The oldest fossile records of organisms are about 3,8 million years old. First organisms that resided on Earth didn't have cells. They were macromolecular systems from which during the chemical evolution first precellular forms, protobionts, were formed. First cells probably originated from protobionts, but the evolutionary pathway from precellular to cellular organisms is not known. It has been suggested that all today living beings have one common ancestor). LUCA (Last universal common ancestor) is at root of the Tree of life, which presents phylogenetic relationships between the species. The Tree has three branches that represent prokaryotic, arheal and eukaryotic domain of life. Viruses are organisms that can exist only as a parasites. They are not considered alive and until recently were viewed as the passive observers of evolution. Radical new hypotheses about viruses were suggested. Methods of molecular biology have shown that viruses are very old and that they were active promoters of evolution of the early organisms. New theories gave viruses leading role in all major evolutionary transitions.