How did life come about on Planet Earth? The origin of life is something that intrigues many people to this day. Since the 15th century, scientists like Francesco Redi (1668), John Needham (1745), Lazzaro Spallanzani (1768), Louis Pasteur (1862), among others, have tried in various ways to explain how living beings appeared, how life appeared on Earth … Did it happen “out of nowhere”? Or does it have an explanation in the light of the science behind it all? Check below certain concepts related to these theories and which were defended by leading scientists who made history on the subject.
The Greek abiogenesis word “a-bio-genesis”, which means non-biological origin, was initially used for many years to talk about spontaneous origin (also called spontaneous abiogenesis, or Aristotelian abiogenesis, since Aristotle was one of the defenders of this theory ).
It was assumed that life arose spontaneously from decomposing materials, such as rotten meat and dirty clothes. For scientists who believed in this theory, it was enough to accumulate dirty clothes or rotten food that in a few days life would arise. At that time, little was known about scientific methods, and many variables were not controlled, which interfered with the reliability of the experiments carried out.
From the emergence of the theory of biogenesis, that of spontaneous abiogenesis was for some time discredited, however with the appearance and improvement of the microscope, and in 1683 with the discovery of microorganisms, the theory of spontaneous abiogenesis returned to gain strength, since the from then on, bacteria and other organisms present in the decomposing materials that were part of the experiments could be observed, even if they were covered with gauze or closed.
In 1745, John Needham carried out an experiment which reinforced the hypothesis of spontaneous abiogenesis. He heated nutrient liquids together with food particles in test tubes, closed them to prevent air from entering with new microorganisms and heated them again.
John Needham | Image: Wikimedia Commons
After several days, a huge amount of microorganisms appeared inside these tubes, which led him to conclude that the beings that appeared after the boil were purely and simply due to a “vital principle” present in the nutrient solution, which gave rise to the life of a non-biological form.
The problem is that as previously mentioned and as we know today, some variables can hinder the experiments. And science is like that: the hypotheses tested by one or more scientists need to be replicated, and validated, so the theories are accepted or rejected.
An example of how science works occurred in 1768, when Lazzaro Spallanzani, in order to test Needham’s findings, boiled some closed vials with nourishing liquid for an hour, and after a few days he realized that there was no sign of life inside. bottles.
Lazzaro Spallanzani | Image: Wikimedia Commons
With this, he demonstrated that the microorganisms that were born inside the tubes of Needham, in fact arose due to insufficient time of boiling of the tubes, that is, Needham did not boil his tubes long enough for the high temperature to cause the death of all microorganisms present in the air and in the solution inside the tube and then they started to multiply inside the tube over the days.
However, this was not enough to completely rule out the hypothesis of spontaneous abiogenesis, Needham further criticized Spallanzani’s findings and suggested that by heating liquids at high temperatures for a prolonged period, the “vital principle” could be destroyed or weakened and this did not appear new microorganisms. Despite this hypothesis, at the time, still being accepted by the population, Spallanzani’s experiments served as a basis for Louis Pasteur’s findings (see throughout this text).
Currently, the word “abiogenesis” has been used to talk about chemical origin (or chemical abiogenesis), also known as biopoesis, chemical evolution or chemosynthesis. Many contemporary scientists argue that chemical abiogenesis occurred only once, some 4.4 billion years ago, and gave rise to what we now call life.
According to this theory, an ancestral cell to all living beings with the ability to reproduce itself, originated from abiotic matter and as time passed, through evolution, gave rise to all the biological diversity that we have on Earth.
The idea is that chemical abiogenesis occurred under different conditions than we had in the 15th century, or even today, and that it occurred in a much longer period of time than the times of experiments carried out by scientists in ancient times. In addition, contrary to the concept of spontaneous abiogenesis, chemical abiogenesis is not about the spontaneous origin of complex life forms (such as flies, rats …) but rather the simple origin of life, the most unique one can imagine.
Biogenesis of the Greek “bio-genesis” means biological origin, that is, origin of life from another life. In 1668, Francesco Redi was one of the first scientists to oppose the concept of spontaneous abiogenesis theory and then defend the theory of biogenesis.
Francesco Redi | Image: Wikimedia Commons
He noticed that larvae (what they called worms at the time) appeared in places with decaying organic matter, often visited by flies. Then, in order to test the hypothesis that these worms originated from eggs of adult flies, he placed meat and other organic matter in eight glass jars, some covered with gauze and others open, without gauze.
He realized that after a few days, the larvae appeared only in open pots. With that, he concluded that the idea that it was enough to have rotten material to originate life was not valid, because if that were true, flies would appear in both closed and open pots, which in fact did not happen.
But Louis Pasteur’s experiments carried out in 1862 represented a watershed. It was at this time and due to Pasteur that spontaneous abiogenesis was refuted both in the microscopic and macroscopic world.
Louis Pasteur | Image: Wikimedia Commons
Contrary to Needham’s argument (who had claimed that boiling for a long time and at high temperatures could have destroyed the vital principle contained in the nutritive liquid), Pasteur devised an experiment using a glassware called the “swan neck” (named so, due to its shape, which looks like a swan neck).
Experiment performed by Pasteur | Illustration: Reproduction / Site Only Biology
This glassware kept the liquid sterile, since the boil killed the microorganisms present in the liquid, and the contaminated air passed through a “filter” formed by the droplets of water located in the neck of the flask while cooling. When the glassware “neck” was broken, the microorganisms returned to colonize the liquid. In this way, he proved that boiling did not incapacitate the solution of maintaining life, it was enough to provide the contact between microorganisms and liquid again.