Evolution from unicellular to multicellular lifeforms
Unicellular Lifeforms
According to the latest knowledge, organic life developed on ALL PLANETS in the same order. Without bacteria and archaea, a biome cannot provide a stable basis for multicellular life. Many soil bacteria are indispensable for important geochemical material cycles because they act as descomposers and make nutrient salts available for plants. In the same way, bacteria occur as symbionts in the intestine or in other organs of many multicellular organisms and are involved in digestion and other important physiological processes. Many of the known bacteria can also adapt and multiply to different extreme conditions such as temperature, pressure and gravitation. Some also produce oxygen and can even make underground cavities or subglacial biomes habitable for multicellular lifeforms.
Multicellular Lifeforms
Multicellular life develops in a sufficiently stable environment and with sufficient time. As with unicellular organisms, evolution is subject to a certain order. Due to the different factors of planets such as chemical composition of the atmosphere, temperature, air pressure, gravitation, luminosity of the sun or day length, there are huge differences to lifeforms on Earth. For example, many life forms primarily produce carboxylase in their liver to be able to breathe in a carbon dioxide atmosphere. This requires biotin which is absorbed through food. Excess carbon is metabolized to malic acid in the body and then excreted via the urinary tract and exhaled as hydrocarbons such as ethane. Multicellular species on gas giants mostly have a very strong electromagnetic field, a slow metabolism and complex filtration methods to regulate their energy balance with the little oxygen or hydrocarbons.
Evolutionary Factors
They denote the processes by which the gene pool (the total of gene variations of a species) is changed. These processes are the central cause of evolutionary changes. The most important evolutionary factors that change the gene pool are mutation, recombination, selection and gene drift.
Mutation
The biggest factor in the evolution of exospecies may be the mutation when considering the transpermic exotardigrades, but this has not yet been researched. On planets where abiogenesis took place, mutations are mostly spontaneous changes in the base sequences of the DNA and new genes arise. If a mutation takes place in a cell from which later germ cells emerge, the altered gene is transmitted to the offspring via the fertilized cell and thus changes the gene pool of the species. The new genetic makeup leads to characteristics that have not previously existed within the species. Whether there is a lasting change in the gene pool depends crucially on how the selection affects the new characteristic. Genetics that lead to disadvantageous characteristics disappear from the gene pool or rarely remains.
Recombination
Recombinations occur in the context of the presumably phylogenetically older parasexual recombination of prokaryotes and some fungi and in the sexual reproduction of almost all plants and animals. Through recombination, which takes place through meiosis in the formation of germ cells and the fusion of the nucleus during fertilization, the gene dispositions of the parents are recombined so that offspring are created with individual combinations of gene dispositions. In the case of sexual reproduction, a distinction is made between intrachromosomal recombination by recombining alleles within chromosomes (as a result of the crossover on the occasion of the first meiosis) and interchromosomal recombination by recombining entire chromosomes in the chromosome set. When recombined, the relative abundance of genes in a species remains unchanged, but the phenotypic variability of individuals is effectively increased. Recombination does not happen in asexual reproduction.
Selection
This means the natural selection by its environment. The prerequisite for selection is the variability of a species caused by recombination and mutation. It occurs when individuals with traits that are beneficial for survival and reproduction can produce more offspring than individuals without these traits. In this way, the species can make better adjustments to their environmental conditions over the course of the generations. Within such changes in characteristics, a species can also split into new species as part of species formation. Sexual selection is a special case and only occurs intraspecifically. It sets characteristics whose presence is directly correlated with the copulation success through preferred partner selection. Traits evolved through sexual selection are particularly common in male species. Although these characteristics can reduce the survival probability of individual males (mating calls or bright colors), the reproductive success of such males is normally higher.
Genetic drift
Genetic drift is a random change in the gene pool. It is more effective in small populations than in large ones. For example, in the event of a natural disaster or a plague, a group of carriers of certain characteristics can suddenly become extinct. The surviving part of the species spreads with a slightly different genetic composition. If individuals with disadvantageous genes accidentally survive, even these spread. Another example of gene drift is the settlement of a new habitat by a small start-up population. The new species shows the frequency distribution of the genetic makeup of the founder population, which may differ from the original species. Whether natural selection or gene drift have the greater influence on the development of new mutations depends on the size of the population and the strength of the selection. Natural selection dominates in large populations, gene drift in small ones. The effective population size therefore has a major impact on the course of evolution. For example, if a species goes through a temporarily very small population size, it also loses a large part of its genetic variability. The population as a whole becomes more similar and loses many rare variants. Such events can be caused by natural disasters, climatic fluctuations, migration or division of populations.
Secondary Factors
These evolutionary factors have a less significant role than the above processes. They tend to be rare and have little impact on the entirety of a species. These factors are called gene flow and isolation.
Gene flow
In evolution, gene flow denotes the exchange of genetic material between two populations of a species and within one population. Modern Humans serves as an example. In Homo sapiens, the gene flow of archaic humans to Homo sapiens has been demonstrated (between Neanderthals and Homo sapiens and between Denisovans and Homo sapiens) because these species had not yet distanced themselves too far genetically. If the gene flow is prevented by geographic isolation for example, the populations develop differently. Then allopatric species formation can occur.
Isolation
Isolation in evolution is the so-called reproductive isolation. This means an interruption of the gene flow between populations of the same species. These can then no longer produce fertile offspring with members of other populations. Geographic and ecological isolation separates different populations of one species from each other, which may develop into new species. However, this insulation is briefly reversible when a contact is made again after a short period of time. Complete reproductive isolation in which the separation is maintained even with direct contact is only the result of long-term evolved isolation mechanisms.
Phylogenetic Systematics
There are countless planets with life in the universe, but organic abiogenesis always has the same natural elements available (on gas giants in traces by meteorites) and always evolves according to its system. The trait comparison of organisms in the context of biological systematics has long shown that the features do not appear in any combination, but in a system of graded similarities. Characteristic groups can be delimited from each other, based on which the recent organisms can be classified into taxa and hierarchically arranged. A special case of homologous features are morphological traits or even behaviors that no longer serve any recognizable purpose for today's wearers, such as the remains of the hind leg skeleton in snakes and marine mammals. In both cases, these rudiments indicate the lineage of four-legged animals.
