The changes that were (necessary) in the Earth's atmosphere to allow complex organisms to evolve was a collision from above such as a meteor and an increase in volcanic activity.
Your correct answer . . .
[tex]\boxed{\boxed{A \ collision \ from \ above \ such \ as }}[/tex] . . .
[tex]\boxed{\boxed{a \ meteor \ and \ an \ increase \ in \ volcanic \ activity.}}[/tex] Hope this helps. ~Jurgen
It had to develop major quantity of oxygen and ozone.
Explanation:
At the beginning, Earth's atmosphere was constituted by methane, ammonia, water vapour and neon. So, at that time, the absence of ozone and major quantity of oxygen didn't allow to create complex organisms.
