THE BRAIN FROM TOP TO BOTTOM

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Our Evolutionary Inheritance

Free Radicals and Aging: More Complicated Than We Thought

Like all the other cells in the human body, neurons need energy to function. In fact, neurons consume an especially large amount of energy. The brain accounts for only 2% of the human body's weight, but almost 20% of its energy consumption.

This energy is produced in the mitochondria, elongated organelles dispersed throughout the cell body. These mitochondria use oxygen to extract energy from sugars and fats and to produce molecules of the energy-storage compound adenosine triphosphate (ATP). These ATP molecules are then used to fuel the various chemical reactions that take place within the neuron.

THE ORIGINS OF MITOCHONDRIA

Mitochondria are cell organelles that are shaped like little sausages and that supply energy to the neurons. These mitochondria evolved through a very special process. In fact, if the event through which mitochondria originated had not occurred, it is very unlikely that multi-celled organisms would ever have evolved, and even less likely that organisms with nervous systems would have done so.

We human beings thus owe a great deal to this key event in the history of evolution. Interestingly, this event resulted not from a process of competition, but rather from one of co-operation: all the evidence indicates that mitochondria were early bacteria that were incorporated into larger cells.

The term symbiosis is used to describe this kind of co-operation, which greatly benefitted both parties concerned. The bacteria —"the ancestors of our mitochondria" thereby obtained a stable, nutrient-rich environment in which to live. For their part, the larger cells into which the bacteria were incorporated obtained the cheap energy that these bacteria could produce using the oxygen in the Earth's atmosphere —oxygen that the larger cells not only could not use on their own, but that was actually toxic to them.