The Earth is 4.6 billion years old and microbial life is thought to have first appeared between 3.8 to 3.9 billion years ago. 80% of Earth's history was exclusively microbial life. Undoubtedly, microbial life is still dominant form of life on Earth.

Three domains of life proposed by C.R. Woese et.al (1997) is an evolutionary modelof phylogeny based on differences in sequences of nucleotides in the cells ribosomal RNA (rRNA, most prominently 16s rRNA genes) as well as cell membrane lipid structure and it's sensitivity to antibiotics. "Domains" are the top level of classification which categorizes life in most simpler way.

The three domains of life are:

Archea

• They are the oldest of all.


• They are prokaryotic, with no nuclear membrane but distinct biochemistry and RNA.


• Cell membrane is composed of branched hydrocarbon chains attached to glycerol by other linkages.


• They are extreme halophiles (can survive in very salty water) and hyperthermophiles ( can withstand high temperature)


• Being highly resistant to chemical and antibiotics they can survive in extreme environmental conditions.


• Eg: methanogens ( which produce methane), thermoacidophiles( which thrive in highly acidic and high temperature water).

Bacteria 

• They are also known as true bacteria or eubacteria.


• They are prokaryotic, i.e. lack true nucleus.


• They have Peptidoglycan in their cell wall.


• Cell membrane is composed of unbranched fatty acid chains attached to glycerol by ester linkages.


• RNA has molecular region distinctly different from Archea and Eukarya.

Eg: cyanobacteria, mycoplasma.

Eukarya

• They are youngest in the history of Earth and superior to all 


• They are eukaryotic.


• They have well defined nucleus with chromosome, DNA and organelles.


• They may be unicellular or multicellular.


• Cell membrane is composed of unbranched fatty acids attached to glycerol by ester linkages as in bacteria.


• They are further sub- divided into five kingdoms Monera, Protista, Fungi, Plantae and Animalia.

Note: A quick overview for all five kingdoms will be given in upcoming note, so please remain updated.

Thankyou!!