A long time ago, Great White Shark thrived. They ate lots of seals and had plenty of other things to eat.
100 years later, However as time wore on, the seals, who were losing mass numbers to sharks, adapted and were able to stay on land in order to eat and survive, and almost never had to go into the water.
Almost a 100 years later, The Great White Sharks, facing a massive loss in numbers because of a lack of food, slowly developed a gene within their gils that allowed them to be in fresh water, so many great white sharks started to swim inland to try and find a reliable food source.
As time continued on, many of the sharks struggled to find reliable food sources in the river, but since they had already adapted to fresh water, they continued to hunt in fresh water, looking for food.
As time continued on, the sharks who had traveled into the fresh water were starting to migrate their ways into fresh water bays and looking for food.
Finally, almost 200 years after the great white sharks were almost at an all time high in numbers and thriving off the seal population, they landed in a fresh water bay, and began to thrive again, eating many fish. This was all because their ancesters long ago had adapted to fresh water, allowing them to migrate into fresh water.