system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If dolphins and whales breathe through lungs, what are the reasons that make them aquatic? Alternatively, why have dolphins and whales not developed the ability to live in land?
gpt
Dolphins and whales evolved from land going creatures. Over millions of years they returned to the sea. This probably started as hunting along shorelines, and adaptations occured that let subsequent generations hunt further and further out until you have the completely aquatic creatures we have today.

We have excellent transitional fossils from what were four legged animals to modern whales. The nostrils move progressively further back on the head, and the limbs form flippers. Dissections of modern whales show vestigial leg bones buried deep in their bodies.