Australia is an amazing continent. There is a lot here that is unusual for the inhabitants of the Northern Hemisphere: when moving to the south it gets colder, and to the north it gets warmer. But the main “visiting card” of Australia is marsupials.
The ancient Romans were right when they said that "everything is from the egg." The difference between oviparous and viviparous animals, which include mammals, is only that in viviparous the egg remains inside the mother's body until a calf hatches from it (the fetal bladder of mammals is the transformation of the egg).
In higher animals, intrauterine development lasts a rather long time, the supply of nutrients in the yolk sac is not enough for this time, so the fetus receives nutrition from the mother's blood through the placenta. But the placenta did not appear immediately.
At first, the fetus was simply inside the mother's body, having no connection with it. Under these circumstances, babies have to be born when the nutrients run out and they are born immature, not quite ready for extrauterine life. Therefore, an intermediate stage was required - the stay of the calf in the pouch.
Thus, marsupials represent an intermediate evolutionary link between oviparous and placental ones.
The fate of the marsupials
Contrary to popular belief, marsupials are not only found in Australia. Rat opossums live in Peru, Chile, Colombia, Venezuela and Ecuador. American possums live in Canada, the USA, Argentina and the Lesser Antilles. But these species can be called "the remnants of the former luxury" in comparison with the kingdom of the marsupials that could be observed in the Mesozoic period.
Australia was then connected either by an isthmus or by a chain of islands with Southeast Asia, which allowed marsupials to move there.
But now the marsupials have competitors in the face of placental animals. Their cubs were born more mature, they had a better chance of survival, so the marsupials lost the evolutionary race, the placentals drove them out on most continents.
By that time, the contours of the continents had already changed, the "connection" between Australia and Asia had disappeared. Australia was isolated, and the placental animals did not get there. The absence of competitors allowed marsupials to exist and evolve in peace. Australia has become a "sanctuary" for marsupials.
Alternative evolution
The "experiment", staged by nature itself in Australia, convincingly proves the immutability of the laws of evolution. Australian "alternative evolution" gave rise to almost the same species of mammals as the evolution of placentals on other continents: marsupial wolves, marsupial anteaters, flying squirrels, koala marsupials, marsupial mole, very similar to the African gold mole.
Only one order was not generated by the Australian evolution of marsupials - the order of primates. One can only guess what human history would look like if an "alternative humanity" - marsupials - emerged in Australia.
