Final answer:
Free oxygen was not believed by scientists to have been present in the early Earth's atmosphere, as evidenced by the Urey and Miller experiment and other scientific theories regarding the conditions of primitive Earth.
Step-by-step explanation:
Scientists believe that free oxygen (b. Free oxygen) was not present in the early Earth's atmosphere, according to the evidence and theories about the conditions on primitive Earth. The early atmosphere of Earth contained ammonia, methane gas, and water vapor, but it lacked significant amounts of free oxygen. It was not until the appearance of photosynthetic organisms, such as cyanobacteria, that free oxygen began to accumulate in the atmosphere, which was a major turning point in Earth's history and set the stage for aerobic life forms.Scientists believe that free oxygen was NOT present in the early Earth's atmosphere. The ancient atmosphere contained ammonia, methane, water vapor, and carbon dioxide but only a trace of oxygen. The absence of oxygen allowed for complex chemical reactions to occur that led to the production of amino acids and other building blocks of life.
The early Earth's atmosphere was vastly different from our current atmosphere, as evidenced by scientific experiments like the Urey and Miller experiment. This experiment demonstrated that organic compounds essential for life could have formed under the reducing conditions believed to be present on early Earth. It used a mixture of gases that included methane, ammonia, hydrogen, and water vapor, simulating the conditions of the early Earth, but notably, it did not include free oxygen in the experimental setup.