The economist, Robin Hanson, proposed a solution to the Fermi Paradox about 20 years ago. Commonly referred to as the Great Filter, Hanson theorized a set of potential barriers to intelligent life in the universe, with the implication that at passing through at least one of the steps must be improbable, the result being that technological species are rare.
I recently read an essay by Neil deGrasse Tyson that quoted an estimate: that in the history of life on Earth, there have been around ten billion species. I’ve looked for a source for this; the estimates that I’ve found range over several orders of magnitude, but ten billion is a nice round number, and it works for the sake of the argument that I want to make (feel free to substitute your own number instead).
We frequently see phenomena, both natural and man-made that follow power laws; in particular, we see this with the size of cities (there are a small number of very large cities, and far more smaller ones), with the size of individuals in a population (there aren’t that many seven foot tall people), and with the size of companies (there are relatively few companies worth hundreds of billions of dollars, and many companies that are much smaller).
IF intelligence follows a similar power law (which it seems to – Earth has one technological species, several precursors to humans, then the next tier down are a slightly larger number of species, such as great apes, cetaceans, cephalopods etc, and by the time you get down to unicellular life, there are hundreds of millions – perhaps orders of magnitude more – species), then perhaps there is a strong connection in terms of the number of species that exist during the course of life on a given planet, and the likelihood of there being a technological civilization there. If a planet only supports (say, for example) one billion species during the period in which life exists on it, perhaps that planet will have a few relatively smart tool-using species, but in all likelihood, no technological one.
The thing is, I can see lots of scenarios that could result in a world that supports life, but with far less diversity than Earth. This doesn’t even include hostile scenarios that are actually inimical to life (such as the destruction of said world by its star).
Picture a living world that receives ten percent less energy from it’s sun. Perhaps that energy means that everything ambles along more slowly, evolution included. Perhaps this bucolic world has plants with vast leaves to soak up the sun, but little animal life.
Or picture a world that has no large moon, no tides, and consequently no tidal pools where oceanic life can gradually adapt to the shore. Perhaps that world has oceans that team with life, but far fewer species that live on the land, resulting in an overall lower species count.
Or a world of mono-cultures, where a particular set of unicellular life proliferates and regulates the environment to the extent that other forms of life cannot thrive (this is a scenario that happened in the early history of Earth, when some forms of life started releasing free oxygen, which was toxic to certain earlier species).
It is possible (we really need more data points!) that there are more ways for a world to be marginally life-bearing than for it to be non-life bearing. Could this alone (plus a simple power law) be the reason for the Great Filter?