South America is now the continent with the lowest proportion of giant animals. Africa is obviously the den of charismatic megafauna, which is at least strange for two reasons: First, because it was in Africa that humans emerged, and if we are the great cause of megafauna extinction, as seems to be the case in good part of the world, Africa should be the most affected place. When in fact, the most affected is the place where humans arrived most recently: South America.
Second, after Antarctica and Australia, South America is the continent least densely inhabited by humans, housing only 5% of humanity in this immense territory. So why is South America the continent least rich in megafauna, and not the most? Since it is the least densely and most recently occupied by humans?
The explanation is that our megafauna was recently massacred, not only by humans, but by other animals, and all the giants native to South America have just disappeared , during an ecological catastrophe called “The Great American Interchange”. Incredible as it may seem, this little piece of land that connects the Americas is a relatively recent geological feature of the planet, having appeared only 3 million years ago. Before the formation of much of Central America, it was almost impossible for non-flying animals to cross the ocean that divided South America from any other continent.
I say almost, because some animals managed to get here, the most notable group being the monkeys, who found our continent being carried by rafts of vegetation, which are large floating wads of plant debris thrown into the sea during storms, which sometimes end , containing animals. Here, the apes diversified into the New World apes, highly specialized for life in trees. But these apes are an exception, and most mammals were incapable of inhabiting both the North and the South.
The last continents that South America had maintained close contact with were Antarctica, and by extension, Australia. At the end of the age of dinosaurs, 66 million years ago, the three living groups of mammals already existed: the monotremes, such as the platypus and the echidine, which lay eggs and have no nipples, forcing the young to lick the milk from the mother's belly. mother, as if it were a sweat.
Marsupials, such as opossums and kangaroos, that do not have full-term pregnancies and the fetus is born prematurely, attach to a nipple and finish its development inside a pouch of skin in the mother's belly. And placentals, like all other mammals, reproduce in the way we are used to. When the asteroid hit and the dinosaurs became extinct, mammals quickly diversified.
North America, Africa, Europe and Asia were dominated by placental mammals, while South America, Antarctica and Australia had a parallel mammalian age, with a diversity dominated by marsupials and some herbivorous placental groups. For almost 70 million years, South America was an island, which allowed unique and bizarre fauna to evolve here, but when 3 million years ago, the movement of the continents restored South America's contact with the rest of the world. world, animals from the north invaded the south and vice versa.
The result was almost always the extinction of South American forms and replacement by northern groups. Even the animals that we see today as characteristic of our territory, such as jaguars and wolves, are recent immigrants. But why did the marsupials not resist the placental onslaught?
Are they evolutionarily inferior? What happened to the megafauna of South America? Where are the giant sloths, gliptodons, toxodons, macrauchenios and other animals that inhabited these lands?
To answer these questions, I brought today an essay by Stephen Jay Gould, I translated and adapted this book into a more accessible language, called the panda's thumb, and I hope you like it! I hate that the destructive habits of my own species kept me from seeing a live Dodo, because a pigeon the size of a turkey was certainly something unique. And stuffed and taxidermied models just don't cut it.
Those of us who see value in diversity tend to pin Homo sapiens as the big culprit, the biggest catastrophe since the Cretaceous extinction. But still, I would argue that the uplift of the Isthmus of Panama, over the past 3 million years, ranks among the greatest ecological tragedies in the recent history of life. During the Tertiary period, South America was an island continent, isolated from all other landmasses, as Australia is today, and like it, South America was home to a fauna of mammals that only existed here.
. But Australia doesn't even compare to the immense diversity and variety of South American mammals. Many of them survived the onslaught of invasive species from the North when the two continents connected.
Some even went the other way and proliferated and prospered, such as the Opossums, which have already reached Canada, and the Armadillos are still conquering spaces further and further north. Despite the success of some, the annihilation of the most dramatically different forms in South America is the greatest effect of contact between mammals on the two continents. Two entire orders became extinct.
All mammals can currently be divided into 25 orders. Think how richer our zoos would be with the Notoungulates, a large and diverse group of herbivorous mammals, ranging from the size of a rhinoceros, like Toxodon, to the rodent analogues represented by Typotherios and Hegetotherios. There were also the Litopterans and their two subgroups: the large, camel-necked Macrauchenids, and the Proterotherians, analogues to horses.
