Why humans (or something very similar) may have been destined to walk the Earth

What would happen if the hands of time were turned back to an arbitrary point in our evolutionary history and we restarted the clock? American paleontologist Stephen Jay Gould proposed this famous thought experiment in the late 1980s – and it’s one that still grips the imagination of evolutionary biologists today.

Gould reckoned that if time was rewound, then evolution would drive life down a completely different path and humans would never re-evolve. In fact, he felt humanity’s evolution was so rare that we could replay the tape of life a million times and we wouldn’t see anything like Homo sapiens arise again.

His reasoning was that chance events play a huge role in evolution. These include mammoth mass extinction events – such as cataclysmic asteroid impacts and volcanic eruptions. But chance events also operate at the molecular scale. Genetic mutation, which forms the basis of evolutionary adaptation, is reliant on chance events.

Put simply, evolution is the product of random mutation. A rare few mutations can improve an organism’s chance of survival in certain environments over others. The split from one species into two starts from such rare mutations that become common over time. But further random processes can still interfere, potentially leading to a loss of beneficial mutations and increasing harmful mutations over time. This inbuilt randomness ought to suggest you’d get different life forms if you replayed the tape of life.

Of course, in reality, it’s impossible to turn back the clock in this way. We’ll never know for sure just how likely it was to have arrived at this moment – for us to have written this article, and for you to be reading it. Fortunately, however, experimental evolutionary biologists do have the means to test some of Gould’s theories on a microscale with bacteria.

Microorganisms divide and evolve very quickly. We can therefore freeze billions of identical cells in time and store them indefinitely. This allows us to take a sub-set of these cells, challenge them to grow in new environments and monitor their adaptive changes in real time. We can go from the “present” to the “future” and back again as many times as we like – essentially replaying the tape of life in a test tube.

Evidence of evolutionary fate

Many bacterial evolution studies have found, perhaps surprisingly, that evolution often follows very predictable paths over the short term, with the same traits and genetic solutions frequently realised. Consider, for example, a long-term experiment, in which 12 independent populations of Escherichia coli founded by a single clone, have been continuously evolving since 1988. That’s over 65,000 generations – there have only been 7,500-10,000 generations since modern Homo sapiens appeared. All the evolving populations in this experiment show higher fitness, faster growth and larger cells than their ancestor. This suggests that organisms have some constraints on how they can evolve.

There are evolutionary forces that keep evolving organisms on the straight-and-narrow. Natural selection is the “guiding hand” of evolution, reigning in the chaos of random mutations and abetting beneficial mutations. This means many genetic changes will fade from existence over time, with only the best enduring. This can also lead to the same solutions of survival being realised in completely unrelated species.


https://theconversation.com/why-humans-or-something-very-similar-may-have-been-destined-to-walk-the-earth-118346

More to read on the link and is fascinating