The Cenozoic Era began around 66 mya and continues into our present times. It came about following the Cretaceous-Paleogene Extinction Event that killed about 1/2 of all animal species on planet Earth. Among the animals killed in this extinction event were mosasaurs, pterosaurs, plesiosaurs, ammonites, belemnites, and others. This massive extinction event saw the killing of all dinosaurs excluding birds so it is claimed. Mammals became the dominant species on Earth and the earliest rodents appeared. This era also saw the evolution of the Creodonts which were an important group of carnivorous mammals.
The Cenozoic also saw the appearance of the earliest primates. Large, flightless birds also appeared at the beginning of this era as did armadillos and the first bats. Around 25 mya the first deer appeared and around 30 mya the earliest pigs and cats appeared. About 10 mya grasslands and savannas appeared and 6.5 mya the first hominins (Sahelanthropus) appeared. Around 5.6 mya Ardipithecus appeared (considered an Australopithicine by some). About 4.8 mya the first Mammoths appeared and by 4 mya the first known Australopithecus appears in the fossil record.
Sometime around 2 mya in the Cenozoic Era the first members of the genus homo appear in the fossil record. This was Homo habilis, “tool man,” “handyman.” By about 1.7 mya the australopithecines were extinct and by 1.2 mya Homo antecessor appears in the fossil record as the last members of Paranthropus go extinct. Short faced bear appeared around 800 kya and become abundant in North America. Then 350 kya the Neanderthals evolve. Gigantopithecus comes on the scene around 300 kya and sometime near 250 kya the first anatomically modern humans appear in the fossil record in Africa. Sometime around 50 kya these AMH migrate and spread out colonizing other continents. This was Homo erectus.
By about 30 kya the Neanderthals went extinct and we also find the dawn of the first domestic dogs. Then 10 kya the Holocene epoch begins after the last Late Glacial Maximum.
At the beginning of the Cenozoic Period atmospheric oxygen content levels were about 25%. Since then oxygen concentrations have declined to where they are today which is about 21%. Thus, there is a big decline in oxygen concentrations on our planet between the Carboniferous and Cenozoic Periods by about 15%.
The Cenozoic is known as the “Age of Mammals” and also as the “Age of Birds.” This is because both have dominated this era. Following the Cretaceous-Paleogene Extinction (also known as the K-Pg Event) the planet became dominated by small fauna including small mammals and birds and smaller reptiles and amphibians thanks to the absence of the great dinosaurs (in large numbers I would like to qualify).
The Cenozoic is divided into several different epochs which are the Paleogene (66 mya to 23 mya), the Neogene (23 mya to 2.58 mya), and the Quaternary (2.58 mya to present). The Neogene is divided into two epochs which are called the Miocene (23-5.3 mya) and the Pliocene (5.3-2.58 mya). The Quaternary Period is divided into two epochs also and they are known as the Pleistocene (2.58 mya to 11.7 kya) and Holocene (11.7 kya to present) epochs.
The Neogene (23-2.6 mya) is the period in which we find the emergence of Ardipithecus (5.6 mya) and their apparent disappearance around 4.4 mya. It is also the period in which we find the emergence of the Australopithecines (4.5 mya) who lasted into the Quaternary going extinct around 1.9 mya. The first of genus homo, Homo habilis, emerged during the beginning of the Quaternary period around 2.1 mya and seems to have disappeared around 1.5 mya.
In terms of climate the Earth entered into a drying and cooling trend which culminated in the glaciations we find during the Pleistocene Epoch and the continents came to their places in which we see them today for the most part. During the Neocene (23-2.6 mya) grasses spread and dominated large portions of the planet. This resulted in the diminishing of forests. It was during the Neocene’s Miocene Epoch era that apes developed and they evolved into at least 30 species. Arid and mountain lands dominated much of the Earth
During the Niocene’s Pliocene Epoch period there were dramatic climatic changes! The Mediterranean Sea dried up for several million years during this time and this was around the time Australopithecus was developing in Africa. This is also the time that the isthmus of Panama formed connecting North and South America. Savannas increased and continue to spread today. This is also the period in which we find the beginnings of the Sahara Desert and the deserts of East Africa as well as the development of the Indian monsoons.
Turning to the Quaternary Period the first epoch was the Pleistocene (2.58 mya to 11.7 kya). This epoch was marked by Ice Ages due to cooling trends that began in the Mid-Eocene (an epoch of the Paleogene Period 66-23 mya) which began around 56 mya and lasted until about 33.9 mya. The Quaternary has seen at least four separate glaciations (ice ages) marked by advancing ice caps as far southward as 40 degrees N latitude in mountainous areas. Africa during this time became more arid. It was also during the Pleistocene that the first Homo sapiens developed around 100 kya or earlier and it was during this time also that some of the worst droughts in Africa came to an end. Also during the Pleistocene there was another mass extinction event that killed much of the planet’s megafauna including the Neanderthals. All of the continents were affected by this extinction event but less so in Africa.
The last epoch of the Quaternary is the Holocene which began 11.7 kya at it includes our present time. This is typically considered the beginning of written History while anything prior to 11.7 kya is deemed as “pre-history.” Human activity has been blamed for a mass extinction event that began around 10 kya and this is sometimes called the “6th Extinction.” Granted human activity may have helped along some of these extinctions but as in all other eras our climate is changing and climate change on a scale LARGER than human activity also contributed to these extinctions. Ten thousand years ago the human population was nowhere near the 7 billion it is today!
Turning to ancient human ancestors below is a chart showing the generally accepted dates and evolution:
As you can see and as has become the acceptable theory now there was much overlapping of these ancient hominids. In other words, one species did not end and another begin, rather the norm has been more than one species of hominid have coexisted at any one time. That is…..until today! Suddenly in our own modern times we claim we are the only species of human on the planet and all other species are extinct. But this is NOT the historical norm! That is….unless we Homo sapiens are hybrids, a combination of all those species that came before us! OR as some put it, have the distinctions between the different species of humans been so blurred by interbreeding and even retrobreeding that clear distinctions between the different species of man are no longer distinguishable. In a sense then the historical norm continues today through the culmination seen in modern humans, a great combination of all previous human species!
Returning to climate change, for the past 11.7 years or so the Earth’s climate has been relatively stable but it has NOT always been that way. In fact, historically our climate has been one of great and sometimes sudden change. Further, we tend to take climate as a sort of backdrop especially in terms of human evolution but the fact is climate is a MAJOR factor in evolution not only in terms of humans but all other living things. Climate has shaped our planet and it has shaped us an it continues to do so even today. Finally, more and more studies are being done in the field of paleoclimatology and that is a good thing as we can perhaps learn from past climates and discover just what important rules they played. And these studies will give us clues of what we might expect in the near future that our survival may well depend on.