Reproduction in all living creatures is never perfect. Variations show up in the offspring, which give them different efficiencies in meeting the challenges of any given environment.

The environment exerts a selective action on the population of offspring: The best adapted survive in the greatest numbers to breed and produce new offspring. Thus the traits that encourage survival in any environment are preserved.

As adaptation to a given environment improves, and as environments gradually change on the planet, the organisms themselves change, adapt, and evolve.

This is the key to the chicken-and-egg paradox. If we trace the evolution of chickens and eggs back far enough, we will not find a first Egg. Instead, we will realize slowly that we are not looking at chickens any more, but at feathered reptiles.

Tracing the line back further, we will see amphibia, bony fish, cartilaginous fish, and invertebrates. The trail, if pursued long enough, leads back to one-celled life.

But where did this one-celled life come from? Is a bacterium-and-spore paradox any less frustrating than the chicken-and-egg?

Unless we suffer from mental fatigue or atrophied curiosity along the way, we must eventually ask "Where did the earliest one-celled life come from?" With such simple organisms, the problem becomes as much chemical as biological.