The increase [in complexity] has been neither universal nor inevitable. Bacteria, for example, are probably no more complex today than their ancestors 2000 million years ago. The most that we can say is that some lineages have become more complex in the course of time. Complexity is hard to define or to measure, but there is surely some sense in which elephants and oak trees are more complex than bacteria, and bacteria than the first replicating molecules.

Maynard Smith, J. and Szathmáry, E. 1995. The Major Transitions in Evolution. Oxford: Oxford University Press. P. 3.