Humans need vitamin C to live. This implies that evolution is true.

So humans need vitamin C to live. We get vitamin C mostly through fruits and vegetables.

But what if humans can't get vitamin C? Lack of this vitamin leads to Scurvy, a disease that leads to death. There is lots of evidence of sailing ships losing much of their crew and passengers on long distance voyages because they didn't have a source of vitamin C onboard.

This leads to an interesting question. What about the Eskimos? The Inuit and Yupik live in the Arctic. During the summer these people had access to grasses, berries and seaweed, and could get vitamin C from that. But winters in the Arctic are long and dark. Plants became unavailable to them.

So these people got vitamin C from animals. Seal liver and whale blubber both have good concentrations of vitamin C. (Only if eaten raw! Cooking vitamin C destroys it!)

The Inuit didn't get Scurvy.

This leads to another interesting question. Why do these animals have vitamin C in them, and we don't?

It turns out that most mammals don't have to eat foods rich in vitamin C because their bodies make vitamin C naturally.

Ascorbate (the "ascorbic" part of ascorbic acid - the scientific name for vitamin C) is a basic requirement for life by all animals and plants. It is made internally by every plant, and almost every animal on Earth. Dogs and cats make their own vitamin C. You could get vitamin C from fresh Cow liver. (Raw, of course.)

But in apes, monkeys and humans, the ability to make vitamin C is... broken.

And I mean "broken" literally. Animals can synthesize vitamin C from basic carbohydrates through a series of chemical steps in the cell, driven by enzymes. In humans, this sequence of steps is interrupted at the very last step by the lack of one specific enzyme.

Scientists can detect these steps being performed in our cells, and can see what is missing. On investigation, it has been discovered that the gene that makes this enzyme in other animals is not functioning in humans.

At some point, our Simian ancestors suffered a genetic mutation that turned off vitamin C synthesis. But no one noticed, because of all the fruits and vegetables that were being normally consumed as part of a standard diet of anthropoids - apes, monkeys and humans.

This mutation would have been a harmful mutation if circumstances had been different. Our ancestor who couldn't produce vitamin C would have died, leaving no offspring. But vitamin C was still readily available by eating fruits and vegetables rich in vitamin C, and since this was our ancestor's diet this genetic mutation was neutral - not deadly.

This leads me to other questions. Are there other animals that are unable to produce their own vitamin C? The answer is yes. Most bats, all Guinea pigs, some birds. And what is interesting is that their vitamin C generating machinery is "broken" in different ways. For example, Guinea pigs also have the same missing enzyme, but it is due to a different gene malfunction. It's not the same gene as the one in humans.

Another question. We humans are learning how to do "gene therapy". And restoring the process that produces vitamin C in our cells seems like low hanging fruit (excuse the pun). Can we not "fix" humans so that our progeny will produce vitamin C naturally?

I've discovered that there are lots of people looking at this, and some studies and experiments indicate that restoring vitamin C synthesis is possible. But really, we still don't know enough about human cells to guarantee that there are no unintended consequences. Like a higher risk of cancer due to the method of genetic modification used.

And lastly, an observation. The study of why humans don't synthesize vitamin C naturally only makes sense when considered together with the theory of evolution. Without this basic foundation, we are unable to understand what has happened and why.  Instead we would be left with silly ad-hoc non-explanations like, "God did it".