A question from my son which I was unable to answer, regarding monotremes.

Assuming that all mammals have a common ancestor...

Did those very early mammals lay eggs, then over time as mammals diversified they all developed live birth - except the monotremes.

Or: did early mammals give birth to live young, and monotremes have subsequently lost that ability (in the same way that rattites have lost their ancestors' ability to fly)?

What is the current thinking on this?

Concensus is very much that all mammals have a common ancestor, with an early split of the monotremes which have retained the "primitive" characteristic of being egg laying.

There has some debate about whether the marsupial mammals are really the "sister" taxon to eutherians (like us) or not. My understanding is that the current weight of evidence is that they are, with eutherians and marsupials splitting from a common "Therian" ancestor (with Therians and monotremes having an earlier separation).