When did the evolution of life on Earth begin?
The evolution of life on Earth began with the appearance of the first
Cyanobacterial mats and archaea were dominantform of life at the beginning of the Archean period and were a huge evolutionary step of that time. Oxygen photosynthesis, which emerged about 2.5 billion years ago, eventually led to oxygenation of the atmosphere, which began about 2.4 billion years ago.
The earliest evidence of eukaryotes dates from1.8 billion years ago, although they may have appeared earlier - the diversification of eukaryotes accelerated when they began to use oxygen in their metabolism. Later, about 1.7 billion years ago, multicellular organisms began to appear with differentiated cells to perform specialized functions.
About 1.2 billion years ago, the firstalgae, and already about 450 million years ago - the first higher plants. Invertebrates appeared during the Ediacaran period, and vertebrates arose about 525 million years ago during the Cambrian explosion.
During the Permian period from large vertebratesSynapsids, the ancestors of mammals, predominated, but the events of the Permian extinction (251 million years ago) destroyed 96% of all marine species and 70% of terrestrial vertebrate species, including most synapsids.
During the recovery period after this disasterArchosaurs became the most widespread land vertebrates and replaced therapsids in the mid-Triassic. At the end of the Triassic, archosaurs gave rise to the dinosaurs that dominated during the Jurassic and Cretaceous periods.
The ancestors of mammals at that time wereare small insectivorous animals. After the Cretaceous-Paleogene extinction event, which occurred about 66 million years ago, all non-avian dinosaurs became extinct, leaving only crocodiles and birds as archosaurs.
After that, mammals quickly becameincrease in size and variety, since now almost no one competed with them. Such mass extinctions likely accelerated evolution by allowing new groups of organisms to diversify.
Fossil remains show that floweringplants appeared in the early Cretaceous period (130 million years ago) or somewhat earlier, and probably helped to evolve pollinating insects. Social insects appeared around the same time as flowering plants. Although they occupy only a small part of the insect “pedigree”, they currently account for more than half of the total.
Humans are one of the primates that started walking upright about 6 million years ago. Although the brain size of their ancestors was comparable to that of other hominids, such as chimpanzees.
The emergence of life
According to the modern concept of the RNA world,ribonucleic acid (RNA) was the first molecule to become self-replicating. Millions of years could have passed before the first such molecule appeared on Earth. But after its formation, the possibility of the emergence of life appeared on our planet.
The RNA molecule can work like an enzyme, joining free nucleotides into a complementary sequence. This is how RNA multiplies.
But these chemical compounds cannot yet be calleda living being, since they have no body boundaries. Any living organism has such boundaries. Only inside a body isolated from the external chaotic movement of particles can the most complex chemical reactions take place, allowing the creature to feed, reproduce, move, and so on.
The appearance of isolated cavities in the ocean -the phenomenon is quite common. They are formed by fatty acids (aliphatic acids) that get into the water. The thing is that one end of the molecule is hydrophilic and the other is hydrophobic.
Fatty acids trapped in the water form spheres in such a way that the hydrophobic ends of the molecules are inside the sphere. Perhaps RNA molecules began to fall into such spheres.
An isolated cavity created by phospholipid molecules
- First metabolism
The ability to reproduce and the presence of body boundaries -these are not all the signs that distinguish a living being from inanimate nature. To reproduce within a sphere of fatty acids, the RNA molecule had to adjust the metabolic process.
- First cell division
How did the first cells, consisting ofRNA molecules and fatty acid membranes are currently unknown. Perhaps the new RNA molecule built inside the membrane began to push away from the first one.
In the end, one of them broke through the membrane. Along with the RNA molecule, part of the fatty acid molecules left, which formed a new sphere around it.
Precambrian or cryptose
The Precambrian lasted almost 4 billion years.During this period of time, significant changes took place on Earth: the crust cooled down, oceans appeared and, most importantly, primitive life appeared. However, traces of this life in the fossil record are rare, since the first organisms were small and did not have hard shells.
The Precambrian accounts for most of the Earth's geological history—about 3.8 billion years. Moreover, its chronology is much less developed than that of the Phanerozoic that followed it.
The reason for this is that organic residues inPrecambrian deposits are extremely rare, which is one of the distinctive features of these ancient geological formations. Therefore, the paleontological method of study for the Precambrian strata is inapplicable.
