What restrictions does unicellularity impose on the evolution of living organisms?

Single-celled organisms for a long time could not evolve, since all changes could occur only within one cell. As they were not, this was not enough for the emergence of more complex species. Some unicellular organisms were able to combine into more complex forms for a number of reasons. When an organism consists of several cells, it begins to grow, tissues, organs, and organ systems are formed. Therefore, it was thanks to the combination of several cells into the body that evolution began, which led to the emergence of new kingdoms of the living.