Answer:
Oxygen would be played a key role in evolution of complex organisms because this molecule is required to efficiently produce chemical energy during cellular respiration
Explanation:
Chloroplasts originated from endosymbiotic cyanobacteria 1,500 million years (mya) ago. Like chloroplasts, mitochondria originated from cyanobacteria, and also approximately at the same time. Mitochondria are organelles capable of generating energy by burning oxygen during cellular respiration (aerobic respiration). During this period (1,500 mya), complex cells became able to efficiently produce energy by using oxygen in aerobic reactions, which can be attributed to the emergence of mitochondria. Interestingly, it has been shown that during the next 500 mya, different groups of organisms composed of more than 50 different cell types evolved (including algae, fungi and early animals). Therefore, it is possible to hypothesize oxygen would be a critical molecule without which complex organisms couldn't evolve.
Oxygen played a vital role in evolution of complex life because the transfer of electrons from organic molecules to oxygen released a large amount of energy, thus resulting in efficient production of chemical energy during cellular respiration.
For life to exist, energy must be available and abundantly so, in order to carry out the vast array of activities that makes life possible.
At first, only simple life forms which did not depend on oxygen existed. This life forms were collectively known as anaerobes.
Because the electron acceptors they used then were not very efficient in accepting electrons during the oxidation of organic compounds to produce energy, life remained in simple forms as microorganisms.
However, as the atmosphere became progressively richer in oxygen, some anaerobic microorganisms evolved who were able to obtain energy by passing electrons to oxygen.
Endosymbiotic associations which eventually became permanent between aerobic and anaerobic microorganisms gave rise to structures such as the mitochondria, plastids and chloroplasts.
This ability to effectively use oxygen as electron acceptors to produce energy as in mitochondria as well as produce oxygen as in chloroplasts led to the appearance of more complex life forms who were able to carry out more complex life activities due to efficient production and utilization of energy. These life forms include the algae, fungi and early multicellular animals.
Therefore, it can be concluded that oxygen played a vital role in evolution of complex life because the transfer of electrons from organic molecules to oxygen released a large amount of energy, thus resulting in efficient production of chemical energy during cellular respiration.
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