What would happen if the hands of time were reset to any point in our history of evolution and we restarted the clock? The American paleontologist Stephen Jay Gould suggested this famous thought experiment in the late 1980s – and it still takes the imagination of evolutionary biologists to never re-develop. In fact, he felt that the evolution of humanity was so rare that we could play the volume of life millions of times, and we would not see anything like Homo sapiens reappear.
His consideration was that random events play a big role in evolution. These include massive mass extinction events such as cataclysmic asteroid impacts and volcanic eruptions. Random events also act at the molecular level. Genetic mutation, which forms the basis for evolutionary adaptation, is dependent on random events.
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Put simply, evolution is the product of a random mutation. A few mutations can improve the chances of survival of an organism in certain environments over others. The splitting of one species into two begins with those rare mutations that become common over time. However, further random processes may continue to interfere, possibly leading to the loss of useful mutations and an increase in harmful mutations over time. This built-in randomness should indicate that you are getting different life forms as you replay the bond of life.
Of course, it is impossible to turn the clock back in this way. We'll never know how likely it is that we arrived at that moment. Fortunately, experimental evolutionary biologists have the opportunity to test some of Gould's microscale theories with bacteria.
Microorganisms divide and develop very fast. We can therefore freeze billions of identical cells on time and store them indefinitely. In this way, we can extract a subset of these cells, challenge them to grow in new environments, and monitor their adaptive changes in real time. We can go as often as we want from the "present" to the "future" and back again ̵
Many bacterial evolutionary studies may have surprisingly often found this followed very predictable pathways in the short term, often with the same characteristics and genetic solutions. For example, consider a long-term experiment in which 12 independent populations of Escherichia coli founded by a single clone have continued to evolve since 1988. That's over 65,000 generations – there were only 7,500 to 10,000 generations since the modern era Homo sapiens appeared. All developing populations in this experiment show higher fitness, faster growth, and larger cells than their ancestors. This suggests that organisms have some limitations on their development.
Natural selection is the "leading hand" of evolution, which is in the chaos of random mutations and favors beneficial mutations.
There are evolutionary forces that make organisms straight and narrow. Natural selection is the "guiding hand" of evolution, in the chaos of random mutations that favor beneficial mutations. This means that many genetic changes will fade over time, with only the best ones lasting. This can also lead to the same survival solutions being achieved for completely related species.
We find evidence in evolutionary history that species that are not closely related but have similar environments develop a similar trait. For example, extinct pterosaurs and birds developed both wings and a distinct bill, but not from a common ancestor of recent times. Due to the evolutionary pressure, wings and beaks developed essentially twice in parallel.
But also the genetic architecture is important. Not all genes are the same: some have very important tasks compared to others. Genes are often organized in networks that are similar to circuits, including redundant switches and "master switches." Naturally, mutations in "main switches" lead to much larger changes, as all genes under their control feel a kick-off effect. This means that certain sites in the genome contribute more or more to evolution than others, affecting evolutionary outcomes.
But what about the underlying physical laws – do they favor a predictable evolution? On a very large scale, it looks like this. We know many laws that govern our universe and that are safe. Gravity, for example, for which we blame our oceans, dense atmosphere, and nuclear fusion in the sun that shower us with energy, is a predictable force. Isaac Newton's theories, which rely on large-scale deterministic forces, can also be used to describe many systems on a large scale. These describe the universe as completely predictable.
If Newton's view was to remain perfectly true, human evolution was inevitable. However, this reassuring predictability was shaken by the discovery of the contradictory but fantastic world of quantum mechanics in the 20th century. At the smallest scales of atoms and particles, true randomness plays a role – meaning that our world at the most basic level is unpredictable.
This means that the general "rules" for evolution remain the same no matter how many times we have repeated the tape. Organisms that harvest solar energy always have an evolutionary advantage. There would always be an opportunity for those who use the abundant gases in the atmosphere. And from these adjustments, we can predictably see the emergence of known ecosystems. But ultimately, the randomness built into many evolutionary processes will deprive us of the ability to look to the future with absolute certainty.
There is a problem in astronomy that functions as a fitting analogy. In the 1700s, a mathematical institute offered a prize for the solution of the "three-body problem" in which the gravitational relationship and the resulting orbits of the Sun, Earth, and Moon were accurately described.
We may not be sure where we were In the end, we need to rewind time, but the pathways available to evolving organisms are far from unlimited.
The winner essentially proved that the problem could not be solved exactly. Similar to the chaos caused by random mutations, a small startup error would inevitably increase, meaning that you can not easily determine where the three bodies will end up in the future. As the dominant partner, the Sun dictates the orbits of all three to some extent, so we can limit the possible positions of the bodies to a particular area.
This roughly resembles the leading hands of evolution, which attach themselves to familiar stretches of adaptive organisms. We may not be quite sure where we would land if we were to rewind time, but the pathways available to evolving organisms are far from unlimited. Perhaps people would never reappear, but it is likely that the foreign world replacing us is a familiar place.
This article originally appeared in The Conversation and has been released under a Creative Commons license.
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