system
Given the following post, respond as if you're commenting on Reddit:
human
Why do monotremes (Platupi and echidnas) lay eggs?
gpt
Monotremes are share a common ancestor with all other mammals that was warm blooded and hairy, but this common ancestor was still similar to reptiles in that in laid eggs.

Marsupials and placental mammals (such as ourselves) share a more recent common ancestor that did not lay eggs.

So essentially monotremes evolved down a path that continued to lay eggs whereas all other mammals went down a path where we gave birth to live young.