From the very concept Darwin's tree was daring: not only extant, but also all extinct organisms found, in principle, a place on this tree.

Darwin's tree has been grown into a veritable forest. In the late nineteenth century, Ernst Haeckel quickly popularized the tree by putting names on the branches. Interestingly, the only non-verbal information in Darwin's book was a single figure with a hypothetical tree of descent for organisms. Understandably, his theory became a scientific and philosophical revolution. In contrast to Maxwell's book, Darwin's grand design was expressed in simple sentences comprehensible to a lay audience. One could even argue that the holy grail of a great unifying theory was achieved by Darwin and Wallace at a time when Maxwell was unifying physics, the older sister of biology, at the level of the electromagnetic field theory. Suddenly, biologists had a unifying theory for their branch of science. How can biologists cope conceptually and technically with this enormous species number? A deep sigh of relief came for biologists already in 1859 with the publication of Charles Darwin's book вЂOn the Origin of SpeciesвЂ™. To date, zoologists have described more than a million animal species and botanists a quarter of a million flowering plant species ( May 1988 Brusca & Brusca 2003), it is now the turn of microbiologists to add perhaps millions of species to the long list of life. The closer biologists looked into the biosphere, the more species were discovered. Today we continue LinnГ©'s eighteenth-century efforts to put a systematic grid on the diversity of life, which has been for biologists a source of aesthetic fascination and scientific despair. Despite this hopeful start, it took nearly 2000 years before biology became an established branch of science. His books offer careful descriptions, sharp reasoning, the beginning of experimentation and entertaining errors. When reading the books of Aristotle on animals, one gets the impression that the roots of biological research go back to classical Greece.

Despite their probable distinct evolutionary origin, both spheres were and are connected by intensive two-way gene transfers.ДЎ. On earth we have therefore at least two large DNA sequence spaces, one represented by capsid-encoding viruses and another by ribosome-encoding cells. In view of the polythetic nature of current life definitions, viruses cannot be dismissed as non-living material. However, presently viruses do not find a place on the universal tree of life, which is thus only a tree of cellular life. Furthermore, structural biology has demonstrated links between viruses infecting the three domains of life pointing to a very ancient origin of viruses. Comparative genomics has shown elements of vertical evolution in some groups of viruses. Viral genomics have revealed an unexpected size and distinctness of the viral DNA sequence space. Microbial ecologists have demonstrated that viruses are the most numerous biological entities on earth, outnumbering cells by a factor of 10.

The tree concept is challenged by the occurrence of horizontal gene transfer and-as summarized in this review-by the omission of viruses. Darwin provided a great unifying theory for biology its visual expression is the universal tree of life.

Tree of life meaning