Symbiogenesis

Symbiogenesis, also known as the endosymbiotic theory, is the main idea that explains how complex cells called eukaryotic cells came to be. According to this theory, certain parts inside these cells, like mitochondria which give energy, and plastids such as chloroplasts which help plants make food, were once tiny living organisms on their own. These small organisms, called prokaryotic organisms, were taken inside bigger cells and began living together in a helpful way.

Scientists believe that mitochondria are related to a group of bacteria called Rickettsiales, while chloroplasts come from cyanobacteria. This idea was first suggested in the 1800s by researchers like Andreas Schimper. The theory was later developed by Konstantin Mereschkowski and strongly supported with evidence by Lynn Margulis in 1967.

There is a lot of evidence that supports this theory. For example, mitochondria and chloroplasts have their own tiny pieces of DNA and can copy themselves by splitting in two, just like bacteria do. They also have special kinds of molecules, like cardiolipin, that are found in bacteria but not in other parts of our cells. All of these clues help scientists understand how life on Earth became more complex over time.

History

The idea of symbiogenesis was first suggested by the Russian botanist Konstantin Mereschkowski in 1905. He proposed that complex life-forms came from two episodes of symbiosis, where bacteria were incorporated to form nuclei and chloroplasts. Other scientists, like Paul Jules Portier and Ivan Wallin, also supported the idea that mitochondria came from symbiosis.

These ideas became more popular in the 1960s when scientists discovered that mitochondria and chloroplasts have their own DNA. Lynn Margulis provided strong evidence for the theory in 1967, arguing that eukaryotic cells evolved through communities of interacting organisms. Today, the theory that mitochondria and chloroplasts originated from symbiosis is widely accepted.

From endosymbionts to organelles

Organelles are parts of cells that have special jobs, and they often start out as whole organisms living inside other cells. As these inner organisms change over time, they give most of their genes to the host cell. This means the host cell must find ways to give the inner organism the proteins it needs.

Mitochondria, which give energy to cells, are closely related to certain bacteria called Pelagibacterales. Plastids, which help plants make food, are related to cyanobacteria. Both of these have much smaller genomes than their free-living ancestors, showing how much they have changed.

Endosymbiosis of protomitochondria

Further information: Eukaryogenesis

The idea behind symbiogenesis is that a very simple cell, called a proto-eukaryote, took in a special kind of bacteria. This bacteria became an important part of the cell called a mitochondrion. Mitochondria help the cell make energy, similar to how a power plant makes electricity for a town.

Mitochondria have their own tiny bit of DNA, which suggests they were once free-living bacteria before they moved into the proto-eukaryote. Over time, the mitochondria and the host cell worked together so well that they share parts and divide at the same time. This teamwork helped both the mitochondria and the cell survive and grow.

Organellar genomes

Some genes from the tiny organisms that live inside our cells have stayed inside special parts called organelles. These genes help make important proteins and handle important jobs like making energy. Scientists have different ideas about why these genes stay inside, but one big reason is that some proteins are hard to move between parts of the cell.

Non-photosynthetic plastids, which are parts in cells that don’t make food from sunlight, still keep a small set of genes. One reason is that these plastids need special RNA pieces to make certain important molecules, and these can’t move to the main part of the cell. Another idea is that when cells evolved to have only one of these plastids, it became riskier to move genes out because losing the plastid would hurt the whole cell.

Evidence

Scientists have found many clues that show mitochondria and plastids, like chloroplasts, came from bacteria. For example, these tiny parts of cells can only make new copies through a process called binary fission, which is how bacteria divide. Also, special proteins called porins, found in the outer layers of mitochondria and chloroplasts, are also in bacterial cell membranes.

Studies of DNA and genes suggest that mitochondria are closely related to certain bacteria called Alphaproteobacteria, and plastids are related to cyanobacteria. Both mitochondria and plastids have their own small circles of DNA, similar to bacterial DNA, and they also contain ribosomes that look more like bacterial ones than those in the rest of the cell.

Secondary endosymbiosis

Primary endosymbiosis is when one cell engulfs another and keeps it alive inside. Secondary endosymbiosis happens when a cell that already has an engulfed organism inside gets taken in by another cell. This process has happened many times and led to many different types of algae and other living things.

One example of this happening right now is in a tiny organism called Hatena. It usually eats other cells, but when it takes in a green alga, the alga stays alive inside it. Hatena then changes to get energy from sunlight instead of eating, and it can move toward light. Even though there are many different kinds of organisms with these special parts inside them, scientists think they all came from one original event long ago.

Nitroplasts

A tiny ocean alga, _Braarudosphaera bigelowii, has been discovered with a special cyanobacterium living inside it. This cyanobacterium, called Candidatus Atelocyanobacterium thalassa or UCYN-A, forms a structure known as the nitroplast. It helps the alga by fixing nitrogen, a process described in more detail on the fix nitrogen page. This cyanobacterium divides when the alga divides, showing it is becoming more like an organelle inside the cell.