2. From about 4 billion years ago to about 2.5 billion years ago, oxygen gas 2 points became more common in Earth's atmosphere. Oxygen was a toxic
compound to most life forms from that time period. Today, oxygen is
required by all but some of the most primitive life forms. Which statement
best explains how early life forms that were harmed by oxygen evolved
into today's life forms that require it?
A Warming temperatures allowed the colonization of land environments, where
oxygen was more plentiful
B. Over time, many species gradually adapted to a more oxygen-rich environment,
allowing them to survive in shallow water and on land
C. Carbon dioxide-using species all became extinct, which allowed oxygenusing life
forms to flourish in shallow oceans.
D The growth of seas allowed early species to survive by using dissolved carbon
dioxide in seawater rather than using it directly from the air
3. Which sequence of events best describes the current theory of the
origin of life on Earth?