Archaeologists often extrapolate the behaviors of ancient hunter-gatherers by studying modern hunter-gatherer groups. do you think this is a valid approach? is it useful to learn about modern hunter-gatherer groups in order to understand early societies?

I think that extrapolating the behaviors of ancient hunter-gatherers by studying modern hunter-gatherer groups is a valid approach. 

Modern hunter-gatherer groups learn how to do their daily tasks from the lessons and teachings imparted to them by their elders. Their elders also learned from their elders and so on. This is how knowledge and skills are passed down from one generation to another. One way or another, the methods and applications that modern hunter-gatherer groups use originated from the behaviors of ancient hunter-gatherers. There may be changes but these changes are usually applied in order for the group to adapt to the conditions that they are living in at that time. 

In short, learning about modern hunter-gatherer groups is vital in understanding early societies because the practices of today are the results of the practices of yesterday with variation to ensure longevity.

  the domino theory states that  if one country in a region came under the influence of communism, then the surrounding countries would follow in a  domino  effect.