Which of the following appeared on the Earth with the arrival of plants and was absent in the past?

Formation of which complex molecules was observed by Urey and Miller when they subjected substances like $NH_3, CH_4, H_2O$ and $H_2$ to electric discharge?

What was the result of the expansion of the universe?

According to which theory does life originate from grass and mud?

Which organism was the first to create an oxidizing atmosphere on Earth?

$A$ : During evolution,the early time period was without an ozone layer.
$R$ : The ozone layer absorbs $U.V.$ rays to some extent.