Life first appeared on Earth around 3.8 billion years ago—not long after the Earth itself —and life forms have been changing and diversifying ever since. How is it possible for scientists to know about the history of life? What methods do they use to study life and how it has evolved over time? The methods scientists use to reconstruct the story of living things and how they change over time also change over time. New techniques and instruments are constantly being developed to look more deeply into the world of cells and DNA.