I am going to present a different "take." It is something I firmly believe, but it is just that, an unverifiable "belief."
This mainly pertains to Type 2.
I believe that when we abandoned hunter-gathering and became settled farmers, we invented "processed foods."
- Domesticating cattle and drinking what comes from their udders.
- Cultivating grain, grinding it, and making bread or pasta from the flour.
- Much later, importing a strange tuber from the New World, the potato.
- Cultivating rice in flooded paddies.
I could go on for a long time. The key thing here is that these things took a lot of human ingenuity, and a lot of time (often at least an entire year of cultivation, even longer to bring up a cow). So it is quite recent, only the past 10,000 years or so.
But modern
homo sapiens evolved slowly. Ten thousand years is the blink of an eyelid in terms of how much adaptation you can expect the body to make.
In the pre-agricultural age our diet was naturally low-carb, with some exceptions. Fruit, for example. But even that was probably quite a rare treat, except in some parts of the world where wild fruits are really abundant.
So my notion is that the human pancreas just wasn't suited for the carb load it began to bear about 10,000 years ago -- and the body as a whole, which would explain "insulin resistance." This explains why diabetes is such an ancient disease. It also explains why a handful of humans who have low-carb traditions (Inuit, Masai) apparently have very low Type 2 diabetes rates.