Most organisms that have ever existed, and still the large majority of the organisms that exist today, consist of a single cell, or even a strand of RNA or DNA lacking the full suite of features that mark a cell, e.g.
viruses. As the neuroscientist Joseph LeDoux explains in The Deep History of Ourselves: The Four Billion Year Story of How we got Conscious Brains, a first step on the path towards complex organisms was the symbiosis among specialized organelles within cells, including mitochondria, nuclei, and cell walls.
It took about three billion years for evolution to hit on the innovation of multicellular organisms with specialized cells and body parts.
Read more on psychologytoday.com