It is made internally by almost all organisms although notable mammalian group exceptions are most or all of the order chiroptera (bats), guinea pigs, capybaras, and one of the two major primate suborders, the Anthropoidea (Haplorrhini) (tarsiers, monkeys and apes, including human beings).
Because a mutation turned off our precursor's ability to do so, and (due to the abundance of vitamin C in their diet), wasn't immediately deleterous but instead managed to spread throughout the entire population.
Probably not a tradeoff. Scurvy is devastating and Wikipedia says an animal like a 70 kg goat makes ~13 grams of Vitamin C a day (which isn't very much of anything).
Sure. I was responding to the tradeoff question. It just doesn't seem very likely that some animal survived because it wasn't making Vitamin C (the other case is some animal surviving even though it didn't properly make it).
It is made internally by almost all organisms although notable mammalian group exceptions are most or all of the order chiroptera (bats), guinea pigs, capybaras, and one of the two major primate suborders, the Anthropoidea (Haplorrhini) (tarsiers, monkeys and apes, including human beings).
So not all primates.