These are groups that evolved independently after the extinction of the dinosaurs, throughout the age of mammals. Even so, Proterotherios repeated some of the evolutionary trends of true horses: Diadiaphorus, which had three toes, was ancestor of Thoatherium, which had only one toe, like modern horses. They are all gone forever, victims, in part, of an ecological disturbance caused by the uplift of a land bridge between the Americas.
Some Nightungulates and Lithopterns managed to survive here until the ice age, and may have taken the last blow from humans, but I have no doubt that if South America had remained an island, they would be among us. The native predators of these herbivores also completely disappeared. Modern mammals here, like felines and canids, are all North American invaders, and believe it or not, the carnivorous niche in South America was split between marsupials and dinosaurs.
South America was the only place in the world where dinosaurs reoccupied the top terrestrial carnivore niche. Terror birds were among South America's top predators, and they even invaded North America for some time before becoming completely extinct. Marsupial carnivores, on the other hand, ranged from small to the size of bears.
One lineage converged impressively with the northern saber-toothed tigers. The marsupial Thylacosmilus developed long fangs with a protective bony plate, like Smilodon. Though not commonly cited, marsupials aren't doing too badly in South America.
There are marsupial possums in North America, but they are all South American immigrants, descended from a group of about 65 species native to here. The large group of South American marsupial carnivores, the Sparassodonts, became completely extinct and replaced by northern cats. The traditional view, which I dedicate this essay to deconstructing, is that the massacre of South American fauna is due to the superiority of placental mammals to marsupials, and this argument seems quite convincing.
Marsupials flourished only on the continents of South America and Australia, which had a more recent connection between them than with any other land mass, and for a long time these places remained free of placental carnivores. These geological and biogeographical arguments echo the idea that marsupials are more primitive, in addition to being physiologically and anatomically inferior to placentals. The very terms of taxonomy seem to legitimize this idea, separating mammals that lay eggs, monotremes as prototherians (or pre-mammals), placentals winning the prize as Eutheria, or true mammals, and marsupials in limbo as Metatheria, or half-mammals.
. . almost there.
The argument of the structural inferiority of marsupials rests mainly on the presumptuous idea that our mode of reproduction is superior, since we tend to think that different means worse than us. Placentals, as we well know, develop as embryos intimately connected with the mother's body and blood. With a few exceptions, they are born complete and capable creatures.
Marsupial fetuses never developed the special trick that allows them to grow inside the mother. Our bodies have the unique ability to recognize and reject foreign tissue, a necessary protection against disease but a barrier that hinders medical procedures such as transplants. For all the mother's protection, love, and half the mother's genes in the offspring, an embryo remains a foreign tissue.
The mother's immune system needs to be tricked to prevent rejection. Placental fetuses have learned to do this, marsupial fetuses have not. Gestation in marsupials is very short, about two weeks in the opossum, followed by up to 70 days of development in the marsupium.
Internal development does not happen in tandem, and very early in development, the mother's immune system pushes the helpless little fetus out. The marsupial newborn is a small creature, equivalent in development to a very young placental embryo. Its arms and head are exquisitely developed, but the hind limbs are usually almost non-existent and underdeveloped.
They then face a tormented journey, slowly crawling a relatively long distance until they find one of the mother's nipples, inside the pouch that people call a marsupium. It is obvious why well-developed arms are needed. Our embryonic life inside our mothers' wombs seems much easier and more comfortable.
Soft! What, then, could challenge the biological inferiority of marsupials? It turns out that marsupials use another adaptive strategy, one that is different but no worse and no less.
It's true that they never managed to hack their mother's immune system to continue their easy life in the womb, but being born early is an equally valuable strategy. Maternal rejection is not necessarily a design flaw or a missed evolutionary opportunity, rather it may reflect a strategy equally suited to the rigors of survival. Darwin's central argument proposes that individuals compete to maximize their reproductive success by increasing the representation of their genes in future generations.
Many different strategies can be equally successful in unconsciously pursuing this goal. Placentians invest a lot of time and energy in the offspring before birth, this commitment indeed increases the pup's chance of success, but the placental mother also runs a risk: If she loses her little one, she has spent a large part of her life in an effort reproductive without an evolutionary gain. The marsupial mother loses her young more easily, but the reproductive cost is much lower.
