A LL LIFE is made of cells. But to build a complex, multicelled organism from those cells almost always requires them to come in more than one type. This means that as cells multiply in a growing organism they need to differentiate, which they do by expressing different subsets of genes from within the genome they all share. Different patterns of gene expression produce different types of cell.
These complex patterns of gene expression appear to be the preserve of eukaryotes—creatures built of cells that have their primary genomes wrapped up in a complex compartment called a nucleus. Sponges, descended from some of the earliest multicellular creatures, have a handful of cell types. Plants have dozens. Complex animals have hundreds
The number of cell types is, broadly speaking, a function of the degree to which these creatures have bodies made up of organs—arrangements in which the activities of different types of cells are co-ordinated in order to perform a specific function or functions. Sponges do not have organs; they just have voids through which they pump water from which single-celled prey can be plucked. This does not require lots of different sorts of cells. Plants have some organs, but not that many: stems and roots; petals, stamens and pistils by means of which to reproduce; and a few more.
It is in complex animals that organs come into their own as a plethora of sophisticated, functional entities. How large that plethora is in a given animal species lies, to some extent, in the eye of the biologist doing the beholding. As in many other areas of science, some are lumpers and some are splitters. A lumper may see a mammalian ear as an organ. A splitter may want to differentiate all sorts of components, such as the structure in the inner ear called “the organ of Corti”, after the anatomist who first described it. Lists of human organs thus vary in length, but 78 is a number that often pops up in medical texts.