Question

a Early Earth's atmosphere was glacial from the formation of Earth until the evolution of mammals, so it was too cold to sustain life
b Early Earth's atmosphere contains a high concentration of oxygen that would have overwhelmed mammals
c Early Earth's atmosphere contained a toxic mix of gases that couldn't have sustained mammals
d Early Earth's atmosphere was too small to hold the gases that were needed to sustain mammalian life

Answer 1

The correct answer is: c Early Earth's atmosphere contained a toxic mix of gases that couldn't have sustained mammals.

During Earth's early history, the atmosphere was predominantly made up of gases such as methane, ammonia, and other compounds that were not conducive to sustaining complex life forms like mammals. It wasn't until significant geological and biological changes occurred—such as the Great Oxygenation Event—that conditions became more suitable for the evolution of mammals and other complex life forms.