The phrase "earlier generations" refers to people who lived in the past, usually the ones who came before your parents or grandparents. Full definition
The sons and daughters of earlier generations of immigrants are hearing the same racist insults that were once directed at their parents. (politics.co.uk)
As a result, their stars contain fewer of the heavy elements needed to construct planets, since those elements (like iron and silicon) must be created in earlier generations of stars. (sciencedaily.com)
It happens over generations as later generations stay in many ways quite similar but in certain respects become rather different from earlier generations, especially in small, isolated populations facing novel selection pressures. (religion.blogs.cnn.com)