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

What would happen if the hands of time were returned to an arbitrary point in our evolutionary history and we restart the clock? American paleontologist Stephen Jay Gould proposed this famous thought experiment at the end of the 1980s – an experience that continues to surprise the imagination of evolutionary biologists.

Gould thought that if time went up, then evolution would lead life on a completely different path and humans would never revolve. In fact, he felt that the evolution of humanity was so rare that we could replay the cassette of life a million times and we would not see anything like it. Homo sapiens to introduce yourself again.

His reasoning was that fortuitous events play a huge role in evolution. These include massive mammoth extinction events – such as cataclysmic impacts of asteroids and volcanic eruptions. But fortuitous events also operate at the molecular level. Genetic mutation, which is at the root of evolutionary adaptation, is based on fortuitous events.

In simple terms, the evolution is the product of a random mutation. A few rare mutations can improve the chances of survival of one organism compared to others. The division of a species into two parts of rare mutations, which become common over time. But other random processes can still interfere, potentially leading to a loss of beneficial mutations and an increase in harmful mutations over time. This built-in randomness should prompt you to choose different forms of life if you replay the tape of life.

Of course, in reality, it is impossible to go back in this way. We will never know for sure how likely it was at this time – for us to have written this article and for you to read it. Fortunately, however, biologists of experimental evolution have the means to test some of Gould 's theories at the microscopic scale with bacteria.

Microorganisms divide and evolve very rapidly. We can freeze billions of identical cells over time and store them indefinitely. This allows us to take a subset of these cells, challenge them to grow into new environments, and monitor their adaptive changes in real time. We can move from "present" to "future" and vice versa as many times as we wish – essentially by replaying the tape of life in a test tube.

Evidence of the fate of evolution

Many studies on the evolution of bacteria have shown, perhaps surprisingly, that evolution often follows very predictable short-term pathways, with the same traits and genetic solutions often achieved. Take, for example, a long-term experiment in which twelve independent populations of Escherichia coli founded by a single clone, has been evolving continuously since 1988. It is more than 65,000 generations – there are only 7,500 to 10,000 generations since the beginning of the Homo sapiens appeared. All evolving populations participating in this experiment show better fitness, faster growth, and larger cells than their ancestors. This suggests that organizations have constraints on how they can evolve.

There are evolving forces that keep organisms evolving on the right path. Natural selection is the "guiding hand" of evolution, reigning in the chaos of random mutations and encouraging beneficial mutations. This means that many genetic changes will disappear over time, with only the best being sustainable. This can also lead to the realization of the same survival solutions in species totally independent of each other.

We find evidence of this in the history of evolution where species that are not closely related, but share similar environments, develop a similar trait. For example, pterosaurs and extinct birds both have evolved wings as well as a distinct bill, but not that of a recent common ancestor. So basically the wings and beaks evolved twice, in parallel, because of the pressures of evolution.

But genetic architecture is also important. Not all genes are created equal: some occupy very important jobs compared to others. Genes are often organized in networks, comparable to circuits, with redundant switches and "master switches". The mutations in the "main switches" naturally lead to much larger changes, because of the training effect that all the genes under its control feel. This means that some genome sites will contribute to the evolution more frequently, or with a greater effect, than others – biasing the evolutionary results.

Physical laws

But what about the underlying physical laws – do they favor a predictable evolution? On a very large scale, it seems so. We know many laws in force in our universe that are certain. Gravity, for example – for which we owe our oceans, our thick atmosphere and nuclear fusion to the sun that feeds us with energy – is a predictable force. Isaac Newton's theories, based on large-scale deterministic forces, can also be used to describe many large-scale systems. These describe the universe as perfectly predictable.

If Newton's point of view were to remain perfectly true, the evolution of the human being was inevitable. However, this reassuring predictability was shattered by the discovery of the contradictory but fantastic world of quantum mechanics in the 20th century. At the smallest scales of atoms and particles, real chance is at stake – which means that our world is unpredictable at the most basic level.

This means that the big "rules" of evolution would remain the same regardless of the number of times we have replayed the band. There would always be an evolutionary advantage for organizations that exploit solar energy. There would always be an opportunity for those who use abundant gases in the atmosphere. And from these adaptations, we can predict the emergence of familiar ecosystems. But ultimately, randomness, which is embedded in many evolutionary processes, will take away our ability to "see in the future" with absolute certainty.

There is a problem in astronomy that is an appropriate analogy. In the 1700s, an institute of mathematics proposed a prize for the resolution of the "three-body problem," consisting in accurately describing the gravitational relation and the resulting orbits of the sun, the Earth, and the moon.

The winner, Joseph-Louis Lagrange, essentially proved that the problem could not be solved exactly. Just like the chaos introduced by random mutations, a small number of departure errors would inevitably increase, which means you could not easily determine where the three bodies would end up in the future. But as a dominant partner, the sun dictates the orbits of the three to a certain extent, which allows us to reduce the possible positions of the bodies in a range.

It looks a lot like the guiding hands of evolution, which adapt organisms to known routes. We may not know exactly where we will end up if we go back in time, but the avenues available to evolving organizations are far from unlimited. And maybe humans will never reappear, but it's likely that everything the extraterrestrial world replaces ours would be a familiar place.