About 315 million years ago a group of vertebrates called amniotes, who protected their young with shell eggs and amniotic membranes, gave way to two lineages : one branch led to reptiles and animals such as lizards, snakes, dinosaurs and birds and the other, to mammals.
The first appeared 225 million years ago , although for the next 150 million they remained in the shadow of the dinosaurs, were the size of a squirrel and possibly moved in the dark of night. From them various lineages appeared; some have become extinct while others are still alive : today’s platypus belongs to the oldest lineage of mammals , the monotremes, which still retain reptilian characteristics such as oviparous reproduction: the female lays an egg the size of a head – which she carries in a opening in its belly – and when it hatches the newborn feeds on milk that oozes from its mammary glands (female platypuses don’t have nipples).
140 million years ago the evolution of mammals produced two branches that would be the most successful of all . One was that of the marsupials, such as the kangaroo, the opossum and the koala; the other, that of placentals, such as dogs, rabbits, horses or human beings. There aren’t many mammalian fossils older than 65 million years , but those few seem to indicate that the lineages from which modern mammals descend began to appear 100 million years ago. The first is to which anteaters, armadillos and sloths belong, which lack some characteristics that other placentals share: they do not have a cervix and their metabolism is slower. The rest of the lineages appeared around 80 million years ago: insectivores (moles, shrews…), carnivores (dogs, bears…), ungulates (camels, elephants, whales), glires (rabbits, rodents) and archonta (bats, primates).
But 66 million years ago the ecological status quo changed . A rock of 34,000 million tons and 12 kilometers in diameter hurtled towards the Earth at a speed of more than one hundred thousand kilometers per hour, hitting the surface of a shallow ocean with the energy corresponding to a billion atomic bombs in the past. which today is the Yucatan Peninsula, in the Gulf of Mexico. The vaporized rocks rose into the atmosphere causing terrifying fires, huge tsunamis devastated the coasts and a cloud of dust covered the sky plunging the planet into eternal night . Temperatures plummeted and phytoplankton and plants, in the absence of sunlight, died, and with them the ecosystems on which they depended, which was all of them. When the smoke cleared months later, the planet was still a cold, dark place . The impact had vaporized the sulfate deposits in the area, which when mixed with oxygen formed clouds of sulfur dioxide, carriers of acid rain. At the same time, the carbon raised by the shock combined with the oxygen in the atmosphere to form carbon dioxide, the greenhouse gas par excellence, to which an exacerbated volcanism that spread throughout the planet due to the collision collaborated. The climatic disaster that caused the impact wiped out two-thirds of the planet’s species from the surface.
When everything returned to normal, the Cretaceous was over and with it the giants that had dominated that period left. It took a few thousand years for plankton in the oceans and plants on land to reappear. Among the ruins of that lost world we also find surviving animals: of the dinosaurs only the birds, but crocodiles, snakes… and mammals were also lucky. These were badly decimated; only 10% had survived.
The greenhouse gases released caused the planet to warm up again , eventually reaching temperatures almost 15 degrees above the current average. A warm climate that extended to the poles, and jungles and forests covered most of the emerged lands. And as has happened after each mass extinction, life took a breather and began a new evolutionary race where the prize was to occupy the dominant position in the ecosystems that emerged after the disaster. Thanks to different genetic studies, we know that the ancestor of modern mammals that appeared in the Cretaceous did not begin to evolve rapidly until 10 million years after the disappearance of the dinosaurs . Mammals were initially small , about the size of a raccoon, and scratched through leaf litter while fleeing predators the size of a weasel. It only took time, a few million years, for mammals to evolve into the 20 orders we know today, plus many others that have become extinct.
The extraordinary triumph of mammals was the result of a series of extremely successful adaptations , such as internal regulation of temperature, which allowed them not only to colonize all available environments but also to adopt nocturnal habits impossible for reptiles and amphibians. What’s more, having a constant temperature inside the body continuously provides optimal working conditions for the cells. The control of the new ecosystems was also possible thanks to an improvement in the sense organs and in the development of an efficient digestive system , which decreased the digestion time and increased the level of utilization of food. Fur was also very important to early mammals because it helped them maintain body heat.
One more evolutionary innovation gave them the ability to maintain a high rate of activity: the four-chambered heart , when the reptilian heart only has three. This extra breakdown means that oxygen-rich blood from the lungs does not mix with blood returning from the rest of the body before it is pumped to the cells by the heart. In this way the cells constantly receive large amounts of oxygen in order to maintain a high metabolic rate.
Mammalian teeth also provide them with an evolutionary advantage . Unlike other groups of animals, each mammal has a different number of teeth in its mouth, and the types vary from species to species. Human beings, for example, have incisors, canines, and molars, each with its own well-defined function that allows us to eat a wide spectrum of foods.
However, the main feature that allowed mammals to quickly take over any habitat was inside their heads. First, the sophisticated and precise hearing system they have. The first mammals developed the ossicles of the middle ear (hammer, incus and stirrup) from the lower jaw. As Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh has put it, “Reptiles have a jaw full of mammalian auditory bones, and mammals have an ear full of reptilian jaw bones.” Thanks to this they became more sensitive to a much greater hearing range. What’s more, the fossils also show an improvement in smell and the rest of the other senses, including the appearance of a highly sensitive mustache. This excess of information from the outside required a larger brain, which the mammals used both to enhance their perception and to incubate a nascent intelligence, the epitome of which we humans are.
Once mammals became the dominant species on the planet, they never stopped being . Of course, as the history of life on Earth has shown, one day his reign will end. Until then, we’ll see if they last longer than the dinosaurs
