The last common ancestor. When the earth formed some 4.6 billion years ago, it was lifeless and inhospitable to living organisms. 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.
All life is cellular.
The major elements of covalently bound biomolecules are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. The principle macromolecules are proteins, lipids, carbohydrates and nucleic acids. 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.