origin of the first molecules
3 billion and 800 million years ago
Essential characteristics of living beings
Everything that surrounds us can be divided into two groups: living beings (or organisms) and non-living beings (or anorganisms).
Living beings are made up of innumerable parts arranged to form a single organism, called organs, capable of performing some functions that together are the basis of life, which, in turn, are formed by smaller bodies of dimensions microscopic, called cells.
Just as Lavoisier had guessed and substained, each living organism is a system which, in order to remain as such, requires a continuous exchange of matter and energy.
This occurs through the action of chemically active organic compounds: enzymes.
The latter, however, in order to maintain their functions, need a continuous supply of energy, produced by exploiting the solar radiations that reach the Earth or, the chemical reactions of the elements that make up the environment.
Furthermore, living beings react to the stimuli of the surrounding environment and are able to reproduce, generating other living beings similar to them.
Non-living beings, also called minerals, do not possess any of these characteristics.
Energy production - Autotrophic life
In general, as regards nutrition and, therefore, the production of energy, living beings are divided into:
- autotrophic organisms that regenerate by producing radiant energy, obtained with holophytic nutrition, that is, assuming gaseous or inorganic substances in solution;
- heterotrophic organisms which produce energy with holozoic nutrition, that is, assuming living beings: plants, animals, bacteria.
Since plants are builders of living matter, while animals are consumers of it, the former must be of a much greater quantity.
Historical theories
The most important theory of Western civilization on the origin of life on Earth, was that of the creation described by the Bible which was opposed, since the time of Aristotle, that of the spontaneous generation of living beings.
First organic molecules (3 billion and 800 million years ago)
About 3.8 billion years ago an intense volcanic activity began with very intense orogenetic phenomena, rains of incandescent materials that gave rise to the first mountain ranges made up of green rocks, which gave rise to the Huronian mountain ridge.
The hot humid climate due to the water vapor coming from volcanic activity is responsible for the formation of the primordial broth, underlying an atmosphere almost devoid of oxygen and without carbon sources (originating from methane), but very rich in hydrogen and nitrogen (in the form of ammonia).
The first organic molecules originated in the first ocean that surrounded the only primordial continent (called Pangea) due to the action of solar radiation, charged with ultraviolet rays not yet filtered by ozone and the electrical discharges of lightning.
The latter, combining with each other, gave rise to nucleid acids and amino acids which, in turn, joining together, gave life to ribonucleic acid (Rna), a component present in all forms of life known and capable, for the first time on earth, to reproduce.
Later, deoxyribonucleic acid was also formed, which still constitutes the genetic material of all life forms on Earth (with the exception of viruses), marking the transition from inanimate matter to the first biological molecule: a primitive protein capable of to reproduce and evolve to the point of originating cells capable of leading autotrophic life.
The temperature of the Earth, in the meantime, had dropped below 100 degrees centigrade for which the condensation of water vapor and the formation of the hydrosphere was favored, which ended up enveloping the whole planet.
First living beings - Autotrophic bacteria (3 billion and 800 million years ago)
Due to the metamorphosis undergone by the rocks, which have erased any organic residue, the fossil remains of the first living beings are practically nil.
Nevertheless, it is now the opinion of all paleontologists that the first organisms to have populated the waters and muds of the only ocean were tiny plants, cells in the free state which, 3 billion and 800 million years ago, exploited the chemical substances present in water: nitrogen, calcium, carbon (essential for the life of all living beings), iron, phosphorus, magnesium, potassium for the production of vital substances.
The absence or, almost, of oxygen on Earth, for the entire Archean period, also authorizes us to suppose that the first Bacteria, related to the current Cyanobacteria, to colonize our planet, were anaerobes, able to live, that is, without oxygen.
The development of the first (unicellular) organisms was due to their extraordinary ability, (still active in living species), to modify the mechanisms active in the production of vital substances.
Evidence of ancestral life forms can be recognized in calcareous and ferrous formations: Autotrophic bacteria capable of metabolizing iron and calcium salts.
These unicellular organisms, with sedimentary structures of organic origin, enclosed layers of filamentous bacteria, subsequently able, through photosynthesis, to produce energy directly from the sun's rays.
