explain " single-celled organisms became multicellular. "

Dear Student,

The slow change in the body design of an organism over a long period of time is termed as evolution. It helps an organism to survive in its surroundings. Through evolution,single-celled organisms became multicellular. 

The earliest cells probably arose about 3.5 billion years ago in the rich mixtures of organic compounds. These cells were chemoheterotrophs.Then a very significant event of evolution occured in which these cells developed the photosynthetic ability to fix CO₂ into more complex organic compounds. Cyanobacteria (unicellular organisms) are the modern descendants of these early photosynthetic O₂ producers. Then, slowly these prokaryotic cells evolved into eukaryotic cells.

 Around I.0 billion years ago multicellularity developed i.e. Unicellular organisms evolved into multicellular organisms.The transition of unicellular microbes into multicellular organisms such as fungi, plants and animals was a huge step in the evolution. The early step in the evolution of multicellular organism that occured was association of unicellular organisms to form colonies. Around 223 million years ago, a species of unicellular green algae began forming aggregates of cells stuck together by a glue of secreted proteins and sugars. Then, finally multicellular organization was obtained.