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.
As we all know, in reality, it's impossible to turn back the clock in this way. We will never know for sure just how likely it was to have arrived at this moment as we are. Fortunately, however, experimental evolutionary biologists do have the means to check a number of Gould's theories on a microscale with bacteria.
Microorganisms divide and evolve very quickly. We can also freeze billions of identical cells in time and store them indeterminately. This allows scientists to use a set of those cells, challenge them to grow in new environments and monitor their adaptive transformations in real-time. We can go from the "present" to the "future" and back again as frequently as we would like - predominantly replaying the tape of life in a test tube.
Many microorganism evolution studies have found, maybe surprisingly, that evolution often follows very predictable paths over the short term, with the same traits and genetic solutions frequently cropping up.
Consider, as an example, a long-term experiment, in which 12 objective populations of Escherichia coli founded by a single clone, have been continuously evolving since 1988. That's over 65,000 generations. All the evolving populations during this experiment show higher fitness, faster growth and larger cells than their predecessor. This suggests that organisms have some constraints on just how they will evolve.
At the same time, Isaac Newton's theories, based on large scale deterministic forces, can be used to describe many systems on large scales. These describe the universe as perfectly predictable.
If Newton's point of view was to remain an absolute truth, then the evolution of humans was inevitable. However, this comforting certainty was crushed by the discovery of the contradictory, however, fantastical world of quantum physics, late in the 20th Century. At the most minute of scales of atoms and particles, true chance is at play - which means our world is unpredictable at the utmost basic level.
