What is Endosymbiotic Theory?

Endosymbiotic theory suggests that the eukaryotic cell’s organelles, such as mitochondria and chloroplasts, evolved as a result of early endosymbiosis between prokaryotic endosymbionts and the eukaryotic host cell.

The Endosymbiotic hypothesis is one of the oldest evolutionary hypotheses still in use today. It is assumed that the early living forms formed an endosymbiotic relationship. This type of symbiosis entails a bigger cell acting as the host and a smaller cell acting as the endosymbiont. The bigger cell absorbed or took in the smaller one, according to the endosymbiotic hypothesis. The bigger cell is a modern eukaryotic cell, whereas the smaller one is a prokaryotic cell.

The presence of membrane-bound cellular components termed organelles distinguishes a eukaryotic cell from a prokaryotic cell. The organelles mitochondria and chloroplasts, according to this view, are the early prokaryotic endosymbionts that were taken in. They spent so much time within the host cell that they evolved into the semi-autonomous organelles we know today.

Endosymbiosis

Endosymbiosis is one of many different types of symbiotic connections (symbioses) that may exist between or among organisms. The endosymbiont dwells inside the host’s body in endosymbiosis. Endosymbiosis still occurs in nature today. The biological relationship between Rhizobium and plant legumes is one example. Rhizobium is an endosymbiont found in the roots of legumes that fixes atmospheric nitrogen into a form that the beans can use. Rhizobium metabolites like malate and succinate are produced by photosynthesis in the legume.

Endosymbiosis formed the basis of the Endosymbiotic Theory in evolutionary biology, which was initially conceived by the botanist Konstantin Mereschkowski (4 August 1855 – 9 January 1921) and then supported by empirical data by Lynn Margulis, 1938–2011.

Endosymbiosis became the mechanism by which organelles such as mitochondria and chloroplasts within eukaryotic cells arose, according to the Endosymbiotic Theory.

This hypothesis proposes that 1.5 billion years ago, a bigger cell took in tiny free-living prokaryotes (bacteria), and the prokaryotes lived as endosymbionts inside the cell.

The mitochondria evolved from proteobacteria (such as the SAR11 clade), whereas the chloroplasts arose from cyanobacteria, according to research findings (particularly the nitrogen-fixing cyanobacteria).

The same characteristics shared by these organelles and their prokaryotic predecessors provide evidence that this idea is realistic.

The following are some of the qualities they share:

• Mitochondria and plastids are both capable of self-replication via a mechanism similar to prokaryotic binary fission.

• Both mitochondria and plastids have a single circular DNA that is comparable in size and structure to that of bacteria but differs from that of the cell’s nucleus.

• Porins in mitochondrial and chloroplast outer membranes are comparable to porins in bacterial cell membranes. Cardiolipin is a membrane lipid found solely in the membranes of bacteria and the inner mitochondrial membrane.

Other Evolutionary Theory

i. Miller-Urey Experiment

The Earth’s age is believed to be about 4.54 billion years, with life appearing around 3.5 billion years ago or earlier. According to the current view of abiogenesis, life on Earth began when the first living organisms ingested non-living elements. They utilised these organic chemicals to make biomolecules and other life-supporting components. Self-replication, self-assembly, autocatalysis, and cell membrane creation are likely biochemical mechanisms that lead to the development of living organisms. These processes were thought to be gradual and made up of a series of occurrences.

The findings of the Miller-Urey experiment showed that the simulated-primitive Earth promoted chemical syntheses of the cell membrane’s basic structures. Amino acids are created by combining the gases methane, ammonia, hydrogen, and water, then electrically electrifying them.

ii. Prebiotic Soup

The Earth was unfriendly to life some four billion years ago. Due to the severe circumstances, no living forms could exist. Simple organic compounds eventually developed. The prebiotic (primordial) soup is a hypothesised model of the early Earth with circumstances that led to the synthesis of simple organic molecules. The notion of the heterotrophic origin of life hypothesis was conceived by Alexander Oparin (1894–1980) and John Burdon Sanderson Haldane (1892–1964), who separately created hypotheses that formed the heterotrophic origin of life theory. They both hypothesised that the early Earth’s atmosphere was chemically decreasing. It helped in the creation of organic molecules.

These chemicals are collected and create a so-called prebiotic soup as they are generated. These simple chemical molecules have evolved into increasingly sophisticated organic polymers throughout time. Life came into being in the end. To flourish and live in the prebiotic soup, the earliest living forms ingested and utilised organic materials. They hypothesised that the first forms of life were heterotrophic in nature. However, new evidence shows that autotrophs were the earliest creatures.

iii. RNA World Hypothesis

Nucleic acids (RNA, DNA), carbohydrates (a variety of sugars), lipids (fats), and amino acids are the four main macromolecules required for life (constituents of proteins). Because RNA may act as both a genetic material and a catalyst, it is thought that primordial life was RNA-based. The evolution of primordial life forms into single-celled living beings happened over millions of years.