All life on earth - everything from bacteria to mushrooms to hippos - shares an astonishing range of detailed biochemical similarities, including the structure of heredity in DNA and RNA , and the universal use of ATP as an energy-storing compound. Two possible scenarios, with markedly different implications for the nature of life, might explain these regularities: either all earthly life shares these features because no other chemistry can work, or these similarities only record the common descent of all organisms on earth from a single origin that happened to feature this chemistry as one possibility among many.
Fossil residues of ancient life-forms discovered in the rocks do not reveal a simple beginning. Although we may care to think of fossil bacteria and fossil algae and microfungi as being simple compared to a dog or horse, the information standard remains enormously high. Most of the biochemical complexity of life was present already at the time the oldest surface rocks of the Earth were formed.