We Have a Broken Gene for Making Vitamin C

Most mammals produce their own vitamin C. Dogs, cats, rats — they just make it internally and never have to think about it. We can't do that. Somewhere around 61 million years ago our ancestors got a mutation that broke the gene responsible, and it never got fixed.

The weird part: the broken gene is still there. It's called a pseudogene — same location, same structure as a working gene, just nonfunctional. A fossil of something that used to work.

It probably broke because our primate ancestors were eating so much fruit that there was no pressure to keep making it internally. Evolution doesn't maintain things it doesn't need. So it just... stopped working, and nobody died from it, so it stayed broken.

What this makes me think about is everything else in our genome we don't understand yet. There are around 15,000 pseudogenes in humans. Broken genes, leftover sequences, things that look like they should do something but apparently don't.

Or maybe they do something and we just haven't figured it out yet. Some "junk DNA" turns out to have functions we didn't expect. I find that more exciting than unsettling — the idea that the genome is full of things we dismissed too quickly.