The simplest level at which life may exist is the cell. Cells are the basic unit of structure and function in living organisms and represent the lowest level of biological organization capable of exhibiting all characteristics of life, including metabolism, reproduction, response to the environment, and evolution
. Regarding minimal complexity, the smallest living microorganisms, such as certain bacteria and archaea, define the lower boundary of life complexity. These organisms have genomes estimated to contain around 300 genes, roughly corresponding to about 375 kilobase pairs of DNA, which is considered the minimal genetic complexity for cellular life capable of independent existence
. Life requires not only complexity but also a certain degree of structural order; neither complexity alone nor order alone suffices to characterize living matter. The combination of complexity and order, along with the ability to undergo Darwinian evolution, distinguishes living systems from nonliving matter
. In summary:
- The cell is the simplest organizational level at which life exists.
- The smallest living cells (microorganisms) have minimal genomes estimated at about 300 genes.
- Life is characterized by a balance of complexity and order, enabling metabolism, reproduction, and evolution.
This understanding excludes entities like viruses, which lack cellular structure and independent metabolism, and thus are generally not considered fully living
. Therefore, the simplest life forms are single-celled microorganisms such as bacteria and archaea, representing the minimal functional and organizational unit of life.
