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One of the oldest questions for mankind is how the Earth was formed. However, no one has an exact answer. First by the best estimates it occurred over 4 billion years ago before any life appeared. So there are no eyewitness accounts and other pieces of evidence.
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Scientists believe that lightning triggered chemical reactions in Earth's early atmosphere. The early atmosphere contained gases such as ammonia, methane, water vapor and carbon dioxide. The scientists created the hypothesis that these gases created a "broth" of organic molecules from inorganic chemicals.
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It begin to appear systems of interacting molecules well delimited and closed. These systems had control over the entry, retention, exit of molecules and also over internal chemical reactions. At this time, those prokaryotic cells become important because of their ability to capture energy and self-replicate
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This happens thanks to one of the oldest organism on the Earth, the stomatolites, which apperars 3500 years ago, they are colonial microorganisms that live in some waters in Australia and other places in the world, and were the first that make oxygen by photosintesis, and tranform the old atmosphere to the atmosphere of nowadays. But oxygen was but for them and most part of them day and the number of heterotrophic bacterias appears.
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For passing from prokaryotic to eukaryotic, it is explained in the endosymbiosis hypothesis, that says that there were bacteria from different sizes, one using the solar energy and the others one survived eating organic matter (other bacteria’s). This hypothesis talks about the formation of eukaryotic cells from the union of eukaryotic cells. Depending of the type of eukaryotic cells that are joining, we will have plant cells or animal cells.
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The unicellular life continued generating oxygen until reaching a certain critical level that accepted a new type of life, the multicellular, much more demanding in the requirements of the appreciated gas. Because higher multicellular animals need large amounts of oxygen to maintain constant body temperature, to maintain a higher metabolism, for the complete oxygenation of their tissues and to carry out the synthesis of collagen.
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The first invertebrates developed in the oceans. They were soft-bodied animals with a shell or carapace, such as these trilobites, in the period of the Cambrian explosion, that resulted in the divergence of most modern metazoan phyla.
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The first known vertebrate fossils date back to the early Cambrian. These early vertebrates, such as Haikouichthys, are small, tapered, streamlined animals showing eyes, a brain, pharyngeal arches, a notochord, and rudimentary vertebrae. Vertebrates appear to have radiated in the late Ordovician, about 450 million years ago. However, most Ordovician fossil vertebrates are rare and fragmentary.
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The first fossil fish are known from rare and very ambiguous fossil remains.
The 518 million-year-old Metaspriggina walcotti fish was about 6 cm long, had a pair of large, protruding eyes and small paired nasal capsules. He breathed through seven pairs of external branches. His ability to swim rapidly was, without a doubt, a key factor in his success while living in precarious seas inhabited by large predators such as Anomalocaris. -
Plants paved the way for the evolution of land animals by simultaneously increasing the percentage of oxygen in the Earth's atmosphere and decreasing the percentage of carbon dioxide, a powerful greenhouse gas.
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Amphibians were the first group of vertebrates to develop limbs and to be able to leave the water to conquer the land. Even if they are seen as simple and primitive animals by most people, amphibians show a wide diversity of survival strategies which have allowed them to occupy most terrestrial and fresh-water habitats.
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At present the living tetrapods form two groups: the amphibians and the amniotes, the latter integrate reptiles and birds; in its evolution, the amniotic egg allowed oviparous reproduction and non-dependence on the aquatic environment. Exactly how the evolution that led amphibians to reptiles took place is not completely known.
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The earliest mammals might have been small, nocturnal creatures that inhabited tropical forests. But, assuming they did exist that long ago, numerous forms could have evolved prior to the fateful cataclysm that wiped out the dinosaurs 65 million years ago.
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Scientists often agree on two hypotheses that could explain the extinction of the Cretaceous: an extraterrestrial impact, for example an asteroid or a comet, or a period of great volcanic activity. Either of the two scenarios would have shot the skies with remains that deprived the Earth of sun energy, preventing photosynthesis and extending the destruction up and down the food chain.
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70 million years ago, between different mammals, different types of monks called primates were developed. The first primates were small animals, of nocturnal habits, that lived (almost always) on the trees. Over time, some of these were changing their habits and physical characteristics: their skull was bigger, their brain grew, they could take objects with their hands, adapt to the day and feed themselves on fruit and vegetables.
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He was the first hominid to walk on his two hind legs, or rather, to walk on his two legs; clearly differentiating itself from the rest of large apes, such as the chimpanzee or the gorilla, and lived in the tropical zones of Africa for about four million years until its disappearance, about two million years ago.
