He explains that by any measure, the most common organisms have always been, and still are, the bacteria. The complexity distribution is bounded at one side (a living organism cannot be much simpler than bacteria), so an unbiased random walk by evolution, sometimes going in the complexity direction and sometimes going towards simplicity (without having an intrinsic preference to either), will create a distribution with a small, but longer and longer tail at the high complexity end.
The expert emphasizes that, by any measure, bacteria have always been and continue to be the most common organisms. The spectrum of biological complexity is bounded on one side (living organisms cannot be simpler than bacteria), so an unbiased evolutionary process will result in a distribution with a small but growing tail at the high complexity end.