Domains of Life
There are three domains of life on Earth: Bacteria, Archaea, and Eukaryotes
Prokaryotes
Organisms without an organized nucleus or organelles, e.g. Bacteria & Archaea
Dominant form of life on earth
Heterotrophic and autotrophic forms
Aerobic (O2-rich) and anaerobic (O2-free) environments
The oldest living organisms on Earth (at least 3.5 billion year old)
Diversity
There are two (2) distint groups of prokaryotes on Earth: Bacteria and Archaea
Prokaryotic, unicellular & colonial
Possess DNA, but not membrane-bound organelles
Prokaryotic, unicellular
Possess DNA, but not membrane-bound organelles
Possess enzymes and transcription factors more similar to eukaryotes
n.b. Some evidence supporting the endiosymbiotic theory indicates that origin of eukaryotes came about through an archaean host, which engulfed a bacterium. This bacterium became the first organelle, such as the mitochondria
Geologic Age
Paleoarchean (maybe Eoarchean) - present
Above: Categories of life; this is an organizational image, not an evolutionary tree
Below: A cladogram of the domains of life
Eukaryotes
Organisms possess organelles such as a nucleus
e.g. "protists", plants, fungi, and animals
Eukaryotes probably evolved around 2.0 billion years ago (Paleoproterozoic)
Both microscopic and macroscopic forms exist on Earth
Autotrophs (e.g. plants and photosynthetic algae) & heterotrophs (e.g. fungi / animals)
Eukaryotes are dependent on an aerobic (oxygen-rich) environment
Diversity
Four major groups of eukaryotes:
Archaeaplasta (Plants)
Unikonts (Fungi & Slimemolds)
Chromalveolates (Various algae groups)
Excavates (Various unicellular protists)
Above: Biomass of organisms on Earth as represented by the amount of carbon in each group
n.b. Plants account for 83% of all carbon found in Earth's organisms (Bar-on, Phillips, and Milo 2018)
Viruses or Virions
Genetic entities that are in a "grey zone" between living and non-living states.
Possess a single strand of genetic information inside a protein capsule
Require a host to reproduce
Lack most of the internal structure and machinery, such as biosynthetic processes
Viruses are not constructed as cells
Questions for thought
How do the three domains differ and how are they similar?
What are examples of each domain?
What is multicellularity? What is colonial?
What makes a virus living or non-living
Additional Reading
How did multicellularity evolve? - Niklas, K. 2014. American Journal of Botany
Microbes in Central Park Soil very diverse (Ramirez et al. 2014)
Virus origins suggest they are alive (Nasir and Caetano-Anollés 2015)