Question

In the early stages of life, how did the different kingdoms of life evolve in reference to differentiation?

This response was obtained from an article in the publication Natural History, "Life's Expanding Realm", by Andrew Knoll, June 1994

The relationships in the tree of life at its earliest stages seemed to be defined by an organism’s use of oxygen. Lower branches of the tree of life are populated by organisms that cannot utilize oxygen in their metabolism; in fact, for many oxygen is toxic. Species higher up on the tree, reflecting their evolution after the great oxidation event in earth’s history, are able to use oxygen respiration. In the new environments created by the oxygen revolution, bacteria diversified to form many aerobic lineages that are ubiquitous on the modern earth. More ancient, anaerobic microbes retreated along with their environments, from which they continued to play a central role in the cycling of carbon and other elements through ecosystems.

The earliest organisms were prokaryotic - single celled creatures whose genes do not reside in a membrane-bound nucleus. But at some point, other kinds of organisms evolved. These eukaryotic (nucleated) organisms evolved before the oxygen revolution took hold, but were probably only a minor part of early communities. Oxygen started eukaryotes out on the road to ecological prominence, not because they themselves had evolved the respiration pathway, but because they could consume the bacteria who had. Some aerobic bacteria became symbiotically incorporated into nucleated cells, in time evolving into mitochondria.
October 30, 2001