A Brief History of Mitochondria.

Mitochondria are derived from almost the earliest form of life on Earth.

The Earth was formed about 4.5 billion years ago, (4,500 million or 4,500,000,000).

About 3.5 billion years ago, life on Earth started with simple, single celled organisms. They are called heterotrophs because they "eat" their "food" from their surroundings - rather than producing it. These cells, called "Prokaryotic Cells", have a simple internal structure, and examples are the bacteria found today. Obviously these bacteria have DNA.

Photosynthesis evolved about 2.5 billion years ago. Certain strains of bacteria developed which were able to convert sunlight into energy. These cyanobacteria (blue-green bacteria) living in the oceans gradually increased the amount of oxygen in the atmosphere to about 2%.

As the oxygen levels increased, about 2 billion years ago, other strains of bacteria evolved which could tolerate the oxygen and use it in the process of respiration, the breakdown of food using oxygen.

About 1.8 billion years ago, Eukaryotic cells evolved. It is thought that several different types of bacteria lived together, each gaining benefits, known as symbiosis. Eventually, smaller bacteria started living inside larger bacteria, (or small ones were endocytosed by the larger ones). The larger bacteria gradually formed a variety of internal structures, such as a nuclear membrane to enclose DNA. The smaller bacteria maintained it's own outer membrane, it's own unique chemistry and it's own DNA - but lived inside the larger cell and became other internal structures of the Eukaryotic cell.

This process occurred with a variety of different bacteria. One small endocytosed bacteria would eventually become the Chloroplasts in plants which undertake Photosynthesis - the production of ATP from sunlight which is then used as fuel for the plant to produce sugar and starch. Another small endocytosed bacteria would eventually become the Mitochondria in animals, where sugar is broken down to generate ATP.

Mitochondria therefore, are about 1.8 billion years old.

However, what we recognise as plants and animals takes a very long time. These single celled organisms thrived in the oceans for about a billion years. Gradually, evolution led to the diverse range we can see today :-
0.7 billion years ago - simple plants and animals formed from colonies of cells.
0.5 billion years ago - fish evolved
gradually the oxygen level increased to 20% and an ozone layer developed which protected the land from harmful ultraviolet radiation,
0.4 billion years ago - first land based plants evolved
0.3 billion years ago - first amphibians, first insects
0.2 billion years ago - first reptiles, first dinosaurs. Huge forests would eventually become coal.
0.1 billion years ago - large dinosaurs, first flowering plants, first mammals, first birds
0.065 billion years ago - (65 million years ago) dinosaurs became extinct, mammals become abundant
0.003 billion years ago - (3 million years ago) first human-like mammals evolved (Australiopithicus).
0.00005 billion years ago - (about 50,000), the first real humans evolved (Homo sapiens).
0.00001 billion years ago - (about 10,000), humans started agriculture

There are many lines of evidence to show that Mitochondria are derived from bacteria - principally that Mitochondria have their own circular DNA. Over immense periods of time, probably as each new species evolved, the DNA of the Mitochondria has either merged with, or been replaced by, the DNA of the host cell - leaving just a small portion of Mitochondrial DNA still in the Mitochondria.

See Also
ATP
Eukaryotic
Evidence mtDNA has bacterial origins
Prokaryotic

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