How organisms are came on to the earth
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As far as I know, many scientists are actively working to answer this. I will try and give you a gist of the existing theories though.
In the previous answer @Charlotte Grayson has mentioned Pasteur's Swan Neck experiment. However the Swan Neck Flask Experiment was designed to disprove the theory of Spontaneous Generation.
According to this obsolete theory (Spontaneous generation), some living organisms were thought to be growing from other unrelated living organisms or even non living matter. Quoting the Wikipedia article, "the idea was that certain forms such as fleas could arise from inanimate matter such as dust, or that maggots could arise from dead flesh."
I'm not sure if Louis Pasteur meant to support Biogenesis as an evolutionary theory or was merely shutting down people who thought themselves dying and becoming tapeworms. But accepting the theory of biogenesis brings on this question: If all life came from another life, where did that another life come from? One can probably trace this backwards to single celled organisms, but the question remains. One theory that answers this question is Panspermia, which proposes that life developed somewhere else on the universe and were brought to earth by some astronomical event. But then again, where and how?
A contradicting theory, Abiogenesis, proposes life evolving from non living matter due to chemical and geographical conditions present on earth billions of years ago. If you study biology deeply, all life processes are inherently chemical in nature. Metabolism, neurological functions, reproduction, even genetics and genetic flaws can be attributed to chemical structure and reactions. So it is possible that all organisms are only complicated chemical machines built upwards from simpler elements of nature. An experiment conducted by Stanley Miller, Miller–Urey experiment, supports the hypothesis that "conditions on the primitive Earth favoured chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors".
I am sure abiogenesis is also riddled with questions and doubts and, unless we can build a huge time machine, I doubt if we will ever know for sure.
In the previous answer @Charlotte Grayson has mentioned Pasteur's Swan Neck experiment. However the Swan Neck Flask Experiment was designed to disprove the theory of Spontaneous Generation.
According to this obsolete theory (Spontaneous generation), some living organisms were thought to be growing from other unrelated living organisms or even non living matter. Quoting the Wikipedia article, "the idea was that certain forms such as fleas could arise from inanimate matter such as dust, or that maggots could arise from dead flesh."
I'm not sure if Louis Pasteur meant to support Biogenesis as an evolutionary theory or was merely shutting down people who thought themselves dying and becoming tapeworms. But accepting the theory of biogenesis brings on this question: If all life came from another life, where did that another life come from? One can probably trace this backwards to single celled organisms, but the question remains. One theory that answers this question is Panspermia, which proposes that life developed somewhere else on the universe and were brought to earth by some astronomical event. But then again, where and how?
A contradicting theory, Abiogenesis, proposes life evolving from non living matter due to chemical and geographical conditions present on earth billions of years ago. If you study biology deeply, all life processes are inherently chemical in nature. Metabolism, neurological functions, reproduction, even genetics and genetic flaws can be attributed to chemical structure and reactions. So it is possible that all organisms are only complicated chemical machines built upwards from simpler elements of nature. An experiment conducted by Stanley Miller, Miller–Urey experiment, supports the hypothesis that "conditions on the primitive Earth favoured chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors".
I am sure abiogenesis is also riddled with questions and doubts and, unless we can build a huge time machine, I doubt if we will ever know for sure.
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