The primordial Amazon River: origin of the fauna of South America
According to the theory of continental drift, until approximately 90 million years ago South America was connected to Africa and North America. In that remote past, the so-called Pangea was the supercontinent of the Mesozoic era that brought together all the emerging lands.
The latest geology studies applied to plate tectonics confirmed that, until that period, a colossal river flowed from Africa to the west and flowed into the western part of South America. This river, whose sources were located in the current Lake Chad, was the largest watercourse of all time, with a length of approximately 14,000 kilometers.
It was the primordial Amazon River.
According to current knowledge, before the South American landmass separated from Africa and North America, many animals had already settled there and therefore remained isolated for the next few million years.
About 90 million years ago, the South American tectonic plate began, slowly but inexorably, to separate from Africa, thus giving rise to the Atlantic Ocean. Large expanses of sea prevented other animals from Africa from colonizing South America. Those who had already adapted to the neotropical, or South American, environment found easy living conditions, precisely because they were able to develop away from the ruthless competition that reigned on the old continent.
Approximately 65 million years ago, one of the fundamental events in the entire history of life on Earth took place: the extinction, probably caused by a catastrophic event of exceptional severity, of the large reptiles that had developed in the Jurassic era (that of the dinosaurs).
Successively, from the beginning of the Cenozoic era, the kingdom of mammals began.
When the South American plate collided with the Pacific plate, the Andes mountain range slowly began to form. In this way, a huge freshwater basin was created in the current Amazon. When, many millions of years later, the Andes reached considerable heights, the water flowed into the Atlantic Ocean, forming the bed of the Amazon River.
In South America, two large groups of mammals had already established themselves: the marsupials and the placentals. While the former are characterized by giving birth to their young early, which then feed on milk secretions inside the mother's marsupium, the latter keep their children for much longer within the mother's womb, to give birth to them when they have reached a high degree of development. These mammals came from North America, which was long connected to South America through an isthmus very different from the current, much wider territory of Panama.
Among these ancient immigrants were the glyptodonts, ancestors of the armadillo; however, it was a felid, therefore a placental, the macrauchenia (macrauchenia patachonica), which had the appearance of an enormous llama with a tapir's snout, which fed on tree leaves located three meters above ground level.
After this primitive group of mammals arrived in South America, the corridor they had used to enter there sank into the ocean and remained submerged for millions of years.
The consequent isolation of South America caused a particular evolutionary situation: the lack of selective pressure from predators (marsupial carnivores were more inclined to necrophagy and feeding opportunism than to true predation), meant that the mammals of the neotropical region were much slower and less cunning than their relatives, the inhabitants of Eurasia and North America.
In the internal sea that corresponds to the current Amazon (called by some Brazilian scholars the Pebas Sea) the proto-cetacean was already present that successively gave rise to the Amazonian dolphin, also called inia or pink dolphin (inia geoffrensis, boto for Brazilians and bufeo in Spanish-speaking countries). This is deduced from the fact that, today, a slightly different species of inia (inia geoffrensis boliviensis) is found above the Teotonio Falls, in the Madeira River. Precisely this location, beyond the waterfalls that are impassable for dolphins coming from the sea, is an indication of the archaic presence of this animal in South America, when the tectonic movements that originated the Andes had not even occurred.
In the Pebas Sea there was also an immense crocodile called purussaurus (up to 20 meters in length and approximately 5 tons in weight), and turtles weighing up to 150 kilos.
However, the isolation of South America was not total, since during the upper Eocene and Oligocene (from 55 to 34 million years ago) chains of islands not very distant from each other allowed the passage of apes and primitive rodents of North to South America. In general, the apes of South America are less evolved than those of the Old World and have peculiarities such as a prehensile tail and other types of teeth, which clearly distinguish them from their African and Asian cousins. Also the rodents that arrived in South America during the Eocene found virgin lands and ecological niches that were easy to occupy. What happened was that some of them were able to develop too much, like the capybara (Hydrochoerus hydrochaeris) which is the largest rodent that exists now.
During the period of isolation, enormous birds were also developed, such as the argentavis magnificens, the largest of all time. It had a wing spread of about 7 meters and could fly for hours at more than 70 km/h. Their hunting territory was more than 500 square kilometers.
While South America remained isolated from the rest of the world, life evolved, perfected and specialized.
About 5 million years ago, starting from the Pliocene, using the land bridge that had barely emerged again, that isthmus of Panama, already very similar to the current one, modern, well-differentiated and specialized animals entered South America. They were deer, mastodons, tapirs, horses (hippidion saldiasi), camelids (llamas, alpacas, vicuñas and guanacos), boreal raccoons, mustelids, canids, felines (such as the saber-toothed tiger, smilodon) and bears.
These latter carnivores replaced marsupial carnivores such as thylacosmilus, except for the most primitive ones such as didelphimorphs. Camelids occupied the niches abandoned by primitive ungulates such as macrauchenia. The tayasuids prospered, omnivores capable of adapting to both jungles and grasslands.
From an animal point of view it was a catastrophe, because many species succumbed and were replaced by others.
The last major extinction of animal species took place about 11,500 years ago (beginning of the Holocene) probably after the end of the last ice age. The megafaunal animals, which had survived the colonization of the Pliocene, such as the eremotherium, the glyptodont, the toxodon and others that arrived just during the Pliocene, such as the saber-toothed tiger and the American horse (hippidion saldiasi) did not managed to adapt to climate changes or increasing human pressure after the arrival of the abundant flow of Sapiens men from Eurasia.
Today, in South America, modern, specialized and “victorious” animals such as the jaguar, the condor or the spectacled bear coexist with authentic living fossils, descendants of the “ancient immigrants”, such as the armadillo, the giant anteater and the sloths. South America, although threatened by increasing human pressure, is a paradise where many animals still reign.
Unfortunately, all that is changing rapidly. As we know, the tropical rainforest is shrinking at a rate of 15,000 square kilometers per year and deforestation continues everywhere to use land for soybean cultivation or cattle ranching.
The great Amazon River, which in the very distant past flowed in the opposite direction, from Africa to the Pacific Ocean, is today in danger. Large ships pass through it daily and controls to ensure that they do not contaminate it ara scarce. In large cities such as Pucallpa, Iquitos, Manaus, Santarém and Belem, the population is increasing rapidly and polluting emissions from factories and means of transportation are not monitored and controlled as they should.
Man, who arrived in South America approximately 60 millennia ago, has occupied only a very brief chapter (less than 0.1%) in the history of this continent since it separated from Africa, 90 million years ago. And yet, this short period is influencing the fauna and flora of the continent like never before.
The great extinctions of the past, some caused by climate changes and others by the arrival of more evolved animals or better adapted to the environment, should make us reflect. Currently, we are causing another of the great animal and plant extinctions in the History of the Earth, just with our obtrusive presence. We could implement less polluting transportation systems and factories but what we see, instead, is an evil race towards economic profit, which does not take into consideration animals or delicate environmental ecosystems. Blinded as we are by the competitiveness of production and consumerism, we do not realize that the next candidates for extinction are precisely us, Homines Sapientes.
YURI LEVERATTO
