Study the graph about the oxygen content of Earth’s atmosphere.
Oxygen Content of Earth’s Atmosphere Line Graph – X-axis: Millions of Years Before Present – Y-axis: Oxygen in Volume Percent The line starts at 1000 million years before present at 4 percent oxygen, remains level until 625 million years before present, increases quickly to 13 percent oxygen, then increases slowly to 18 percent oxygen. The line jumps to 35 percent oxygen near 300 million years before present, drops quickly to 15 percent oxygen, then increases to 31 percent oxygen near 100 million years before present, before dropping down to 20 percent oxygen. Which describes oxygen content as Earth evolved over time?
A. Oxygen levels sharply declined about 400 million years ago.
B. Oxygen levels stay above 15 percent starting 200 million years ago.
C. Oxygen levels remain below 5 percent starting 1000 million years ago.
D. Oxygen levels raise significantly between 1000 and 700 million years ago.
The Correct Answer and Explanation is:
The correct answer is D. Oxygen levels raise significantly between 1000 and 700 million years ago.
Explanation:
The provided graph depicts the changes in the oxygen content of Earth’s atmosphere over a span of 1,000 million years (1 billion years) to 100 million years ago. The oxygen levels start at approximately 4% at 1,000 million years before present, indicating a relatively low oxygen atmosphere early in Earth’s history. This initial level of oxygen remains stable until around 625 million years ago.
At this point, there is a notable increase in oxygen content, rising quickly to about 13%. This rapid change signifies a significant biological or geological event that contributed to increased oxygen production, likely linked to the proliferation of photosynthetic organisms such as cyanobacteria and later, plants. Following this quick increase, oxygen levels continue to rise slowly, reaching 18% by around 400 million years ago.
The statement “oxygen levels raise significantly between 1000 and 700 million years ago” accurately reflects this trend observed in the graph. During this period, there was not only an increase in the percentage of oxygen but also a major shift in Earth’s atmosphere that set the stage for the evolution of more complex life forms.
To consider the other options:
- A is incorrect because while oxygen levels did drop from 35% to 15%, this was after 300 million years ago, not a decline that sharply defines the 400 million-year mark.
- B is inaccurate as oxygen levels fall below 15% after 300 million years ago, not remaining above that threshold.
- C is false since oxygen levels were not consistently below 5% but instead were higher, beginning at 4% and later significantly increasing.
In summary, the graph illustrates the dynamic changes in Earth’s atmosphere, emphasizing the significant rise in oxygen levels between 1,000 and 700 million years ago, marking a crucial evolutionary phase in Earth’s history.