What is the maximum concentration of NaCl for life in the oceans to be possible?
Was it fortuitous that the concentration was originally no higher, in which case the evolution of life on Earth would not have occurred?
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There are organisms called halophiles which thrive in conditions of high salt concentration - and, indeed, require high salt concentrations in order to grow. According to Wikipedia:
Halophiles can be found in water bodies with salt concentration more than five times greater than that of the ocean ...
Since the earliest life on Earth originated in the oceans over 3.5 billion years ago, it is likely that it was adapted by evolution to suit the salinity of the oceans as they were then. If the salinity of the oceans had been significantly different then life would have evolved in a different direction.