Before Placentals evolved, there was a widespread group of more basal Eutherians who were not Placental.
They were more similar to modern placental mammals in anatomy than to modern marsupials, however they had some small differences in the anatomy of their feet, they still had epipubic bones, and gave birth to underdeveloped "joeys" like modern marsupials.
How do you think the history of Earth would have progressed if True Placentals simply never evolved?
Would the fauna of Eurasia, North America, and Africa resemble that of pre-Great American Interchange South America and pre-human-contact Australia?
Would non-placentals develop a sapient species by the present day?