On the topic of eating raw fish and sashimi So it has been widely accepted that raw fish or "Sushi-graded" fish must be frozen below $-37C$ for a certain period before they can be defrosted and eaten raw. I have heard that even fresh fish must go through the freezing process to ensure bacteria are killed

I am just wondering can't we do this with just regular frozen fish in the supermarket provided it doesn't have other preservatives? 

I mean in ancient times, the Japanese people didn't have commercial freezers and thermoeters to make all these measurements, so what did they do to ensure that their fish is "sushi-graded"? Something tells me the answer is simply just eating it fresh and raw. 

Any sushi-chefs or someone who prepares a lot of sashimi can help?
Present day Japanese chefs are taught which sea foods are affected by which particular parasites during their apprenticeship. So they handle those accordingly. In ancient times Sushi wasn't made out of raw fish. The fish was cured and cooked before being consumed. Using Salmon for sushi is a new concept dating back around 30-40 years. Which is why Sushi was all that safe in ancient Japan.

You can find a consuming discussion on the same topic on Reddit. 
https://www.reddit.com/r/sushi/comments/2g5mhe/noob_question_how_did_the_japanese_of_old_not_get/