The Endosymbiotic Theory

The German botanist Heinrich Anton de Bary coined the term ‘Symbiose’ to designate this coexistence. The concept of symbiosis, that two different organisms stably coexist and even give rise to a new type of organism, is attributed to Simon Schwendener.

19th Century

• In 1905, Russian biologist Konstantin Mereschkowski suggested that the origin of eukaryotic cells involved the engulfment of smaller prokaryotic cells.
• Around the same time, German botanist Andreas Schimper proposed that chloroplasts in plant cells might have originated from independent photosynthetic organisms.

1960s-1970s

• In 1967, Lynn Margulis proposed that eukaryotic cells, with their complex structures and organelles, resulted from symbiotic relationships between ancestral prokaryotic cells. He argued that mitochondria, the cellular powerhouses, were once free-living bacteria capable of aerobic respiration. Similarly, she suggested that chloroplasts originated from photosynthetic bacteria that became incorporated into host cells. 

1980s-1990s

• Researchers discovered striking similarities between the DNA of organelles like mitochondria and chloroplasts and the DNA of modern-day bacteria.
• Protists like single-celled organisms are found to host certain bacteria fixing nitrogen (nitrogen-fixing bacteria).

2000s-Present

•  In the 21st Century, endosymbiosis forms the basis of the origin of mitochondria, chloroplast, and other organelles. It also explains the development of complex multicellular organisms and the coevolution of symbiotic partners.

According to the endosymbiotic theory, the symbiotic origin of eukaryotic cells is a multi-event process.

1. Primary Endosymbiosis

It refers to the initial engulfment of a free-living bacterium by a host cell, creating a new organelle within the host cell. The most prominent examples of primary endosymbiosis are the origin of mitochondria and chloroplasts; both were once free-living cells. Here, two membranes surround the organelles; the inner is obtained from the bacterium, and the outer is derived from the host cell.

• Origin of Mitochondria: A eukaryotic cell engulfed a bacterium capable of aerobic respiration. This bacterium provides a valuable energy source through respiration. Over time, the host cell and the engulfed bacterium developed a mutually beneficial relationship. The bacterium became the mitochondrion, retaining its own DNA and membrane structure while working in tandem with the host cell.
• Origin of Chloroplasts: Here, an ancestral host cell captured a photosynthetic bacterium, which could convert sunlight into energy through photosynthesis. As with mitochondria, a symbiotic partnership formed, with the photosynthetic bacterium evolving into the chloroplast. It allowed the host cell to harness the power of photosynthesis for energy production.

2. Secondary Endosymbiosis

It involves a eukaryotic cell engulfing another eukaryotic cell already undergoing primary endosymbiosis. This secondary engulfment results in a more complex cellular arrangement, leading to the diversification of eukaryotic lineages and the emergence of new types of organelles.

• Formation of Plastids: These organelles involved in photosynthesis are found in various algae and plants. Different groups of algae have acquired plastids through secondary endosymbiosis, which consists of the engulfment of photosynthetic eukaryotic cells. In contrast to the two membranes of primary organelles, four membranes surround chloroplasts obtained by secondary endosymbiosis. In most cases, the nucleus of the engulfed cell disappears, with the remains of this nucleus still found lying between the two pairs of membranes. This structure is called a nucleomorph.

Thus, the endosymbiotic theory explains the presence of double-membraned organelles within protists.

There are several proofs to support the Endosymbiotic Theory. However, the discovery of independent DNA (from the host) in mitochondria and chloroplasts supported the theory the most. The other evidences are as follows:

1. Structural Similarities: Mitochondria and chloroplasts share structural characteristics with free-living bacteria, such as double membranes and DNA. Both the organelles are almost of the same size as the bacterial cell.
2. Reproduction: Mitochondria and chloroplasts replicate within the cell independently, similar to how bacteria reproduce.
3. Genetic Evidence: The DNA within mitochondria and chloroplasts is more similar to bacterial DNA than the host cell’s nucleus.
4. Evolutionary Relationships: Analysis of genetic sequences shows that mitochondria and chloroplasts are more closely related to specific groups of bacteria than eukaryotic cells.

However, the statement that mitochondria and chloroplasts are much larger than prokaryotic cells does not support the endosymbiotic theory.

It is important because the theory explains the origin of the eukaryotic cells. It also describes how chloroplast and mitochondria might have originated from once free-living prokaryotes. This understanding has reshaped our perception of how fundamental cellular organelles came to be and how multicellular life forms arose.

The endosymbiotic theory also highlights how cooperation and symbiosis have played pivotal roles in shaping cellular evolution.