XXIII.1 All known species of organisms living on the Earth evolved from a common ancestor in the course of phylogenesis
To the present day, 1-2 million species of living organisms and hundreds of thousands of extinct organisms have been described.However, most living species and even more extinct species have not yet been identified and probably never will be.Generally accepted estimates place the number of species now living on the Earth at several tens of millions.While the latest results suggest that, at the very least for insects, these estimates will probably be excessive (Novotný et al. 2002), it seems, on the other hand, that the biodiversity of parasitic organisms could be substantially underestimated.
All these species apparently evolved in the past from a common ancestor.It is not clear how many times life evolved on the Earth or how many times it was transported from the surrounding universe.In the light of the speed with which it appeared as soon as conditions became favourable for its existence, i.e. after the end of the period of massive bombarding of the surface of the Earth by cosmic projectiles and after a solid cooling crust was formed, it is quite possible that life evolves relatively easily and thus that this occurred repeatedly on the Earth.No surprising events can be completely excluded; nonetheless, it is highly probable that the representatives of only a single line have been preserved to the present day, either by accident or as a result of mutual competition.Available information, especially related to the uniformity of the basic molecular apparatus of organisms and the similarity of basic biochemical processes, such as replication of nucleic acid or proteosynthesis, unambiguously indicate that all the known species of organisms have a common ancestor.This common ancestor most probably lived on the Earth 3.8 billion years ago (Holland 1997).However, we have no proof that the traces indicating the existence of life found in rocks of this age were actually left by the ancestors of modern organisms, and not by the members of some other, extinct line.However, it is almost certain that life did not cease to exist on Earth from the most ancient times to the present day and that organisms have been present on the Earth for at least 3.5 billion years in such large numbers that they fundamentally affected the development of the atmosphere, hydrosphere and geosphere of the planet.What this life looked like and when, not the first, but the last common ancestor of all contemporary forms of life, i.e. the organism from which modern bacteria, archea (archeobacteria ) and eukaryotes, branched off at a certain moment and in a certain order, remains an interesting and still unresolved question.This hypothetical organism is designated by the abbreviation LUCA (Last Universal Common Ancestor) in the English literature.On the basis of current knowledge in comparative molecular biology, it rather seems that this was a quite advanced organism with a modern-type, fully developed proteosynthetic apparatus.According to some hypotheses, it could have lived on the Earth at a relatively late date, possibly 2 billion years ago (Schopf 2000).