1.1 Major Groups of Living Organisms

Groups of Life

In the recent past, scientists grouped living things into five kingdoms—animals, plants, fungi, protists, and prokaryotes—based on physical traits, such as the absence or presence of a nucleus, the absence or presence of cell walls, multicellularity, etc. However, in the late 20th century scientists realized that comparing DNA sequences was a more accurate method for grouping organisms according to their evolutionary history.

In 1977, Carl Woese and his colleagues proposed that all life on Earth evolved along three lineages, called domains. The domain Bacteria comprises all organisms in the kingdom Bacteria, the domain Archaea comprises the rest of the prokaryotes, and the domain Eukarya comprises all eukaryotes—including organisms in the kingdoms Animalia, Plantae, Fungi, and the protists.

 

A phylogenetic tree showing that life is divided into three domains -- Bacteria, Archaea, and Eukarya.
Carl Woese constructed this phylogenetic tree using data that he obtained from sequencing ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms lacking intracellular organelles. (16S Tree is in the public domain).

Properties of all cells

There are many differences between prokaryotic and eukaryotic cells. However, all cells have four common structures: the plasma membrane, which functions as a barrier for the cell and separates the cell from its environment; the cytoplasm, a complex solution of organic molecules and salts inside the cell; a double-stranded DNA genome, the informational archive of the cell; and ribosomes, where protein synthesis takes place.

Prokaryotes

Two of the three domains—Bacteria and Archaea—are prokaryotic. Prokaryotes were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. These organisms are abundant and ubiquitous; that is, they are present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively contaminated regions, such as Chernobyl. Prokaryotes reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an important role in the preparation of many foods.
Three organisms from the three domains of life. From left to right: Sulfolobus, an archaeon, Staphylococcus aureus, a bacterium, and Ranoidea caerulea, a eukaryote. (Sulfolobus by Xiaoyu Xiang is in the public domain; Staphylococcus aureus by Janice Haney Carr is in the public domain; Magnificent Tree Frog by LiquidGhoul is in the public domain).

Eukaryotes

Eukaryotes are organisms whose cells have a nucleus, as well as other membrane-bound organelles. Eukaryotes used to be classified into four kingdoms (Animalia, Plantae, Fungi, and Protista), but molecular data has shed light on the relationships between eukaryotic groups, and demonstrated that this classification scheme is incorrect. Eukaryotes are currently classified into five supergroups (Opisthokonta, Amoebozoa, Excavata, Archaeplastida, and SAR), but this may change with new information.

Some eukaryotes are unicellular, and others are multicellular. Multicellularity has evolved independently many times in eukaryotes, including in the groups that gave rise to animals, fungi, and plants. All animals and plants are multicellular, and some fungi are. We use the word “protist” as an informal name to refer to eukaryotes that are not animals, plants, or fungi. Protists are extremely diverse, and can be either unicellular or multicellular.

A sampling of protists. Clockwise from top left: red algae (Chondrus crispus); brown algae (Giant Kelp); ciliate (Frontonia); golden algae (Dinobryon); Foraminifera (Radiolaria); parasitic flagellate (Giardia muris); pathogenic amoeba (Acanthamoeba); amoebozoan slime mold (Fuligo septica).1
Text adapted from OpenStax Biology 2e and used under a Creative Commons Attribution License 4.0.
Access for free at https://openstax.org/books/biology-2e/pages/1-introduction

1. Red algae by Franz Eugen Köhler is in the public domain. Giant Kelp by Claire Fackler is in the public domain. Frontonia by Wiedehopf20 is used under a Creative Commons Attribution-ShareAlike 4.0 license. Dinobryon by Frank Fox is used under a Creative Commons Attribution-ShareAlike 3.0 Germany license. Radiolaria by Patrick De Wever is used under a Creative Commons Attribution-ShareAlike 4.0 license. Giardia by Dr. Stan Erlandsen is in the public domain. Acanthamoeba by Jacob Lorenzo-Morales et. al is used under a Creative Commons Attribution license. Fuligo septica by Urmas Tartes is used under a Creative Commons Attribution-ShareAlike 4.0 International license.

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