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Prokaryote Evolution

Compares the three classification domains.

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Prokaryote Evolution

What are the most numerous organisms on the planet?

Bacteria. And all it takes is one to quickly grow, under just the right conditions, into millions and billions. Luckily, we know how to control bacteria when necessary. But bacteria do serve many important purposes. In fact, we could not survive without them.

Evolution of Prokaryotes

No doubt you’ve had a sore throat before, and you’ve probably eaten cheese or yogurt. If so, then you’ve encountered the fascinating world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny and sometimes bothersome, but they are the most numerous organisms on Earth. Without them, the world would be a very different place. Prokaryotes are the simplest organisms. The first cells and organisms to evolve would be classified as prokaryotic.

Prokaryotes are currently placed in two domains. A domain is the highest taxon, just above the kingdom. The prokaryote domains are Bacteria and Archaea (see Figure below). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells.

The three domains of life

The Three Domains of Life. All living things are grouped in three domains. The domains Bacteria and Archaea consist of prokaryotes. The Eukarya domain consists of eukaryotes.

It’s not clear how the three domains are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share (see Table below). How can this be explained? One hypothesis is that Eukarya arose when an Archaean cell fused with a Bacterial cell. The two cells became the nucleus and cytoplasm of a new Eukaryan cell. How well does this hypothesis fit the evidence in Table below?

Characteristic Bacteria Archaea Eukarya
Flagella Unique to Bacteria Unique to Archaea Unique to Eukarya
Cell Membrane Unique to Bacteria Like Bacteria and Eukarya Unique to Eukarya
Protein Synthesis Unique to Bacteria Like Eukarya Like Archaea
Introns Absent in most Present Present
Peptidoglycan (in cell wall) Present Absent in most Absent


  • Prokaryotes include Bacteria and Archaea. An individual prokaryote consists of a single cell without a nucleus.
  • Bacteria live in virtually all environments on Earth.
  • Archaea live everywhere on Earth, including extreme environments.


  1. What are prokaryotes?
  2. What are two major differences between Bacteria and Archaea?

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Archaea Domain of prokaryotes; includes single-celled organisms that live in extreme environments.
Bacteria Domain of prokaryotes; single-celled organisms; cells lack a nucleus.
domain Taxon in the revised Linnaean system that is larger and more inclusive than the kingdom.
Eukarya Domain of eukaryotes; organisms with a nucleus in their cells.
kingdom Largest and most inclusive taxon in the original Linnaean classification system.
prokaryote Single-celled organism that lacks a nucleus or membrane-bound organelles.
taxon (plural, taxa): A grouping of organisms in a classification system such as the Linnaean system; for example, species or genus.

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