Eukaryotes have a very complex structure due to their nuclear membrane which surrounds their nucleus, and that preserves the genetic material in eukaryotic cells. But, the most characteristic part of a eukaryotic cell is the specialized organelles performing a specific function hypothetically inherited by a prokaryotic cell, in this case, mitochondria and chloroplast. And, according to the theories on multicellular life, the complexity of organelles instigates the evolution of multicellular life, including mammals, plants and also Homo Sapiens. If so, how did eukaryotic cells possess organelles, including mitochondria and chloroplast?
There is one more question left to answer before proceeding with how eukaryotic cells engulfed organelles, including mitochondria and chloroplast, what is a cell? The answer is as follows. “Practically all living things, from the very smallest to the greatest – from the microscopic amoeba and single-celled algae to huge whales and redwood trees – are made up of cells. Many animals and plants consist of one cell, which is the whole individual. Others have passed far beyond this unicellular, or one-celled, stage. They are made up of a great number of cells that are joined together. Such animals or plants are called multicellular, or many-celled. The exact significance of many in this case can be seen from the fact that more than six million cells may be found in a drop of human blood.(1)” In other words, cells are the building units of living things, such as plants, animals or even algae. Eukaryotic cells and prokaryotic cells have the same fundamental biochemistry, but eukaryotic cells are more complex than prokaryotic cells. So, the aforementioned explanation suggests that the procaryotic cells are the first primitive cells(as suggested by their name) and that more advanced eukaryotic cells evolved from them later, gradually accruing complexity(2). As described in the endosymbiosis theory, procaryotic cells(free-living bacteria) must have been engulfed by eukaryotic cells to gain the abilities they do not have as compared to that free-living bacteria have. Many features of eukaryotic cells support the fact that one organism(an organelle) actually lives inside the host organism(eukaryote). And, it is therefore called endosymbiosis.
There is compelling evidence that organelles were once primitive cells, or free-living bacteria, especially mitochondria and chloroplast, as described in the endosymbiosis theory. Every time a new organelle, engulfed by eukaryotic cells, joined to the structure of eukaryotes, the ingested primitive cells have changed and affected eukaryotes’ DNA patterns by injecting their DNA into the host cell’s stream, as well as their messenger RNA, transfer RNA and ribosomes. And, eukaryotic cells have changed the ingested primitive cells, such as mitochondria, indeed. After two billion years of shared evolution, despite its atavistic features, mitochondria have lost almost all their formal independence(3). Any bacterial genes necessary for survival inside the primitive cells have quickly moved to the nucleus of eukaryotes. Hence, thanks to the primitive cells(free-living bacteria) genes, eukaryotes took the next step of evolution by perpetuating and improving the skills of free-living bacteria to survive, such as using oxygen to generate energy or making food through light energy. According to the German biologists Schmitz, Schimper and Meyer, chloroplasts were derived from cyanobacteria, as one of the basic tenets of the endosymbiosis theory. Self-food production is therefore explained by the endosymbiosis theory. The reason is as follows. In photosynthesis, energy in the form of radiation, or light from the Sun, is converted into the chemical energy to produce food in the form of sugar(4).
In addition to the opinions on the evolution of life as regards the endosymbiosis theory, sex-cell production predicted as pangenesis by Darwin may have been stimulated by mitochondrial respiration. “According to the hypothesis of pangenesis, every unit or cell of the body throws off gemmules or undeveloped atoms, which are transmitted to the offspring of both sexes, and are multiplied by self-division. (5)” If the creation of sexes is due to mitochondrial respiration, then the endosymbiosis theory explains why eukaryotes exist in this way.
(1) What is life?. “Cells." The New Book Of Popular Science Vol-3. Grolier Incorporated, 1988. 382-383. Print.
(2) Lane, Nick. "Last Ancestor in an Age Before Oxygen." Oxygen: The molecule that made the world. Oxford: Oxford University Press, 2016. 153. Print.
(3) Lane, Nick. "Last Ancestor in an Age Before Oxygen." Oxygen: The molecule that made the world. Oxford: Oxford University Press, 2016. 154. Print.
(4) Schultz, Beth. “Photosynthesis." The New Book Of Popular Science Vol-4. Grolier Incorporated, 1988. 84-85. Print.
(5) Darwin. "Principles of Sexual Selection." The Descent of Man: and Selection in Relation to Sex. Ed. Tom Griffith. London: Wordsworth Classics of World Literature, 2013. 216-217. Print.
Figure - 10.1 https://www.newscientist.com/article/dn27762-one-gene-may-drive-leap-from-single-cell-to-multicellular-life/
Figure - 10.2 https://multicellularorganims7thblock.weebly.com/survival.html
Figure - 10.3 https://biologywise.com/cytoplasm-function-in-cell
Figure - 10.4 By Mariana Ruiz Villarreal LadyofHats - the diagram i made myself using adobe illustrator. as a source for the information i used the diagrams found here:, , , , ,  and ., Public Domain, https://commons.wikimedia.org/w/index.php?curid=8152684
Figure - 10.5 https://learn.genetics.utah.edu/content/cells/organelles/images/endosymbiosis.jpg
Figure - 10.6 By Kelvinsong; modified by Sowlos - Own work based on: Mitochondrion mini.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27731882
Figure - 10.7 By Daniel Mayer (mav) - original imageVector version by Yerpo - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=20722530
Figure - 10.8 https://www.nature.com/news/why-humans-develop-sex-cells-as-embryos-but-corals-don-t-1.21218