About 3500 mya.
Unicellular organisms evolved first; and from them evolved the multicellular organisms. But that leads onto another question as to why multicellular organisms evolved.
Unicellular plants and animals were first forms of life on Earth
unicellular
A unicellular organism is a life form consisting of a single cell.
Bacteria
"Unicellular" refers to life, in which each organism only has a single cell.
The gametic life cycle exists in which the haploid form is always unicellular as in humans.
The gametic life cycle exists in which the haploid form is always unicellular as in humans.
How unicellular organisms may have given rise to the first animals?
It is believed to have taken about 2 billion years for unicellular organisms to evolve into multicellular organisms. The transition from unicellular to multicellular life forms is thought to have occurred around 600 million years ago.
The first scientist to classify unicellular organisms was Antonie van Leeuwenhoek. In the late 17th century, he utilized his own improved microscope to observe and describe a variety of microscopic life forms, including bacteria and protozoa, which he referred to as "animalcules." His detailed observations laid the groundwork for microbiology and the classification of unicellular organisms.
The gametic life cycle exists in which the haploid form is always unicellular as in humans.