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> Egg laying animals already grow their babies externally, yet mammals dominate. Why?

There are mammals that lay eggs, for example, the platypus [1].

[1] https://en.m.wikipedia.org/wiki/Platypus

given that only 2 branches of montremes exist in the present day. Its kind of a miracle we get to come on the scene as a species in time to see them because they are really not positioned to survive much longer wether or not we are around.

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