This entry is part of a recurring series called Use It Or Lose It that looks at those things that didn’t make the evolutionary cut and have been lost in modern species.
‘Eat your carrots, they’re good for your eyesight’ was a common refrain at the dinner table when I was growing up. As an adult I now know this was a child-friendly explanation that Vitamin A deficiency causes blindness, and this can be prevented by a healthy diet that includes carrots. Over the course of evolutionary history, humans, like many insects and animals, have lost the ability to synthesize the vitamins they require to live. This represents an evolutionary process known as compensated trait loss, whereby an organism loses an essential ability but continues to survive because some aspect of their environment or other behaviour compensates for the loss.
In the case of humans and many other mammals, a diet rich in vitamins compensates for the loss of vitamin-producing abilities: eating carrots compensates for not being able to synthesize Vitamin A. By contrast sap-feeding insects like leafhoppers, aphids, and sharpshooters, who subsist on little more than sugar water would seem unlikely candidates to lose the ability to synthesize vitamins in their own cells. The same goes for blood-feeding insects like tsetse flies. Despite their extremely uniform, vitamin-poor diets, these insects are nevertheless unable to synthesize most vitamins and even some essential amino acids. It’s a perfect example of how evolution can take many roads to the same destination. In this case, the key to surving an evolutionary breakdown in vitamin biosynthesis is not a rich diet, but evolutionary outsourcing. Over millions of years, symbiotic relationships have developed between these organisms and bacteria living within their cells that are collectively called endosymbionts. For example, the tsetse fly has bacteria belonging to the genus Wigglesoworthia*, living freely within its cells. These endosymbionts have the ability to produce Vitamin B, which is then transferred to the fly. This obligate association between the bacterium and the insect has driven the tsetse flies to lose the ability to synthesize the vitamin, and allowed it to succeed with a restricted blood-only diet that does not provide all of its essential nutrients. The story of sharpshooters, leafhoppers, and aphids is much the same: all of these insects feed exclusively on plant exudates or sap that are poor in nutrients, and compensate for their own inability to synthesize vitamins by harnessing the vitamin manufacturing power of endosymbionts.
The exploitation of endosymbionts as vitamin factories is not limited to insects either: examples of similar dependencies can be found among animals and fungi too. However, by far the most extreme examples of evolutionary outsourcing come from deep below the sea. There are few species that are able to survive in deep ocean sediments, where there is little oxygen, no sunshine, and few available nutrients. Oligochaete worms like Olavius algarvensis thrive in this harsh environment by using endosymbiont bacteria that live under their skin to do the heavy lifting. The relationship between these two organisms has become so specialized that the worms are wholely dependent on their associated bacteria for their nutrition. The endosymbionts are chemoautotrophic, meaning they harness chemical energy to produce sugars from carbon dioxide. The host then digests its own endosymbionts to survive. The evolutionary result of this complete dependence is a no-frills organism that is reduced to only the essentials. Over time, Olavius algarvensis has lost all of the structures related to a normal munching and crunching lifestyle: namely its mouth, gut, anus, and even its nephridia, which function like kidneys. The worm’s nitrogenous waste that would normally be excreted as ammonia is instead recycled by the bacteria and used to fuel further metabolism. In this case, the adage ‘use it or lose it’ most certainly rings true. Evolutionary outsourcing can lead to not just the loss of biochemical pathways, but the loss of entire organs in extreme cases.
*Wigglesworthia is the best bacterial genus name I have heard in ages