In the dark waters of a limestone cave in northeastern Mexico, a blind fish makes its home. Washed into the cave at some time in the distant past, the fish had no choice but to survive by eating bat droppings and not much else. With no light, their distinct pigmentation was useless and was lost to evolution. With nothing to see, their eyesight – and eventually their eyes and a significant part of their brains – were the next things to go. Biologists in Sweden report that these losses save the fish energy and were probably vital to the species’ continued survival.
The “expensive tissue hypothesis” has been a difficult one on which to gather data, because it is impossible to compare the energy use of long-gone ancestors to creatures that exist today. However, the blind Mexican tetra’s ancestors still populate surface-level rivers in parts of Texas and Mexico. By comparing these sighted relatives to their cave-dwelling counterparts, the scientists showed that the loss of the visual system substantially lowered the amount of energy the fish expended on “expensive” neural tissue.
In humans, a related hypothesis has been used to explain our evolution of bigger brains. It attributes our large brain size to the invention of cooking, which allowed our ancestors to get more energy from food and therefore, to expend more energy on brain tissue. As human population increases and resources become scarce around the world, perhaps humans will go the way of the blind Mexican cave fish, experiencing an evolutionary reduction in brain size to cope with our ever-constricting diets.
Via The Guardian