It's interesting when you read about how the evolution happened. There are researchers and scientists that would be better able to explain it but I remember reading about it years ago.

The Indian subcontinent was an island in the ocean at one point separated by a large body of water from Asia. These creatures lived in the area where Pakistan is today but on land and near the water. As the Indian continent moved in, it started to create a shallow sea. The creatures evolved over millions of years to live in the new shallow water. They became dependent on the water and evolved more and more to live in water. Then the shallow sea started to dry up as Indian merged with Asia. The creatures were pushed out and by that time, they had become fully evolved to live in the open ocean.

I always found it neat because it was a clear example of how geology directed evolution.