The Study of Evolutionary Relationships of Living Organisms

• Over 1.5 million different organisms have been identified to date. 

• Many similarities among living organisms:

- Made up of cells surrounded by a plasma membrane.

- Use ATP as energy source.

- Store genetic information as DNA.

- Ribosomes are the site of protein synthesis.

• Both differences and similarities among organisms are caused by natural selection (Darwin, 1858). 
• Organisms can be classified into taxonomic categories (taxa), based on the differences and similarities among them 
• Ancient Greeks classified all living organisms into two groups

-Kingdom Plantae

-Kingdom Animalia 

- In 1850s bacteria and fungi were incorrectly placed in 

the Plant Kingdom.   

- In 1860s Kingdom Protista was proposed to include 

bacteria, fungi, algae, and protozoa, but many scientists 

still classified bacteria and fungi as plants. 

- Intense disagreement over classification of bacteria and 

fungi persisted over 100 years. 

- In 1930s electron microscopy made it clear that 

bacterial cells lacked a nucleus.The term procaryote was 

introduced in 1937.

- In 1959 Kingdom Fungi was established.

- In 1961 the current definition of the term procaryote 

was established.

- In 1968 the Kingdom Procaryotae was accepted by 

biologists.

- In 1969 Robert Whitaker proposed a five-kingdom 

system of biological classification for all living 

organisms.