A quick summary of the evolutionary mistake that means you need to eat oranges, but almost every other animal doesn’t.
Biochemistry is an incredibly complex and awkward subject, mainly because of the sheer quantity of biochemicals there are. Countless different chemicals are in your body right now, being made and broken and changed into new stuff all the time, all to keep the mechanics of your body running so you don’t die.
Your body uses special proteins called enzymes to make chemical reactions happen at a useful speed. There are several thousand unique enzymes found in your body, each with a specific job: breaking down fat, replicating DNA, making important chemicals from the nutrients you eat, etc etc. Most of these enzymes are really important, and without them you would get sick or die pretty quickly.
However, sometimes the enzymes do nothing useful at all. Like your appendix or wisdom teeth, these enzymes are vestigial relics that survive in the body even though they became useless millions of years ago.
The enzymes of the vitamin C synthesis pathway in humans are a great example of this.
Vitamin C is a vital chemical in many different organisms, including all animals and plants. It has a whole host of different functions, from aiding the immune system in mammals to allowing plants to do photosynthesis.
Nearly all plants and animals make their own vitamin C: and why wouldn’t they? It’s pretty easily made from glucose, a common nutrient used by every organism.
However, because vitamin C is so common in fruits, animals that eat lots of fruit don’t really need to make it themselves. So when a frugivore (fruit eating animal) has a genetic mutation that means they can’t make vitamin C anymore, evolution often doesn’t notice, and the animal survives to spread the mutation throughout its species.
This has happened to the guinea pig family of rodents, the fruit-eating bat family, and to the ancestor of monkeys and apes.
Around 60 million years ago, not long after the dinosaurs died out, a single gene mutated in an early primate that turned out to be our distant ancestor. This mutation happened to be on the gene for the enzyme L-gulonolactone oxidase (GULO), which is responsible for making Vitamin C in mammals.
Because this ancient primate was living in the jungle and eating mainly fruit, this mutation went unnoticed, and the primate was able to pass on his genes and eventually a whole host of different primate species were unable to make their own vitamin C.
Their livers still made all the precursors for vitamin C, as the early steps of the reaction chain were unaffected. But the very last step was broken, and so what they made was almost, but not quite, a useful vitamin.
Fast-forward 60 million years and suddenly there was a new kind of primate, an ape that left the trees and sometimes didn’t eat as much fruit as it should. Humans are the only primates that commonly have vitamin C deficiency. Sometimes this deficiency gets so bad it causes a really horrible and eventually deadly disease called scurvy.
Scurvy was first recorded over 3500 years ago in the hieroglyphs of ancient Egypt, . The ancient Chinese wrote of scurvy too, and their sailors used ginger ( a source of vitamin C) to treat it. In the 1400s Portuguese explorers ate citrus fruits to cure or prevent scurvy.
French explorers in North America were cured of scurvy when Native people taught them to brew a tea with the needles of evergreen trees, another source of vitamin C.
In 1753 a Scottish surgeon called James Lind finally proved that citrus fruits can be a treatment for scurvy, and within a few decades every Royal Navy ship carried lemons or limes with them (which is the origin of the term “limey” to refer to British people).
It seems that the use of vitamin C-rich plants as a treatment for scurvy has been independently discovered over and over again throughout history, showing just how detrimental the mutation of our distant primate ancestor has been for our special species of ape.
So right now, in your body, a bunch of different enzymes are working together to make /almost/ vitamin C. And then, when they’ve got 99% of the way there, they break it down and start again. For 60 million years, these useless enzymes have been working tirelessly to make their unusable almost-vitamin.
And so we’re stuck eating oranges like chumps.