Fundamental to a deeper understanding of complex biological functions are ideas about how life originated and evolved. They include questions about how the first compounds, essential to life, appeared on Earth; how the first replicating molecules came into being; how RNA and DNA were formed; how prokaryotes and the earliest eukaryotes emerged; how different species, with traits like susceptibility, sentience, perception, cognition, and self-consciousness, and with various patterns of behaviour, evolved; and how, with these developments, the environment and the ecological systems changed. These questions differ in some sense from other problems of natural science. The study of the origin and evolution of different forms of life is both a matter of trying to reconstruct the past, and of studying present structures and processes. Conclusions about how things might have happened are drawn from evidence such as that obtained from fossils, theories based upon knowledge about present biological processes, models based on knowledge about chemical reactions and assumptions about various conditions on the early Earth. The analysis involves a great number of widely differing disciplines, such as chemistry, geology, biology, physics, computer science and philosophy (e.g.