Hannah Critchlow writes:

About 2 billion years ago, evolution performed an improbable experiment. A larger ancestral cell engulfed a smaller bacterium. It should have been a meal. Instead, it became a merger. The bacterium survived inside its host, and together they forged one of the most consequential partnerships in the history of life. The host offered shelter and access to oxygen. The bacterium supplied something revolutionary: a vastly more efficient way to generate energy.

From this intimate alliance emerged the eukaryotic cell – and with it, the possibility of complex life. Every plant, animal and thinking being traces its lineage back to that ancient symbiosis. Our capacity for reflection, imagination and doubt rests upon what was once a free-living microbe. We call these descendants mitochondria.

They persist in nearly every cell of our bodies, hundreds to thousands at a time. In total, we carry an estimated 10 million billion of them – collectively accounting for roughly a 10th of our body mass. Red blood cells are the exception: they lack mitochondria, which maximises oxygen transport. Almost every other cell depends on them absolutely. Neurons are especially demanding hosts. Each contains thousands of mitochondria, occupying up to 40 per cent of its volume.

These rod-shaped structures are often described as the cell’s powerhouses. Through aerobic metabolism, they generate most of the chemical energy that keeps cells alive and functioning – the molecular fuel that sustains every biological process.

Although the brain represents just 2 per cent of body weight, it consumes about 20 per cent of our energy at rest. Every perception, memory, emotion and idea is metabolically expensive. Thought itself is an energy-hungry act. Weight for weight, our brains are more mitochondrial than neural. This is more than a biological curiosity. It suggests that cognition is inseparable from metabolism – that the mind is not only shaped by networks of neurons but by networks of energy. [Continue reading…]