The organisms that are most primitive or the first to evolve on earth are the bacteria. They do not possess a nuclear membrane around their single chromosome. Absence of a well-defined nucleus terms them prokaryotes (pro means primitive, karyon means nucleus).

All bacteria including blue-green algae (Cyanobacteria) are prokaryotes.

Organisms other than bacteria possessing a well-defined nucleus are eukaryotes (eu means true, karyon means nucleus).

Difference Between Prokaryotes & Eukaryotes

Characteristics Prokaryotes Eukaryotes
Size 0.1-10 μm 10-100 μm
Location of DNA DNA in cytoplasm DNA inside distinct nucleus
Organelles No membrane bound organelles Mitochondria, golgi body, lysosomes present
Cell wall Always present Present in plants
Respiration By mesosomes By mitochondria
Reproduction Mostly asexual Asexual and sexual
Examples Bacteria and cyanobacteria Protoctista, fungi, plants, animals