Some blue-green microorganisms, better known with the name of Blue Algae (fossils found in Bitter Springs - Australia) capable of fixing carbon dioxide in sugars, initially had a length of a few millimeters but, subsequently, assumed increasingly complex and branched forms.
Protists (Neither vegetable nor animal)
350 years before the birth of Christ, Theophrastus (372-287 BC) was the first to divide living beings into 2 kingdoms: the Animal and the Vegetable.
But at the end of the 19th century, Haeckel (1834-1919), after having demonstrated the affinities between the Protophytes, the simpler Vegetables, and the Protozoa, the simpler Animals, proposed the introduction of a new kingdom called Protists, to classify living species whose characters are not really Vegetables or Animals.
In fact, the hypothesis according to which both the vegetable world and the animal world originated from a common ancestor is now well established.
Some organisms, the Flagellates, (subphylum Mastigophora or Flagellata) for example, although traditionally classified in the Animal Kingdom among the Protozoa, (phylum Protozoa), denounce the improper classification.
Some species pass from autotrophic to saprophytic nutrition and vice versa, depending on the characteristics of the surrounding environment.
Only among the multicellular beings, Eufite, (phylum Euphyta-Vegetali) and Metazoi, (sub-kingdom Metazoa-Animals) are the differences clearly identifiable.
The first animals - Protozoa (1 - 1.5 billion years ago)
The separation of animal living beings from plant ones took place with an evolutionary process that began 1 - 1.5 billion years ago.
This process can only have been slow, so that in a certain moment of the evolution of living organisms, forms of life with characteristics that are neither completely vegetable nor completely animal have evolved: the Protists.
Of this process we still have evidence today in the Protists, distinct from Animals and Vegetables.
The first animals to evolve were the Protozoa, (from the Greek protos = first and zoon = animal), single-celled organisms capable of leading an isolated life or of gathering in colonies. On their evolution, due to the absence of fossil evidence prior to the Cambrian period, the evolution that differentiated the species has not yet been clarified so, although the findings dating back to this period represent almost all the currently known phylums, scholars still divide into various theories, each of which is object of objections.
Early soft-bodied invertebrates (Segmented worms - 630 million years ago)
However, it took a very long time before evolution gave rise to forms of animal life, albeit in primitive forms.
As they were unable to carry out the photosynthesis that allows them to produce the energy necessary for life directly from the sun's rays, the first forms of animal life appeared only after the spread of protists.
Only towards the end of the Proterozoic period, about 630 million years ago, were the waters populated with forms of real animal life. They were invertebrate beings, that is, without a skeleton, be it cartilaginous or bone, some of which were similar to earthworms capable of digging into the sandy bottom of the sea.
First invertebrates with exoskeleton (Trilobites - 570 million years ago)
The oldest invertebrates with an exoskeleton date back to about 570 million years ago. Many of them were similar to crustaceans, others, however, lacked motor skills or enclosed in shells.
Some of them, such as Extinct Trilobite Fossil - Enter to Buy Fossils Online All Calimenids, had the ability to roll up on themselves.
Very widespread and numerous are the Trilobites of which there are testimonies even in Sardinia dating back to about 570 million years ago.
Most of the Trilobites were of considerable size, but there were also some small and simple ones.
The Trilobites were formed by an external skeleton divided into three parts: the head, the thorax and the tail.
The thorax and tail were made up of segments, each of which had legs for walking and gills for breathing. The eyes, well developed, were placed above the head.
During development, when the external skeleton became too narrow it was abandoned by the animal that provided for its reconstitution as evidenced by the countless fossil remains. The trilobites fed on organic particles found on the seabed on which they moved, sometimes leaving fossil traces in the Cambrian and Ordovician rocks.
In their evolution, the Trilobites transformed into sightless animals capable of digging in the sand or developed enormous eyes and spiked protuberances some of which, arising from the cephalic shield, projected forward.
Their extinction took place definitively at the end of the PALEOZOIC to give way to other Arthropods such as Crabs, Shrimps, ...
