The hosts provides an overview of how life on Earth started. He mentions the eons in which life formed and explains the RNA World Hypothesis, which states that early life relied on RNA rather than DNA.
John Green explains how historians have determined the age of the Earth. He mentions that they created a quantitative chronology of human history by using ancient documents and events. He also states that various thinkers throughout history have used fossils and rocks to determine the Earth's actual age.