subsistence groups = the groups of the people who make a living?

Everybody makes a living, so it's not quite that. A subsistence group is a group that makes its living in a particular way. Here they're contrasting hunter-gatherers, who don't grow their own food, but hunt animals and gather wild plant materials to eat, versus agriculturalists who eat mostly plant materials which they grow themselves rather than gather wild (tho in fact they're likely to eat some wild plants).

What you eat leaves different traces on your bones and teeth. Plant food is much more fibrous and harder to chew, which leads to teeth worn down more and worn down flatter over the course of a lifetime, with striations, grooves, visible under a microscope, whereas more omnivorousness and meat eating leads to less wear and teeth that show microwear more like polishing.

Further, when you put more stress on bones and muscles and tendons, they get thicker and denser as you grow, so skulls will develop differently. You can have two groups of the same human stock that will have different tooth and skull development depending on how they get their food, i.e. are of two different subsistence groups.

Excellent!
Thank a lot.