The Big Bang Theory is the most accepted theory regarding the origin of life on earth and different forms of life in it. According to the theory, the universe is a result of a huge explosion, which occurred 20 billion years ago. It took another 10 billion years for the origin of the earth. Once earth originated, it grew, and the living conditions like the temperature and atmosphere stabilized. It is believed that the first life form came into existence on earth around 2000 million years ago (mya).
Even though there is no clear picture of the evolution of the first living cell, it is assumed that the first cell originated from non-cellular molecules. This cell is divided to form more of them. The evolving cells had few characteristic features which helped them to survive. These features include the ability to trap light energy and derive energy from it and the oxygen releasing capacity of the cells. Finally, single unicellular organisms evolved to form much more complex multicellular organisms.
Once the multicellular organisms evolved, evolution happened at a much faster pace. The further series of development and evolution of new organisms like invertebrates, plants, animals such as reptiles, fish, amphibians, mammals, etc., took place within a time range of 500-300 million years ago (mya). Later these organisms started to evolve in their own way. This commenced the branching of lineages. Environmental conditions like climatic changes, limited resources, predators, etc. played an enormous role in that. During each phase of evolution, one life form dominated over the other. Nature selected one over the other, which was called the fittest of all.
The flow of evolution of animals began with jawless fish (350 mya) before plants (320 mya) invaded the lands. Later, fish which could live on both land and water replaced plants, they fed on them. Amphibians, which were derived from these lobefin animals later evolved into reptiles. Reptiles of different shapes, sizes, and life patterns flourished during that many mya. Certain ones lived in water, e.g., Ichthyosaurs (200 mya), others lived on land. Dinosaurs were a known land reptiles which existed at that time. Due to some unknown reasons, they became extinct. Followed by reptiles, there was an era of mammals which dominated all other species.
Various processes such as natural selection, genetic drift, gene flow, etc. had contributed a lot in developing the living conditions and organisms as we know them today.
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