What were the earliest living things on earth?

Photo by CDC on Unsplash

There are a number of theories concerning the earliest living things on earth. Here I discuss a few prominent ones: eubacteria, archaea, and RNA-based living things.

Bacteria

The oldest DNA-based species are thought to be one-celled microbes, likely of the domain eubacteria, which existed in the earth’s oceans as early as 3.8 billion years ago. Like present-day bacteria, these early bacteria were prokaryotic — meaning that they did not have a DNA-bearing nucleus, nor did they have cell parts enclosed by membranes. Prokaryotes are typically single-celled, though some, like cyanobacteria, will form large colonies.

A species of cyanobacteria — Cylindrospermum sp, under magnification at the Adelaide laboratories of CSIRO Land and Water, 1993. Attribution: CSIRO, CC BY 3.0, via Wikimedia Commons

Cyanobacteria are also notable because they are photosynthetic — they consume carbon dioxide and water, and in a series of chemical reactions, produce carbonaceous food molecules — as well as oxygen.

Over time, these photosynthetic bacteria proliferated and flourished, collectively producing large quantities of oxygen that became part of earth’s atmosphere. This gradual oxygenation of earth’s atmosphere reached a tipping point around 2 billion years ago, wiping out a good portion of the microbes that found…