system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did penguins develop to the stage they are at now?

 Are their flippers residual wings or fins? And how on earth did Emperor Penguins start nesting in Antarctica.
gpt
Their flippers are wings that have been adapted to propel and maneuver through water instead of air. Their ancestors first migrated to Antarctica by flying there when it was a warmer environment.

As the antarctic cooled and vegetation disappeared penguins became more reliant on fishing for their food. Penguins who could swim better survived and had more kids. Over many generations there was a progressive reduction in wing size, which makes diving more efficient and flying less so. Penguin bones also thickened over the ages, as lighter bones that make it easier for birds to fly gave way to more dense bones, which may have helped make them less buoyant for diving. Finally, penguins grew larger as bigger lungs meant they could dive deeper and longer while looking for fish. They also got fatter, because cold.

There's a tradeoff between efficient flying and swimming. Penguins didn't benefit much from flying when there were no big threats left on land, but they really benefited from being good swimmers. Over time they evolved to make the best of their environment.