This is possible thanks to key proteins called syncytins which allow exchanges between the mother and its offspring through a placenta, even rudimental ones such as in marsupials.

[3] The marsupials and the placentals evolved from a common therian ancestor that gave live birth by suppressing the mother's immune system.

[6] The flexible and protruding nose in therians is not found in any other vertebrates, and is the product of modified cells involved in the development of the upper jaw in other tetrapods.

[9] The earliest known therian mammal fossil is Juramaia, from China's Late Jurassic (Oxfordian stage).

However, the age estimates of the site are disputed based on the geological complexity and the geographically widespread nature of the Tiaojishan Formations.