Final answer:
The Great Oxidation Event was the most significant alteration of Earth's atmosphere by living things, but the rapid increase in CO2 due to human activity since the Industrial Revolution has become a primary driver of modern global climate change.
Step-by-step explanation:
The biggest change to Earth's atmosphere was the accumulation of oxygen produced by photosynthetic organisms, such as cyanobacteria, which altered the atmosphere during the Great Oxidation Event approximately 2.4 billion years ago. However, since the Industrial Revolution, humans have had a significant impact on the atmosphere. The carbon dioxide (CO2) concentration has risen markedly due to human activity, notably the burning of fossil fuels, which is a key driver of current global climate change.
Historical data from ice cores, tree rings, and other sources indicate that before industrialization, climate changes were driven by natural processes like volcanic eruptions and changes in the Earth's orbit. Today, the rapid increase in greenhouse gases, especially CO2, is closely linked with human activities. It is clear from scientific evidence that humans are now the primary force behind the altering of Earth's climate and environment.