Large amounts of oxygen gas appeared in earth's atmosphere about years ago.

The cluster of developing cells from conception until birth is called an

In the practice of science, this type of reasoning is used to develop explanations.

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled