Origin of Life

There is not direct fossil-like evidence of how life originated on Earth, the molecular processes that preceded the appearance of cells do not leave such tangible evidence. However, fossils of single-celled microorganisms are present in rocks 3.0–3.5 billion years old (with some scientific controversy over which rocks contain the oldest true bacterial fossils), and chemical traces in Greenland rocks establish that single-celled life existed as long ago as 3.8 billion years. As the Earth itself is approximately 4.6 billion years old, these data suggest that life evolved within 700 million years after the Earth formed. Experiments performed over the past 50 years suggest that many important ingredients of life, including amino acids and nucleic-acid bases (the molecular building-blocks of deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]), could have formed abundantly under conditions that may have existed on the early Earth, and scientists conjecture that the presence of these molecules facilitated the formation of the first actual life.