Final answer:
The early Earth had a very different atmosphere from today's, rich in gases like ammonia and methane but lacking oxygen, and was subject to frequent geologic upheaval and volcanic activity, which together with a stronger greenhouse effect, set the stage for the emergence of life.
Step-by-step explanation:
Most scientists agree that the early conditions of Earth were very different from the conditions today. The early Earth had a very different atmosphere, characterized by an anoxic environment, meaning that it lacked oxygen. This made it inhospitable to the aerobic life forms that predominate today. The early atmosphere contained gases like ammonia, methane, water vapor, and carbon dioxide, but only a trace amount of oxygen. Extreme radiation bombarded the planet, and geologic upheaval and volcanic activity were more prevalent than contemporarily observed conditions.
As Earth continued to cool after its formation, the solid crust developed, and volcanic outgassing contributed to the formation of the early atmosphere. The resulting climatic conditions, despite the faint young Sun, were kept warm due in part to higher levels of carbon dioxide and a stronger greenhouse effect. Consequently, the ancient atmosphere and oceans, although toxic to most of today's life forms, provided the stage for the beginnings of life.
Scientists find the origin of life on Earth and the ability of life to persist through such harsh early conditions intriguing. The resilience of anaerobic organisms and the biochemical traces in modern life forms offer insights into the past. However, due to Earth's dynamic nature through processes like plate tectonics, little direct evidence remains about its earliest history.