The gestation is short and she can mate again in the same season, and in addition, her little newborn does not drain so much energy and guarantees a calm, easy and safe delivery, unlike placental mothers, who not infrequently suffer from serious complications. during childbirth. Perhaps if we were marsupials, this would be used to argue for the biological inferiority of placentals.
Looking at biogeography, the mistaken traditional view may lead one to believe that the southern continents became refuges for inferior animals that could not withstand the competition from the north, when in fact, this diversity is a reflection of the success of marsupials at the time when the continents were separated. separated. Apparently, South American carnivores such as Borhyenids and Australian carnivores such as Thylacines were closely related!
Paleontologists believed that these two groups were distant and that their similarities were explained by evolutionary convergences, when two groups develop similar characteristics from different ancestors, like the marsupial and placental sabertooths I just mentioned. In fact, scientists saw the radiation of mammals in South America and Australia as independent events, but if carnivores from the two continents were close, it means that marsupials crossed from one continent to the other, via Antarctica. These three continents separated from the rest of the world before the extinction of the dinosaurs, and remained connected to each other for millions of years afterward.
This is why each hemisphere of the Earth has had a different era of mammals, with a unique and independent evolutionary history. The history of marsupial groups and the fossil record suggests that these continents were the center of origin rather than the haven of marsupials. But I confess that the anatomical and biogeographical arguments seem to fall apart in light of the most obvious fact: marsupials were slaughtered by competition from the North.
Is this not sufficient argument for the superiority of placentals over marsupials? How to continue defending equality among mammals? Despite the extinction of most native groups in South America, there is no evidence to suggest that this is due to their being marsupials.
I prefer an ecological argument, which predicts a good beating for any group that remained in South America after the extinction of dinos, whether marsupials or placentals. The real victims were the marsupials, but the culprit may not be biology, but the history of these groups. Northern carnivores have been evolutionarily tested in two ways.
Twice, they suffered the brunt of mass extinction events, and new, more generalist and flexible groups dominated the scene. In periods of stability, a great diversity of prey and predators engaged in an intense arms war that created evolutionary trends that increased the efficiency of predators in feeding, being able to eat fast and cut meat efficiently, and in locomotion, developing great acceleration in ambush and resistance predators for those chasing long distances. Predators in South America have never been tested in any of these ways.
They have not suffered major extinctions, and their original representatives have persisted. Diversity never reached Northern levels, and competition was always less intense. It seems that specialization in food and locomotion was always less than in the North American carnivores that lived at the same time.
In other words, the low life of South American carnivores prevented them from being able to compete with their analogues. Anatomical status as marsupial or placental seems to have made no difference, and the groups' history of evolutionary challenges was the crucial factor. If, by chance, the carnivores of the North were marsupials and the ones of the South Placentaries, probably, the result of the competition between north and south would still have been a massacre for the south.
North American faunas were constantly plagued with intense and hostile weather, mass extinctions and fierce competition. South American carnivores never experienced this, and when the isthmus of Panama came along, they were weighed on the scales of evolution for the first time. I hope you enjoyed this essay by Stephen Jay Gould, comment below what you thought, and what you discovered watching this video.
I just want to add that marsupials never dominated South America alone. They co-existed with the placentals here throughout the Cenozoic, and when the northern gate opened, both marsupials and placentals here took a hit. So this really has nothing to do with supposed placental superiority.
And another, that North America periodically connects with Asia, and consequently, Africa and Europe. It means that South America has not only made contact with North America, but with virtually the rest of the world. Which makes the competition even more unbalanced.
And the next time you see a possum walking on the wall, know that it is one of the last representatives of the genuinely South American fauna, and more than that, he is one of our only representatives to succeed and prosper in other continents. Many of our misconceptions about marsupials reflect our own misconceptions about nature and evolution. We tend to see ourselves as the great success benchmark in the competition for survival, and we attach more value to what is most like us.
But we are not more evolved than any other living being. Everything living at this very moment has gone through the same billions of years since the last common ancestor that connects us. Life has no preference for complexity, and evolution does not mean improvement, nor progress, it just means change over time.
And that changes everything. Other living beings have other strategies, and we must stop thinking in this meritocratic way, as if the best or most advanced always win, because the chaos of history is the most imposing force of nature, and sometimes, animals were just in the wrong place. , at the wrong time.
Thank you so much for watching until the end, don't forget to like, subscribe, comment and share with anyone who might be interested in the war between marsupials and placentals! To the next!
