Vital Question: Energy, Evolution, and the Origins of Complex Life

by Nick Lane

Of course, the removal of cells that don’t work well enough only improves overall tissue function if they are replaced with new cells from the stem-cell population. A major problem with neurons and muscle cells is that they cannot be replaced. How could a neuron be replaced? Our life’s experience is written into synaptic networks, each neuron forming as many as 10,000 different synapses. If the neuron dies by apoptosis, those synaptic connections are lost forever, along with all the experience and personality that might have been written into them. That neuron is irreplaceable. In fact, while less obviously necessary, all terminally differentiated tissues are irreplaceable – their very existence is impossible without the deep distinction between germline and soma discussed in the previous chapter. Selection is all about offspring. If organisms with large and irreplaceable brains leave more viable offspring than organisms with small replaceable brains, then they will thrive. Only when there is a distinction between the germline and soma can selection act in this way; but when it does, the body becomes disposable. Lifespan becomes finite. And cells that can’t meet their metabolic requirements will kill us in the end.