Evolution of life on earth has not been a simple process. Some explain it as a process while some interpret it as a result of various natural occurrences. Evolutionary theory tries to convey the message of origin of the earth and how lives on earth evolved. Evolution as a process, explains how the world came into existence. It has resulted in biodiversity.
The Big Bang Theory is the most accepted theory regarding the origin of earth and life forms 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 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 first living cell, it is assumed that the first cell originated from non-cellular molecules. This cell 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 derived 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 were 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, size, 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 reptile 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 to develop the living conditions and organisms as we know them today.
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