When the earth formed some 4.6 billion years ago, it was lifeless and inhospitable to living organisms. One billion years later it was already teeming with prokaryotic life forms, ancestors to all present living things. What would these early progenitors of life be like? If we make the reasonable assumption that the last common ancestor of all presently living organisms must have had those characteristics which are now shared by the organisms which constitute the five living kingdoms, then a listing of the common characteristics of living species also describes the minimum characteristics of the last common ancestor. Harold Horowitz compiled the following list in his book, "Beginnings of Cellular Life" (Yale University Press, 1992)
All life is cellular.
All living things are from 50 to over 90% water, the source of protons, hydrogen and oxygen in photosynthesis and the solvent of biomolecules.
The major elements of covalently bound biomolecules are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
There is a universal set of small molecules: (i.e. sugars, amino acids, nucleotides, fatty acids, phospholipids, vitamins and coenzymes.)
The principle macromolecules are proteins, lipids, carbohydrates and nucleic acids.
There is a universal type of membrane structure (i.e. the lipid bilayer).
The flow of energy in living things involves formation and hydrolysis of phosphate bonds, usually ATP.
The metabolic reactions of any living species is a subset of a universal network of intermediary metabolism (i.e. glycolysis; the Krebs cycle, the electron transport chain)
Every replicating cell has a genome made of DNA that stores the genetic information of the cell which is read out in sequences of RNA and translated into protein.
All growing cells have ribosomes, which are the sites of protein synthesis.
All living things translate information from nucleotide language through specific activating enzymes and transfer RNAs.
All replicating biological systems give rise to altered phenotype due to mutated genotypes.
Reactions that proceed at appreciable rates in all living cells are catalyzed by enzymes.