I'm trying to think of a suitable analogy for the reason there is no other unique genetic structures on earth.

The best I can think of is a sperm cell fertilizing an egg. Once this event happens - conditions in the egg change such that no other sperm is allowed to penetrate.

Perhaps once life got started - the toxic byproducts of consuming energy and eliminating waste made the earth inhospitable to any other unique genetic structures. Consider the oxygen cotastrophe as an example. Once life is started - the only things that can occur are variations on a theme.

Same with fertilization. We don't know exactly how that new life will grow and develop over the long term. The one thing we do know for certain is that the basic DNA signature will never change.

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Spock Jenkins of the Vulcan Jenkins'.