Why would early earth have been toxic for modern day animals?

Early Earth had a harsh environment characterized by high volcanic activity, a lack of oxygen, and extreme temperatures. The atmosphere was primarily composed of toxic gases like methane, ammonia, and carbon dioxide, which would have been harmful to modern-day animals that rely on oxygen for survival. Additionally, the absence of protective ozone layers exposed the surface to harmful solar radiation. These conditions made it inhospitable for complex life forms as we know them today.