The evolutionary tree of life
In modern science, a broad diversity of information helps to feature how life was born on earth, and how and when the different groups now observed in nature originated. Such inferences and reconstruction are mainly based on two very different kinds of data: the fossil record and molecular phylogenies.
Broadly speaking, the surface of our planet solidified about four billion years ago. In a relatively short period (500 million years) prebiotic systems evolved and produced cells surrounded by a membrane, more or less similar to extant bacteria. Typical bacterial constructions, the stromatolithes, are now observed along the seashore, but already existed three billion years ago. The further evolution of Prokaryotes (Bacteria and Archea) was fairly slow with little phenotypic changes. A major event, which took place about two billion years ago, was the emergence of Eukaryotes, that is, of cells with a nucleus and many other cellular characteristics. The origin of Eukaryotes is still debated. Molecular investigations have shown that, according to the kind of genes which are compared, Eukaryotes appear to be closer either to Bacteria or to Archea. Of course the fossil documents are very scarce. A novel theory now spreading among biologists, assumes that the first ancestral Eukaryote arose from a symbiosis between a Bacterium and an Archea: the membrane of the internal symbiont produced the nuclear membrane, and the two genomes progressively fused into a single one, within the nucleus. The acquisition of a bigger and more complex genome was followed by further major changes. We may recall the origin of mitochondria from another cytoplasmic bacterial symbiont and the acquisition of photosynthesis (chloroplasts) from symbiotic cyanobacteria.
For a long evolutionary time, Eukarya remained unicellular but produced a broad diversity of phyla still found in the extant world. These unicellular eukaryotes are often classified in a single cluster, or kingdom, called Protista, although they include a diversity of clades and are obviously paraphyletic. During their evolution, Eukaryotes acquired progressively new properties and organelles, such as chromosomes, mitosis and presumably meiosis. Possible mechanisms can be suggested from a comparison of what now exists in the living world (Maynard Smith and Szathmary 1995) while the fossil record is useless.