How do we measure the diversity of life? One numerical representation of diversity is simply the total number of species of life that have ever existed. Given that we can't even say within an order of magnitude how many species are alive today (estimates range from 10 to 100 million, a very significant proportion of which are beetles!) and the imperfections of the fossil record, it is impossible to state with any confidence how much diversity has existed on Earth. However with some stated assumptions, it might be possible to calculate a rough figure to estimate a possible upper limit of how much diversity the processes of evolution might have produced.
To make such a calculation we need estimates of how long evolution has had to operate and a reasonable figure for expected rate of species production, i.e. how often a new species might be expected to arise from immediate ancestor species.
The universe originated about 20 billion (2 x 1010) years ago. Our planet Earth originated about 4 billion years ago and life originated on Earth about 3.5 billion years ago (see Fig. 24.2, p 487 in Campbell). Thus every extant organism is product of 3,500,000,000 years of evolution, driven by the mindless non-conscious forces of random mutation and natural selection.
Figures of billions of years are used with abandon (as I've just done) to describe the time available for life to evolve to its present day diversity, to accumulate adaptations and genetic change yielding the 10 - 100 million or so different species of organisms alive today. However, the human mind cannot adequately handle such immensely large numbers to get a true appreciation of the enormity of geological time. We have evolved to work with time scales measured in decades or centuries, time scales appropriate to human lifetimes. Let's make a simplifying assumption, one to bring the incomprehensible 3.5 billion years to a more accessible and meaningful figure. Can we reasonably divide 3,500,000,000 (3.5 x 109) years into numerically smaller «evolutionary units»?