Bacteria, archaea and protists make up two out of three taxonomic groups of organisms and also comprise one of the four kingdoms. With the exception of some protists, they are all unicellular organisms that make up the most abundant and diverse group on the earth.
They are ancient organisms, their ancestral lineages are the oldest and earliest to appear on this earth and since then, they have fixed nitrogen from our atmosphere, lived in conditions we could never imagine, invaded other organisms and even help digest our food.
Without bacteria, archaea and protists we wouldn’t have the clear understanding of life on earth we today have, and by understanding these we can begin to understand our early ancestors and our continual evolution.
From Crash Course: Biology on YouTube presented by Hank Green.