Answer
On a cellular level, many functions are necessary in both prokaryotic and eukaryotic cells, such as cellular respiration and metabolism. These are functions essential to the life of a cell, regardless of whether there are many other cells present in the overall organism or not. These would have been adapted in the earliest forms of life and passed on to modern creatures.
Work Step by Step
Through the process of evolution, traits build upon other traits to create increasingly better fitness over time. Unless a trait is more seriously detrimental, it will not be selected out of a population, and the genes that code for it will continue to be passed along in a larger number of cases. Additionally, organisms with vastly different characteristics still need to perform many of the same functions, and those necessary traits will be present in all kinds of organisms.
