The exact mechanisms of abiogenesis are unknown: notable theories include the RNA (RiboNucleic Acid) world hypothesis (RNA-based replicators) and the iron-sulfur world theory (metabolism without genetics). The Darwin theory of evolution does not attempt to explain the origin of life but the process by which different lifeforms have developed throughout history via genetic mutation and natural selection. At the end of the 20th century, based upon insight gleaned from the gene-centered view of evolution, biologists George C. Williams, Richard Dawkins, David Haig, among others, conclude that if there is a primary function to life, it is the replication of DNA (Deoxyribo Nucleic Acid) and the survival of one’s genes.

However, though scientists have intensively studied life on Earth, defining life in unequivocal terms is still a challenge. Physically, one may say that life “feeds on negative entropy which refers to the process by which living entities decrease their internal entropy at the expense of some form of energy taken in from the environment”. Biologists generally agree that lifeforms are self-organizing systems regulating the internal environment as to maintain this organized state, metabolism serves to provide energy, and reproduction causes life to continue over a span of multiple generations. Typically, organisms are responsive to stimuli and genetic information tends to change from generation to generation resulting in adaptation through evolution, these characteristics optimizing the chances of survival for the individual organism and its descendants respectively.

Though the Big Bang model was met with much skepticism when first introduced, it has become well-supported by several independent observations. However, current physics can only describe the early universe from 10−43 seconds after the Big Bang (where zero time corresponds to infinite temperature); a theory of quantum gravity would be required to go further back in time. Nevertheless, many physicists have speculated about what would have preceded this limit, and how the universe came into being. Some physicists think that the Big Bang occurred coincidentally, and when considering the anthropic principle, it is sometimes interpreted as implying the existence of a multiverse.