Ascects of the evolution of the algae
The first living organisms on Earth were bacteria (and archaea, a now rather rare and still little-researched bacteria-like group that differs significantly from both bacteria and eukaryotes (i.e. plants, animals, fungi)). These organisms lived in the sea and obtained their energy from chemical reactions with hydrogen sulphide or hydrogen (the Earth’s atmosphere was still oxygen-free at that time). Relatives of these bacteria still exist today in the ‘black smokers’ at the mid-ocean ridges.
Soon, the first cyanobacteria (‘blue-green algae’) emerged, which were able to perform photosynthesis with the help of the pigment chlorophyll a (there is evidence of the existence of cyanobacteria as early as 3.5 billion years ago, and of true algae 2.2 billion years ago; however, very little is known about the first steps in the evolution of bacteria and single-celled organisms due to the lack of fossils). The cyanobacteria released oxygen into the Earth’s atmosphere, which not only allowed animal life to develop for the first time, but also formed the ozone layer, which blocks the sun’s UV radiation, enabling life on land.
A very important step in the development of life was the emergence of the first eukaryotic cells after more than a billion years(!). These had a protective cell wall and a cell nucleus containing the chromosomes with the genetic information, which made cell division and heredity more effective. They also developed the ability to reproduce sexually by mixing the genes of the parents, which significantly accelerated the evolution. The metabolism and anatomy of cells, as well as the mechanisms of cell division and heredity, have hardly changed during the evolution of multicellular plants and animals, whereas among single-celled organisms there are still different groups with slightly different mechanisms. This shows that these mechanisms were already so optimised at the time of the emergence of multicellular animals and plants that there was little room (or necessity) for improvement afterwards.
The first simple amoeba-like eukaryotic cells took another decisive step forward through endosymbiosis with a simple aerobic bacterium, which was absorbed by the eukaryotic host cell but not digested. Instead, it continued to live in symbiosis with the host cell and, as a cell organelle, took on an important role in the host cell’s metabolism: it developed into the mitochondrion, the cell organelle which is, so to speak, the energy factory of the cell and, by using oxygen from sugars, produces the universal energy carrier of the cells, ATP, and also performs other important metabolic processes. Mitochondria are found in all eukaryotic cells of all plants, animals and fungi (with the exception of a few amoeba forms, which are therefore more or less unchanged relics from the time before the emergence of this endosymbiosis). The endosymbiosis that gave rise to mitochondria took place at the very beginning of the evolution of eukaryotes and enabled their success in the course of evolution.
Soon, a second endosymbiosis occurred, this time between an amoeba-like eukaryotic cell and a cyanobacterium that performed photosynthesis (using the pigments chlorophyll a and various phycobilins), which became another cell organelle, the chloroplast. This is how the first plants came into being, initially as simple algae. Such endosymbiosis did not develop just once, but several times: the chloroplasts of green algae (and higher plants) originated from different cyanobacteria than those of red algae and brown algae. Green algae developed chlorophyll b as a second pigment, but do not possess phycobilins. The pigment composition of red algae corresponds to that of today’s cyanobacteria. They can live in areas where green algae cannot exist, namely in greater ocean depths. Finally, another endosymbiosis occurred: this time a red alga was absorbed by a host cell, giving rise to the brown algae, diatoms and chrysophyta, whose chloroplasts have three or four cell membranes: Here, the chloroplasts are all that remains of an algae cell that was absorbed in its entirety (surrounded by the two usual chloroplast membranes and the former red alga cell membrane). These groups further developed the pigments chlorophyll c and brown fucoxanthins.
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