We have so far analyzed the emergence of the first living forms, and you may have noticed that we have already mentioned, for these forms, some important characteristics for conceptualizing a living being. These early organisms have organic compounds in the constitution of their bodies, are cellular (unicellular, in this case) and have reproductive capacity.
We have not discussed yet another feature of living things: the metabolism. Let us then look at what the likely evolution of metabolic pathways must have been in living things.
Every living being needs food, which is degraded in metabolic processes for the release of energy and the fulfillment of functions. These degraded foods can also be used as raw materials in the synthesis of other organic substances, enabling growth and loss replacement.
Let us then consider how these early beings were able to obtain and degrade food for their survival. Two hypotheses have been discussed by scientists: the heterotrophic hypothesis and the autotrophic.
According to this hypothesis, the first organisms were structurally very simple, and it can be assumed that the chemical reactions in their cells were also simple. They lived in an aquatic environment, rich in nutritious substances, but there was probably no oxygen in the atmosphere or dissolved in seawater. Under these conditions, it is possible to assume that, having abundant food around them, these first beings would have used this already prone food as a source of energy and raw material. They would therefore be heterotrophs (hetero = different, trophies = food): organisms that are unable to synthesize their own foods from inorganic compounds, getting them ready from the environment.
Beings capable of synthesizing their own foods from simple inorganic substances are called autotrophs (self = own, trophies = food), as is the case with plants.
Once inside the cell, this food needs to be degraded. In today's Earth conditions, the simplest metabolic pathway to degrade oxygen-free food is fermentation, an anaerobic process (an = without, aero= air, bio = life). One of the most common types of fermentation is alcoholic fermentation. Glucose sugar is degraded into ethyl alcohol (ethanol) and carbon dioxide, releasing energy for the various stages of cellular metabolism.
These organisms began to increase in number per reproduction. At the same time, the Earth's weather conditions were also changing to the point where no prebiotic synthesis of organic matter occurred. Thus, the food dissolved in the middle would have started to become scarce.
With reduced food and large numbers of individuals in the seas, there must have been much competition, and many organisms would have died from lack of food. At the same time, CO would have accumulated2 in the environment. It is believed that in this new scenario there would have been the emergence of some beings capable of capturing sunlight with the help of pigments such as chlorophyll. Light energy would have been used to synthesize their own organic foods from water and carbon dioxide. Thus would have arisen the first autotrophs: photosynthesizing beings (Photograph = light; synthesis in the presence of light), which did not compete with the heterotrophs and proliferated greatly.
These early photosynthesizing beings were instrumental in modifying the composition of the atmosphere: they introduced oxygen into the air, and the atmosphere would have gone from reducing to oxidizing. To this day, it is primarily photosynthesizing beings that kill the oxygen levels in the atmosphere, which is critical to life on our planet. Under conditions of low availability of organic molecules in the medium, these aerobic organisms would have great advantage over fermenters.
With the availability of oxygen, it was possible to survive beings that developed complex metabolic reactions, capable of using this gas in the degradation of food. Then came the first aerobic beings, who perform the breath. Through respiration, food, especially glucose sugar, is degraded to carbon dioxide and water, releasing much more energy to perform vital functions than in fermentation.
Fermentation, photosynthesis, and respiration have remained over time and occur in organisms currently living on Earth. All organisms breathe and / or ferment, but only a few breathe and photosynthesize.
|Fermentation -> Photosynthesis -> Breathing|