Post by Amanda Bendia
English edit by Katy Shoemaker
Part IV: The dawn of life: How the first living organisms on Earth came to be
We know today that the first living organisms were probably very similar to modern prokaryotic microorganisms, such as bacteria. Prokaryotes are unicellular organisms with a relatively simple cell structure: their genetic material is contained within the cytoplasm, without a nuclear envelope or membrane-bound organelles, such as mitochondria or chloroplasts. Despite this, they are able to perform various metabolic functions including carrying out biogeochemical cycling essential to the planet. Bacteria are found in virtually every environment on Earth, including extreme environments including on ice, in areas of high atmospheric pressure, and around geothermal activity. Fittingly, these are called extremophiles.
Many scientists propose that the first organisms on the planet inhabited hydrothermal vents in the deep ocean. These extremophiles adapted to high temperatures and are called thermophiles. Molecular studies of modern thermophiles indicate that these organisms have long branches in phylogenetic trees, evidence of their emergence early in Earth’s history.
In the fossil record, we have evidence of living organisms dated to approximately 3.5 billion years ago. These microfossils found in Australia have a cell structure very similar to modern prokaryotes. It is likely that these organisms respired sulfur instead of oxygen, since the abundance of oxygen in the atmosphere is a recent advance in Earth's history, forming about 2.4 billion years ago. This “Great Oxygenation” of the planet was due to a group of photosynthetic prokaryotes called cyanobacteria which released oxygen into the atmosphere as a by-product of photosynthesis.
Fossil structures dating back to Earth's earliest
organisms, approximately 3.5 billion years ago.
Oxygen can be extremely toxic to cells, so the organisms that did not have the necessary machinery to metabolize it went extinct. Some prokaryotes that were unable to metabolize oxygen adopted a strategy that would change the course of all life diversity on Earth: they engulfed within their cell another bacterium capable of processing the oxygen toxic to them. In return, the larger cell offered protection and reduced metabolic needs for the encapsulated bacterium.
This revolutionary event in the evolutionary history of life, called endosymbiosis, gave rise to the first eukaryotic organisms on the planet. These hosts encompassed different types of bacteria, giving rise to eukaryotic organelles called chloroplasts and mitochondria.
Chloroplasts were probably primitive cyanobacteria that were encapsulated by the host, while mitochondria were likely derived from bacteria similar to what we classify today as alphaproteobacteria. There is much evidence to support the theory of endosymbiosis, including the fact that chloroplasts and mitochondria have circular genetic material similar to the structure found in bacteria and archaea.
When the DNA of chloroplasts and mitochondria are analyzed, they present many similarities with the genomes of cyanobacteria and alphaproteobacteria, respectively. In addition, division of these organelles occurs independently of the cell and in a similar way as prokaryotic organisms, through binary fission.
The emergence of these eukaryotic cells allowed for a greater genetic and structural complexity of the cell, enabling the diversity of life we see today. If it were not for this revolutionary event in the history of life a few billion years ago, we would not be here to discuss these issues today.
Part V: Astrobiology: Are we alone in the universe?
One of the questions that has intrigued humanity since the beginning of civilization is: did life arise only once on the planet, or were there multiple origins of life? To answer this question we would need to go back more than 3.5 billion years ago, and since this is not possible, we need to turn to the philosophy of science.
There are basically two opposite currents of thought on this subject: contingency and determinism. The contingency hypothesis suggests that, because of the highly specific and rare conditions that propagated a set of chemical molecules to form a living being, life on Earth could only have arisen once.
Determinists, on the other hand, say that these conditions are not that rare, and as chemical and physical laws probably ruled the emergence of life, its appearance would be inevitable. Determinism indicates the origin of life is a plural event: it may have occurred several times on our planet and also beyond it, in other bodies of the solar system and other planetary systems.
Considering that we have already detected billions of stars in billions of galaxies, it seems reasonable to imagine that somewhere in the Universe the necessary conditions arose and life similar to that as we know it, could have also originated. The branch of science that studies this possibility is called Astrobiology. With the modern multidisciplinary tools of Astronomy, Biology, Physics, Chemistry and Engineering, we are always getting closer and closer to discovering if there is any kind of life outside of our Earth.
Questions about the origin of life have been discussed since the earliest days of mankind, and the mystery surrounding it has intrigued the skeptical to the religious. Religion has played a key role in early civilization, as man first started discussing where we came from, who we are, and how life came about.
As our ancestors did not have modern scientific tools, religious and philosophical thinking was essential to the evolution of their knowledge. The knowledge built through philosophical discussion is the foundation of the current scientific and technological tools we develop today, which are enabling us to be ever closer to unraveling the great mystery.
It is important to emphasize that philosophical thought is still fundamental to science. It makes us break paradigms and face the barriers of knowledge. Our ability to think through philosophy and science and produce technology is one of the characteristics that sets us apart from other organisms. We are increasingly advancing our knowledge of the mysteries of the origin of life, yet we still always ask ourselves if we will ever unravel it.
It is difficult to answer this question, since we will never be able to reproduce all of the exact conditions that were present billions of years ago. Whether this mystery is unraveled or not, we will always remain fascinated by the notion that some atoms produced after the Big Bang combined billions of years later to form life capable of questioning its own existence.