Answer

Most scientists think that the evolution due to the lack of land predators made penguins change their wings for flippers. This adaptation was also the consequence that penguins had to find food in the ocean and the wings were difficult to use underwater; therefore they were changed to flippers, which greatly help penguins to swim and made them extremely agile in the water.

Besides, during this transition, penguins adapted further to life in water, changing their brittle bones needed for flying with hard bones which help them stay on land and submerge in water.