Geology Reference
In-Depth Information
The complexity of classification
Modern genetic science has opened new doors to understanding the relationships between ancient
life and modern living organisms. For a long time biologists organized all living things into six kingdoms:
animals, plants, fungi, protists, bacteria, and archaea. Of these six, bacteria and archaea are the only
ones that include simple-celled organisms, or prokaryotes. The other four kingdoms are composed of
organisms with complex, eukaryotic cells. (Protists are
single
-celled, but their one cell is complex; thus
they are eukaryotes.)
You are a complex and multi-celled organism. The cells that compose your body, your houseplants, and
the mushrooms on your pizza are all eukaryotic cells. Eukaryotic cells have a cell membrane around
them and multiple organelles inside, including a nucleus and mitochondria. In contrast, bacteria and
archaea are single-celled organisms with prokaryotic cells. Prokaryotic cells are much smaller than
eukaryotic cells and do not have a thick cell membrane or any organelles.
Based on this similarity alone, it seems logical that archaea and bacteria are more closely related to
one another than they are to any plants, animals, or other eukaryotes. Modern genetic analysis of the
ribonucleic acid (RNA), however, indicates that archaea and bacteria are only distantly related, and
that archaea share a more recent common ancestor with eukaryotes. This relationship is illustrated
on the branching tree diagram that includes the major families within the three domains: Bacteria, Ar-
chaea, and Eukarya.