- Catarchean eon (4.54 - 4.0 billion years ago)
Research of meteorites, rocks and othersmaterials from that time show that our planet was formed about 4.54 billion years ago. Until that time, there was only a blurry disk around the Sun, consisting of gas and cosmic dust. Then, under the influence of gravity, the dust began to collect into small bodies, which eventually turned into planets.
For many millions of years on Earth there was nothere were no life forms. After the Archean episode of melting of the upper mantle and its overheating with the emergence of a magma ocean in this geosphere, the entire pristine surface of the Earth, together with its primary and initially dense lithosphere, very quickly sank into the melts of the upper mantle.
The atmosphere at that time was not dense and consisted offrom poisonous gases such as ammonia (NH3), methane (CH4), hydrogen (H2), chlorine (Cl2), sulfur. Its temperature reached 80 ° С. Natural radioactivity was many times higher than the current one. Life in such conditions was impossible.
4.533 billion years ago Earth is supposedcollided with a celestial body the size of Mars, the hypothetical planet Theia. The collision was so strong that the debris from the collision was thrown into space and formed the moon.
The formation of the Moon contributed to the emergence of life: it caused tides, which helped cleanse and aerate the seas, and stabilized the Earth's rotation axis.
Catarchean eon, 4.54-4 billion years ago,known as the protoplanetary stage in the development of the Earth. Covers the first half of the Cryptozoic. The earth at that time was a cold body with a rarefied atmosphere and no hydrosphere. In such conditions, no life could appear.
During the katarchei, the atmosphere was not dense. It consisted of gases and water vapor that appeared when the Earth collided with asteroids.
Due to the fact that the Moon was tooclose (only 170 thousand km) to the Earth (the length of the equator is 40 thousand km), the day did not last long - only 6 hours. But, as the moon moved away, the day began to increase.
- Archean Eon (4.0 - 2.5 billion years ago)
The first chemical traces of life are about3.5 billion years have been discovered in the rocks of Australia (Pilbara). Organic carbon was later discovered in rocks dating back 4.1 billion years. Perhaps life originated precisely in hot springs, where there were many nutrients, including nucleotides.
Life in the Archean evolved to bacteria and cyanobacteria. They led a near-bottom lifestyle: they covered the bottom of the sea with a thin layer of mucus.
Eoarcheus:
It lasted 4-3.6 billion years ago. Prokaryotes may have appeared already at the end of the Eoarchean. In addition, the oldest geological rocks - the Isua Formation in Greenland - belong to the Eoarchean.
Paleoarchean:
Paleoarchean lasted from 3.6 to 3.2 billion years ago. The oldest form of life dating back to this era is found in Australia - well-preserved remains of bacteria aged 3.46 billion years.
Mesoarchean:
Stromatolite of the Mesoarchean period
Mesoarchean lasted 3.2-2.8 billion years ago. Stromatolites are already found in the Mesoarchean.
Neoarchean:
Neoarchean lasted 2.8-2.5 billion years ago.During this period, oxygen photosynthesis appeared, which caused the oxygen catastrophe that occurred in the Paleoproterozoic. During this period, bacteria and algae actively develop.
What was the progenitor of living organisms?
Scientists at Nagoya University in Japan believe that before the emergence of the first living cell, there was a pre-RNA world based on xenonucleic acids (XNA).
Unlike RNA strands, XNA replication and assembly do not require enzymes. Xenonucleic acid chains are stable enough to carry genetic information.
They are also able to bind to proteins and have enzymatic functions like ribozymes (as scientists call ribonucleic acids that can catalyze biochemical reactions).
The scientists synthesized fragments of the aliphatic (non-cycle) nucleic acid L-threoninol (L-aTNA), which is believed to have existed before the advent of RNA.
They also made a longer L-aTNA chain,which was complementary to the original sequence of fragments, just as two DNA strands complementary to each other create a double helix.
In a test tube under controlled conditions more thanshort L-aTNA fragments come together and bind to each other on the longer L-threoninol chain. This happened in the presence of a compound called N-cyanoimidazole and a metal ion such as manganese, both of which were most likely present on early Earth.
Fragments of L-aTNA could also bind to DNA and RNA. This suggests that the genetic code can be transferred from DNA and RNA to L-aTNA and vice versa.
According to the scientists, the results of the study will help future developments to create artificial life and highly functional biotechnology tools, consisting of acyclic XNAs.
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