Three different types of mammals coexisted in the primeval forests Australian. But only a few of them were gaining ground as their new evolutionary prototypes were improving. Adaptive capacity that tipped the balance was the development of the cerebral hemispheres and thus intelligence.
In those distant forests, one hundred million years ago, there lived different types of mammals who sought to ensure their survival by using different means of reproduction. The monotremes, the oldest of all, were mammals, but laid eggs; the eutherians gave birth to completely developed young; and the marsupials, somewhere between these two extremes, completed their development outside the mother’s body. Competition was extremely tough. The eutherian mammals were victorious in almost all the corners of the Earth. But when Australia became an independent island, around fifty million years ago, none of these new mammals with placentas had yet colonised its lands. And Terra Australis became the kingdom of the marsupials.
Night in the southern hemisphere is full of ghosts from the past.
When darkness falls, shadows come to life in the ancient forests of north east Australia.
The yellow-bellied gliders move silently around the trunks of the dense forest, in search of sap and resin.
When the dinosaurs prowled around the jungles of Gondwana, their cycle of activity took place during the daytime, determined by the sun which activated their gigantic circulatory systems. At that time, the mammals were nocturnal creatures who, whenever possible, avoided encounters with the enormous lizards. The habits of those first marsupials can still be observed in many of the species of present day Australia, and the nights are alive with furtive movements.
The possums, along with the kangaroos, are considered the most evolved of the marsupials. But, despite this, they remain faithful to the nocturnal traditions of their origins. Millions of years in the dark of the jungle has equipped them with excellent climbing skills, and the ability to see and hear in the humid shadows of their environment.
Like the gliders, the possums, such as this bush-tailed variety, lick the sap which oozes from a broken branch or a damaged tree trunk. Climbing is for them synonymous with survival. There are now no large predators in Australia, but the possums are small animals, and this makes them vulnerable to the carnivorous marsupials.
For the European scientists, basing their studies on the original zoological classifications, it was clear that the marsupials were inferior to the mammals that inhabited the old continent of Europe. So much so, that they named the mammals of this European group, ‘eutherians’ which, in Greek, means “perfect mammals”. For these zoologist, the eutherian mammals had displaced their competitors because they were able to keep their young inside their bodies until they were completely formed, whereas the young of the marsupials complete their development in the pouch used for this purpose. But if the first zoological classification had been carried out in Sydney, instead of in Ancient Athens, the conclusions might well have been very different.
The marsupials were, it is true, displaced by the eutherian mammals. But the reasons for this are to be found not in the pouches of the marsupials, but rather at the other end. And to understand why, nothing better than to return to the place where the struggle for survival originally took place.
The jungles of South America were, like those of Australia, once part of Gondwana. Back then, monotremes, marsupials and eutherians coexisted in the primeval forests. But the last of these three, little by little, gained ground, as their new evolutionary prototypes improved. And these monkeys of the New World are the best examples of the final victors.
The ability to adapt, which tipped the balance, was not based on the different ways of giving birth, but rather on the development of the brain: the centre where information and associations are stored. It was not, therefore, a question of the marsupial’s pouch versus the placenta, but something much more effective and decisive – intelligence.
The new mammals not only had placentas, they were also more intelligent, and they took over the majority of the habitats of the monotremes and the marsupials. But defeat was not quite as absolute as people tend to think. Because, in the South American night, old ghosts from Gondwana still hide.