What is the scientific explanation for how life began according to the theory of evolution?
you are confusing abiogenesis with the theory of evolution. The prevailing theory of how life began is that elements released from stars that went supernova gathered on one of the planets that is hospitable for life, such as Earth, and combined to form molecules that combined to form amino acids, which evolved into very simple cells, and thus life began.
It is believed the earliest signs of life appeared 600 million years after the earth formed. These life forms were possibly derived from self-reproducing RNA molecules. The replication of these organisms required resources which soon became limited, resulting in natural selection. DNA molecules then took over as the main replicators. They began to develop inside enclosed membranes which provided a stable environment for replication: proto-cells. 100 million years passed before cells resembling prokaryotes appeared. These organisms were chemoautotrophs.
Another 900 million years passed before photosynthesizing cyanobacteria evolved which produced oxygen. The oxygen concentration in the atmosphere subsequently rose. Eventually, more complex cells began to appear: the eukaryotes. After 2 billion years, the first multicellular organisms evolved. Natural selection fueled the evolutionary radiation that occurred during the last 1 billion years. Homo sapiens (modern humans) didn't appear until about 200,000 years ago.