system
Given the following post, respond as if you're commenting on Reddit:
human
What were the evolutionary steps for the blowhole on whales and dolphins?

I'm watching a dolphin show live right now and I'm wondering, that if they're mammals, did they develop from an earlier form of gills? When in the chain of evolution might have it developed and are there any other mammals that have a vestigial remnant of this feature?
gpt
The blowhole evolved from the nostrils, which migrated to the top of head and (in some whales) merged to form a single opening. Some whales still have two blowholes, while the sperm whale's blowhole is its left nostril, with the right nostril not having an opening.