The Tree of Life - Bacteria, Archaea, Eukaryotes

The modern tree of life consists of three domains: bacteria, archaea, and eukaryotes. Bacteria are unicellular prokaryotic organisms lacking a membrane-bound nucleus. Archaea are also unicellular prokaryotes but have a distinct evolutionary origin from bacteria. Eukaryotes include uni- and multicellular organisms with membrane-bound nuclei, encompassing fungi, plants, and animals.

Darwin's tree compared evolutionary relationships to tree branches. Haeckel proposed three kingdoms based on observable phenotypes in 1866. By 1937, organisms were divided into eukaryotes and prokaryotes. Genetic analysis, particularly 16S rRNA sequencing, revealed that prokaryotes had divergent evolutionary origins, leading to the modern three-domain system in 1990.

16S rRNA sequencing enabled genetic analysis that revealed prokaryotes traditionally classified together actually had genetically divergent evolutionary origins. This molecular evidence showed that archaea differed from bacteria in cell wall composition, coenzymes, and gene sequences, supporting their classification as a separate domain and fundamentally reshaping the tree of life.

Archaea lack peptidoglycan in their cell walls and possess different coenzymes and 16S rRNA gene sequences compared to bacteria. At the molecular level, archaea resemble eukaryotes in their machinery for replication, transcription, and translation, but resemble bacteria in metabolism and energy conversion, reflecting their unique evolutionary position.

The tree of life uses a branching metaphor where all species radiate from a common ancestor at the root. Branch tips represent extant, or living, species, while extinct species are positioned toward the center. This visual representation shows how organisms share common ancestry and have diverged over evolutionary time through types of genetic transfer between organisms.

The tree of life is called 'living' because it continuously evolves as new information emerges. The classification of many organisms, even the distinction between archaea and bacteria, remains contested and occasionally changes. This reflects the dynamic nature of scientific understanding as molecular and genetic evidence provides deeper insights into evolutionary relationships.

Archaea are often found in the most inhospitable environments on Earth, including bogs, wastewater treatment plants, the deepest parts of the ocean, and hot acid springs. Their ability to thrive in extreme conditions reflects their unique evolutionary adaptations and distinguishes them from bacteria, which are found in nearly every terrestrial and aquatic environment.