system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do all extra calories turn into fat?

If I eat an extra 3,500 calories of nothing but fat-free protein powder I will gain a pound.  However I didn't eat any fat.  How do all calories turn into "fat" when we overeat.  

A second question would be why hasn't human evolution changed our metabolisms because we clearly get enough food to not need to store it up like we did a thousand years ago.
gpt
The wording of your question leads me to believe that you have little background in basic biochemistry so here comes your ELI5 answer, starting at the basics.

When you eat food, your body breaks it down into tiny pieces. The fats and carbohydrates(Bread and pasta etc) get turned into glucose mostly. This is what your cells use for energy, and when there is extra left over, it gets turned into body fat. Proteins get turned into little bits called amino acids. These are basically still protein, just in their tiny broken down form. If you eat more protein than your body needs at the time, your body does not want to waste it, so the next step is to turn the amino acids into more glucose, because your body can easily use and store that. It takes a little more energy to do it this way because it has more steps, but it keeps your body from wasting what you have eaten.

TLDR: Everything can turn into body fat if you eat too much of it.


As for your second question. Evolution hasn't changed our metabolisms for two reasons. First, there hasn't been enough time. It takes thousands and thousands of years for a big change like that to happen, and our diets have only changed like this within the last few hundred years. Second, most of us don't die from it before we can reproduce. For evolution to happen on a noticeable scale, it would have to interrupt our reproduction capabilities on a massive scale. Certainly there are some people that are too obease to reproduce, but for most of humanity, even unhealthy fat people can reproduce and therefor pass on their genes.