Cell-to-cell variability in isogenic cell populations is analyzed by taking into consideration causal relations between the parameters that define the state of individual cells. A population of red blood cells is treated as an example to show that it is possible, from the relationships between standard deviations of distribution functions with respect to various cell parameters and their corresponding correlation coefficients, to unravel previously unidentified causal relations between these parameters. It is suggested that this approach can be employed in searching for the causal relations, which, in analogy with the process of self-reproduction of vesicles, could be required for the proper division of cells that grow and divide. On the basis of a vesicular origin of primordial life, it is concluded that the cell-to-cell variability could be a consequence of asymmetric vesicle divisions needed for the sustainability of early evolution.