The author doesn’t suggest that a behavior needs to be around for 250,000 years in order to impact our biology. Rather, the author believes 250,000 years is long enough for biological adaptation to occur. In other words, that timeframe is sufficient for adaptation. But we have no reason to think the author believes it’s necessary for human evolution.

This best captures the purpose, which is to provide support for the iopinion that the assumption that cooking couldn’t have had any impact on biological evolution because the practice is too recent is wrong. The author mentions the 250,000 year figure to show that evidence of cooking goes back far longer than what others might consider “recent.”

The author doesn’t suggest that we became unable to survive on raw-food diets about 250,000 years ago. Rather, the author suggests that the practice of cooking existed 250,000 years ago; our adaptation might have occurred much more recently.

This doesn’t make sense as the purpose, because the author favors the view that cooking affected the human digestive anatomy. So she wouldn’t try to undercut that view.

The author doesn’t mention “earth ovens” because she wants to show the technology that we used to cook food. She mentions “earth ovens” to provide evidence that we cooked 250,000 years ago; the specific technology we used isn’t important to the author’s point. The fact that we cooked is what the author cares about in that line.