Link to the book:
https://www.abc.org.br/wp-content/uploads/2025/03/ABC_Evolucao_redux.pdf
A book produced by ABC based on the work of a multidisciplinary group of specialists aims to compile everything that is most current in different areas of science and that allows us to tell a solid and unequivocal story about the occurrence of the biological evolution of life on Earth. This work reflects ABC’s commitment to promoting and disseminating quality scientific knowledge to society. The theory of evolution is one of the most robust and fascinating pillars of modern science.
A journey from the dawn of life on Earth
The book begins even before life emerges and, therefore, has geologists among its authors. Among them is Professor Umberto Cordani, who brings us to the Hadean period, between 4.5 and 4 billion years ago, when the Earth was still an uninhabitable fireball. It was only after a great cooling, which led to the formation of the first oceans, that life became possible. This happened in the following period, which we call the Archean, when the first evidence of organic carbon and the first bacteria began to appear.
The origin of life and the “RNA world”
But how exactly did life come about? To begin to answer this question, we need to look at the three fundamental molecules of life: DNA, RNA, and proteins. All three are carbon chains made up of small “blocks” that repeat themselves. In the case of DNA and RNA, they are nucleotides, and in the case of proteins, amino acids. In most living things today, hereditary information – that which can be transmitted in reproduction – is stored in DNA, but this molecule alone is not capable of forming an organism. To do this, the information contained in DNA is transcribed into RNA and then translated into proteins, which are capable of performing the functions of our body.
This is only possible thanks to what we call the genetic code, whose discovery was a milestone in the history of biology. In short, each sequence of three nucleotides will always correspond to the same amino acid, allowing it to be possible to look at a DNA or RNA molecule and know exactly which protein it will produce. This code is shared by all living things and is proof that we are all descended from a single common ancestor.
Therefore, at one end we have DNA, which is capable of replicating, but not of forming an organism, and on the other we have proteins, which form organisms and catalyze metabolic processes, but do not replicate on their own. And in the middle we have RNA, which is also capable of replicating and catalyzing metabolic functions – such as ribosomes, the “machines” responsible for translating RNA into a protein, themselves made of RNA.
Because of this, it is believed that the origin of life resides in RNA. According to this theory, RNA could be the first “living” molecule because it can carry information and perform catalysis. So, the idea of the ‘RNA World’ was proposed, a world where cells would have the RNA genetic material. We don’t have this world today, only a few viruses have RNA genetic material, but our cells carry remnants of this world.
Photosynthesis, eukaryotes and the absorption of bacteria
Once this first form of life was established, it would soon encapsulate itself and begin to develop the first metabolic activities, giving rise to the first cells without nucleus – prokaryotes – not very different from the bacteria we know today. For about two billion years, these organisms inhabited the primordial oceans and evolved.
During this period, one of the most notable developments was the emergence of photosynthesis, whereby some bacteria, called cyanobacteria, began to use sunlight to produce energy, releasing oxygen in the process. As this new gas accumulated in the atmosphere, many unadapted bacteria eventually became extinct, but some had enough to use oxygen to generate energy, in the first forms of aerobic respiration.
Then, around 1.9 billion years ago, a new type of cells emerged, larger and divided into “compartments,” including a nucleus for DNA. They were the first eukaryotic cells. One of the first innovations of eukaryotes was the absorption of photosynthesizing and aerobic bacteria, the ancestors of plasmids and mitochondria, respectively. These two cell organelles became responsible for performing these same functions within their new hosts. Throughout this process, a part of the genome of these bacteria migrated to the nucleus, in what we call horizontal transfer. Eukaryotic cells are the result of the fusion of different organisms. We are part bacteria!
Another innovation was pluricellularity, the first evidence of which appears about 1.4 billion years ago. For the first time, a single organism was formed by more than one cell working together. From then on, the stage was set for a true revolution of life on Earth.
The Cambrian Explosion, the Fossil Record, and the Warning of Mass Extinctions
One of the advantages of geology is its power to unravel deep time. Some evidence of primitive life forms is striking billions of years ago, such as stromatolites, characteristic formations of cyanobacteria. These and other types of microfossils dominated the geological record until about 500 million years ago, when, in the period immediately preceding what we call the Cambrian, a profusion of multicellular fossils began to appear.
Many of these organisms are unlike anything we know today, showing that there was a phase of intense “evolutionary experimentation” before the large groups of animals and plants became established. But even after that period, life on Earth never followed a linear trajectory. Another thing that the fossil record shows us are the major mass extinction events, five in all, including the one that put an end to the dinosaurs, which radically altered biodiversity and made room for new groups to diversify.
We Arrive at the Human Being
Just five million years ago, a blink in the evolution of life on Earth, a group of primates diverged from their common ancestor with chimpanzees, walked on two feet, and, after leaving a vast number of raw species behind, became Homo sapiens, the modern human being.
Despite being just a breath in the history of life, human evolution is the most sensitive point in the debate. Even Charles Darwin did not have the courage to address it in his classic The Origin of Species, and did so only in his later book, The Descent of Man, for which he was harassed. Alfred Russel Wallace himself, who shares with Darwin the credit of having first described natural selection, died without accepting the obvious inferences that the theory brought to our own species.
But the conclusion is inescapable. Whether in the traits we share with other primates, or in the fact that we share 98.5% of DNA with chimpanzees, scientific evidence leaves no doubt. The human being is an animal like any other, the product of a process of evolution by natural selection that has been taking place for 4 billion years.
