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Did you ever wonder what happens to old automobile tires?

The sequence from protozoan to jellyfish to trilobite to nautiloid to armored fish to dinosaur to monkey to human is no lineage at all, but a chronological set of termini on unrelated evolutionary trunks. Moreover, life shows  no trend to complexity in the usual sense —only an asymmetrical expansion of diversity around a starting point constrained to be simple. Let me explain that last cryptic remark: For reasons of organic chemistry and the physics of self-organizing systems, life arose at or very near the lower limit of preservable size and complexity in the fossil record. Since diversity, measured as number of species, has increased through time, extreme values in the distribution of complexity can move in only one direction. No species can become simpler than the starting point, for life arose at the lower limit of preservable complexity. The only open direction is up, but very few species take this route. Increasing complexity is not a purposeful trend of an unbroken lineage but only the upper limit of an expanding distribution as overall diversity increases. We focus on this upper tail and call its expansion a trend because we crave some evolutionary rationale for our perception of ourselves as a predictable culmination.

There are more bacteria in the gut of each person reading this essay than there are humans on the face of the earth. And who has a better hope for long-term survival? We might do ourselves in by nuclear holocaust, but prokaryotes will probably hang tough until the sun explodes.

Stephen Jay Gould

1941 - 2002