What system of the lowest level of the organization is already capable of independent existence?

All living organisms on Earth are made up of cells. Therefore, the cellular level of organization is a system that is capable of independent existence. Living organisms are diverse, they can be unicellular (ciliate shoe, bacteria), that is, they consist of only one cell and at the same time perform all the functions of a living organism: breathe, eat, grow, multiply. Living organisms can be multicellular, with developed or undeveloped organ systems: nervous, digestive, excretory and others.