How do biologists learn which organisms we're the ancestors of today's plants?

by classifying into categories that get smaller and smaller  these are the classification steps :  life , domain , kingdom , phylum , class,  order , family , genus , and species every time you narrow it down the option gets smaller and smaller till there's none left except the species it self .

one negative thing it that it can kill water animals. another negative thing is it can cause the destruction of ecosystem. one way water pollution can be reduced is to conserve energy.