ABSTRACT
The fi rst simple cellular organisms, as indicated by the fossil record, originated on the Earth about 3.5 billion years ago (Chapter 1). The genome of these organisms was
probably already composed of a double-stranded DNA molecule. Long before that time a chemical evolution should have developed simple, self-reduplicating creatures like viruses or virus-like forms. Maybe some virus-like and/or proto-bacteria forms were inseminated on the Earth via a meteorite or comet attack in a time when the atmosphere was not yet a suffi cient barrier to prevent life forms from entering. So, the creation of life itself may have occurred somewhere else in the Universe. The fi rst eukaryotic fossils, resembling single-celled green algae, dated from about 1.4 billion years ago. During their life history both prokaryotes and eukaryotes constantly met variable environment, and had to change themselves to survive. In simple terms, these changes of organisms as a response to changing environments are evolution, when considered over in time. The environment is complex and comprises both the abiotic components (water, minerals, etc.) and the biotic components (other organisms and the products of their metabolism). The Genome, as an interrelated unity of all functioning genes of living beings, also had to change dynamically. That is why the causes of evolution are multifactorial. The initial causes of evolutionary changes can originate from inorganic compounds of the environment, as well as from organisms themselves and their genome. These changes, obviously as nowadays, included mutation, recombination, transposition, deletion, duplication, and others. By examining and comparing genomes of organisms of different phyla that exist today, we may imagine what mechanisms are operating and how the genome is changing, or reconstruct its evolution.
