Two competing hypotheses on mammalian
evolution.
Mammals are divided into three sub-classes; eutherians, which include humans, dogs, whales, elephants, etc; marsupials, which include possums, opossums, kangaroos, wombats, bandicoots, etc; and monotremes, consisting of the playtpus and echidnas.
The Therian hypothesis holds that eutherians and marsupials are more closely related to each other than they are to monotremes. The Marsupionta hypothesis holds that marsupials and monotremes are closely related to each other and the eutherians are separate.
Mitochondrial DNA studies seemed to support the Marsupionta hypothesis, but more reliable nuclear genetic studies clearly support the Therian position.
So, for now, the Therians rule.