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Evolution and the brain
Our Evolutionary Inheritance

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Prokaryotes are small, single-celled organisms, about .001 mm in size, which, like bacteria, do not have any nuclei to hold their DNA. Instead, their DNA forms a single chromosome located in a cytoplasmic structure called the nucleoid. The first prokaryotes are believed to have appeared over 3.8 billion years ago.

Eukaryotes have cells about 10 times larger in diameter than prokaryotes. The first fossil traces of eukaryotes date back about 3 billion years. Eukaryotes can be either protozoa (single-celled organisms, such as amoebas), or metazoa (multi-celled organisms, such as plants and animals, whose many cells are grouped into tissues). The first metazoa appeared about 650 million years ago. Eukaryotic cells have nuclei containing DNA grouped into multiple chromosomes. Eukaryotic cells also have several other types of organelles, such as Golgi bodies, a cytoskeleton and, of course, mitochondria.

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The mitochondria in the neurons and all the other cells in the human body resemble bacteria. These mitochondria not only are the size and shape of bacteria, but also have their own kind of DNA that is very similar to bacterial DNA.

This resemblance is no accident. Indeed, scientists generally agree that the mitochondria in human cells come from bacteria that were incorporated into primitive cells about 1.5 billion years ago. More precisely, scientists believe that mitochondria originated when primitive aerobic prokaryotic bacteria were captured by anaerobic eukaryotes, then permanently incorporated into their structure (see sidebars).

To understand the conditions that encouraged this merger, remember that in the early years after the Earth was formed, there was absolutely no oxygen in the atmosphere. Only much later did certain single-celled algae begin to produce oxygen by photosynthesis.

Oxygen is a highly reactive gas, and it was harmful to the first single-celled organisms. These organisms, the ancestors of today's plant and animal cells, produced their energy through a relatively inefficient series of chemical reactions known as glycolysis. At the same time, however, some much smaller bacteria with no nuclei developed a way of using oxygen's corrosive properties to produce energy much more efficiently. But to fuel their energy “"furnaces", these bacteria depended on nutrients that were scattered throughout their environment.

It was for these reasons that a merger proved beneficial to both parties. The bacteria that had learned to harness oxygen were incorporated into larger host cells, for which they then served as tiny energy plants. In exchange, these larger host cells provided their new partners with a stable supply of nutrients of all kinds.

Those are the basics of the theory of the endosymbiotic origin of mitochondria, which represented a major turning point in the evolution of life on Earth.

Organisms that can use oxygen to produce energy are said to be aerobic. Those that cannot are anaerobic.

The first bacteria to appear on Earth, some 3.8 billion years ago, had no choice but to be anaerobic, because the atmosphere at that time contained no oxygen. But about 3.2 billion years ago, certain bacteria began to produce their energy using the light of the sun. In the process, they gave off a toxic by-product: oxygen. Subsequently, about 2.5 billion years ago, bacteria emerged that could use this hazardous substance to their advantage. These were the first aerobic bacteria, and they were very likely the ancestors of the mitochondria now found in human cells.

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