Researchers have newly discovered a surprising and potentially significant reason why eating foods frequently cooked at high temperatures, such as red meat and deep-fried fare, elevates cancer risk. The alleged culprit: DNA within the food that's been damaged by the cooking process.
How high is too high? Didn’t early humans unlock the next tier by basically burning meat?
We do not really know how cooking helped us evolve, but there are many theories. It could have been that cooking our food allowed us to have smaller digestive tracts (compared to our ancestors) meaning we had a better ability to grow our brains. Or possibly it was because cooking allowed for a break down of plant and animal matter which allowed us to have more energy/access to nutrients giving us an evolutionary edge. Or if you want to examine a possible psychological reason, may fire specifically allowed us to think, examine, and memorize for longer periods of time including night.
As for why it was good then and bad now, we have to understand the split in what kills us. Prior to the invention of common sterilization methods, clean drinking water, and antibiotics/antifungals/antivirals, you were far more likely to die of some disease than some chronic disorder like you are today. Now, most people are living well past their 60s in some impoverished countries and many in rich countries far higher in their late 70s, 80s, or even 90s. This gives a lot more time for other factors like cancer to be an issue. Considering cancer is developed because of mutations in our DNA, the longer we live, the more chance said mutations will occur before we can stop the deleterious effects Additionally, our tools are far more extreme than they used to be. Boiling is a fairly new (in the broad scheme of things) invention with things like frying or smoking being even newer. That doesn’t always mean its going to be a bad thing, but in this case, newer cooking methods just involve a higher heat which does indeed present issues.
As far as how high you can cook food without seeing damage, there is no answer for a specific no-no temperature. Just that higher temperatures result in more damage as I discussed here. Boiling seems to be the least bad compared to roasting, grilling, or smoking considering boiling stays around 100C compared to > 200C in other methods. Plant matter seems to be far better than animal matter likely due to the high content of other molecules with less DNA although the authors of the study note that future studies should be undertaken to learn more about these effects with a wider food pool.
TL;DR: we don’t really know all of the specifics for why we evolved. Cooking was likely part of it. Today, there is no correct temperature to cook at, but lower temperatures seem to be better for this particular issue.