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What would happen if the hands of time were turned back to an arbitrary moment in our history of human evolution, and what if we restarted the clock?

American scientist Stephen J. Gould projected this infamous thought experiment around the 80s - and it still clutches the imagination of today's evolutionary biologists.

Gould imagined that if time was rewound, then evolution would push life down an entirely different path and humans would certainly not re-evolve. In fact, he believed that humanity's evolution was so rare that we could replay the recording of life a million times and we wouldn't see anything like Homo sapiens arise again.

His reasoning was that random events play an enormous role in evolution. These involve monumental mass extinction events - like devastating asteroid impacts and volcanic eruptions.

But random events are also found at the molecular level. Genetic mutation, which forms the basis of evolutionary adaptation, is dependent on chance events.

Put simply, evolution is the product of random mutation. A rare few mutations will improve an organism's probability of survival in specific environments over others. The split from one species into two begins from such rare mutations that just happen to become common over time. But more random processes will still interfere, undoubtedly resulting in a loss of helpful mutations and amassing detrimental mutations over time. This intrinsical randomness should imply you'd get completely different life forms if you replayed the tape of life.

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