Major Evolutionary Transitions: From DNA Double Helix to Multicellular Complexity

The advantage of the double helix structure is that the two strands carry the same information, but only one is actually decoded, while the other is used to correct errors during replication, thus offering the genetic system greater stability. In this passage or transition, the impulse of organic life toward greater complexity is evident, which led to dimensions where RNA was unstable, and simultaneously highlights the need for stability in the evolutionary direction, a necessity from which the more stable double helix of DNA arose. The stability of the evolutionary direction with information redundancy to correct evolutionary errors is hardly compatible with the idea of random mutation.

Bifurcation from prokaryotes to eukaryotes

One of the greatest events in biological evolution is the formation of the eukaryotic cell. This cell is incredibly more complex and articulated than its prokaryotic predecessor and allows supporting an incredibly higher degree of complexity and specialization. Based on the chemical composition of the membrane, current living organisms are divided into two groups: bacteria and eukaryotes on one side, archaea on the other. In both, the main lipids of the membrane are phospholipids, but with opposite chirality and according to a profoundly different organizational structure. It is plausible that bacteria and archaea diverged from a common ancestor that did not have true membranes. In this scenario, eukaryotes are seen as a lineage derived from the fusion of an archaeon and a bacterium, the latter converted into mitochondria. The transition from prokaryotic cells, archaea and bacteria, to eukaryotic cells corresponds to a substantial increase in the organizational complexity of a cell. Eukaryotes possess a series of unique traits that reflect a profound evolutionary divide including: phagocytosis and the endomembrane system; a unique cytoskeleton; the nucleus; mitosis and the cell cycle; unique mechanisms for controlling gene transcription and translation and a large number of genes without homologs in bacteria or archaea. Finally, but no less important, eukaryotes have mitochondria, an $ATP$-producing organelle derived from a symbiotic bacterium.

From asexual clones to sexual populations

Sex is a universal property of life that includes any process that incorporates foreign DNA into functional genomes. Sex is essential for the conservation of biological information and, together, is a powerful mechanism of genetic innovation. Sexual reproduction, instead, is a unique process of eukaryotes, consisting in the formation of a new cell, the zygote, by fusion or syngamy of two specialized cells, the gametes. Thus defined, sexual reproduction is actually the opposite of a reproductive mechanism: from two cells comes only one cell, and usually the process requires the collaboration of two individuals. In genetic terms, each parent transmits to the offspring only half of its genes, while asexual reproduction allows a single parent to transmit the entire genotype. Despite the enormous costs and potentially harmful implications, sexual reproduction occurs routinely in most eukaryotes, often alternating with forms of asexual reproduction in which a new organism derives from somatic cells or from a single gamete, without syngamy. The transition from asexual clones to sexual populations represents a very important transition in biological life. The occurrence of meiotic sex in eukaryotes and its absence in prokaryotes is not a mere difference in the mechanism of reproduction, but reflects a profound divergence in the life strategy of the two types of cellular organization. Prokaryotic recombination leads to pangenomes, eukaryotic recombination through sexual reproduction leads to vertical inheritance and prepares for cell specialization.

Cell specialization and differentiation

Despite the unicellular condition being very versatile and efficient, multicellularity has evolved many times independently in both bacteria and eukaryotes, but only in the latter has it reached high levels of complexity. Multicellularity corresponds to the increase of physical form, division of labor and cell specialization. Multicellularity has required the evolution of mechanisms of cellular communication and differentiation: in large multicellular organisms, vascular systems for long-distance transport have evolved. Animals and terrestrial plants have added internal extracellular compartments subject to homeostatic control. In multicellular organisms of closed form, body shape is determined during embryonic development, after which stem cells control cell turnover and isometric growth. Complex multicellular organisms have three hierarchically interconnected levels of organization and therefore necessarily of death: systemic, organic and, finally, cellular.