The Cenozoic Era began 65 million years ago with the extinction of the Cretaceous (end of the Mesozoic Era) and continues to this day. The Cenozoic Age is the geological age in which we are living.
The Cenozoic Age is known as the “Age of Mammals” because of the great expansion of the Cenozoic Era at that time.
Although many say that this statement is ambiguous, because although mammals increased in number of species they already existed long before the entrance to the Cenozoic, that is why some authors do not speak of an “Age of Mammals” since they would be comparing it with the “Age of the dinosaurs”, which appeared and disappeared in that era (Mesozoic).
The Cenozoic can be divided into three Periods:
- Paleogene Period. It is the first period of Cenozoic Era, and in turn, we can subdivide it into Paleocene Period (beginning 65 million years ago), Eocene Period (beginning 56 million of years) and Oligocene Period (beginning 34 million of years).
- Neogene. The second period of Cenozoic, and we can subdivide it into Miocene (beginning 23 million of years) and Pliocene (beginning 5 million of years).
- Quaternary. Last period of the Cenozoic and in which we are now, and subdivided into Pleistocene (2’5 million of years) and Holocene (beginning 11,000 years ago and lasts until today).
We are starting an era of mass extinction again, leaving about 5% of species alive and capable of repopulating the Earth.
After the extinction of the Cretaceous, our planet was in a so-called “nuclear winter”: cold temperatures were almost extreme and there was practically no sunlight.
That the Earth was plunged into total darkness caused temperatures to drop, and with it, many organisms perished (not only did the impact of the meteorite eradicate species, but also the subsequent consequences caused extinctions).
This drop in temperature was later changed to a warm, dry period at the beginning of the Paleogene, much warmer than the current one.
The maximum temperature reached during the Cenozoic was 55’8 million of years (passage from the Paleocene to the Eocene), which consisted of a long drought and which submerged the continents in several deserts, but even in the Paleogene tropical forests appeared.
Later in the Neogene Period, the climate cooled down. Much of the Northern Hemisphere went from large forests to bushes or cold-resistant vegetation, while the Southern Hemisphere seemed like a constant savannah, albeit with fairly cold winters.
The Neogene caused a forced evolution towards the survival of the extreme winter climate, which gave rise to large furry animals such as the woolly rhinoceros or the mammoth.
During the Neogene also appeared the great mountain ranges that we know today due to plate collisions: Spain joined France originating the Pyrenees, Italy joined the rest of the European continent originating the Alps, and India still followed its migration to Asia.
The Quaternary is the last period of the Cenozoic and the one we are currently in. It is characterized by a series of constant ice (North and South Poles) and glaciations.
The appearance-disappearance of ice on Earth at the end of the Neogene and beginning of the Quaternary allowed a bridge of connection between North America and South America, and this one across Antarctica and Australia.
Although the climate is currently warm on the planet, we still have permanent ice from the Neogene (the Antarctic and the North Pole), so we are still in the Quaternary.
Life in the Cenozoic is characterized by the large-scale expansion of mammals on all continents. Mammals can be divided into three types: monotremes, marsupials, and placentals.
Monotremes are a series of primitive mammals that present many characteristics of true mammals, such as bodies with hair, milk secretion, and endothermia.
The difference is that they lay eggs, being oviparous or ovoviviparous.
More similar to true mammals, but the embryonic development does not end in the mother’s womb but in a pouch (pouch).
They are known as true mammals and have special glands for the secretion of milk (breasts) and embryonic development is performed entirely in the maternal uterus.
The origin of the marsupials is different from that of the monotremes (one does not originate from the other), and they appeared in Africa. From there, they migrated and expanded across all continents.
The passage from Africa to Europe or Asia is simple: to Europe, they entered through the Strait of Gibraltar (Spain and Morocco were connected, the Mediterranean Sea was made up of a series of salt lakes) and to Asia through the Middle East.
From Europe, they were able to move from Europe to North America thanks to the glaciers we talked about earlier, although the arrival to Australia was more complicated.
In the current Gulf of Mexico, there was a glacier that appeared in winter and disappeared in summer, acting as a temporary bridge for the passage of species.
This caused the passage of species to be truncated, not continuous like the Africa-Europe passage.
Placentals originated in the Eurasian plate, and just as easily as the marsupials could migrate all over the planet, so did the placentals.
The appearance of the placentals caused the extinction of almost all the marsupials of the planet, except for two points:
South America. The Gulf of Mexico was a temporary bridge, so the passage of placentals was not continuous, allowing some marsupials to resist and survive, although their populations were severely reduced.
The large and medium-sized placentals (foxes, rabbits, and ruminants) did not reach Australia until the men arrived in 1770 by Robert Cook. In Australia the placental mammals did not arrive because the bridge connecting with America was broken, that is, the 3 continents South America – Antarctica – Australia were no longer connected, which prevented the passage of placentals to Australia and the maintenance of the marsupials.
As we get closer to the present, we see how primate species are gaining more knowledge about their existence and their capacity as animals.
Hominids go from being quadrupeds to being bipeds, from having a diet that is only herbivorous or carnivorous to a varied diet, they lose hair, increase their intellect and begin to create societies and cultures until they originate the modern human.
It is not an evolution in line, the typical image that comes to our head of the 4-legged monkey that stands up.
Human evolution (and that of all animals) is based on changes, mutations, apparitions, and extinction of many species, and hopefully, the survival of a few of which in the end only one would survive.
After the appearance of the man, the Earth has changed rapidly.
The man has acted as a driver of climate change, as climate change has always been present, but not in such an accelerated way.
Some scientists now indicate that we are in the Anthropocene, a period of the Cenozoic that began with the Industrial Revolution in the late eighteenth century that caused this accelerated climate change.