At this point we have covered the Hadean, Archean, and Proterozoic Eons, and we are
working our way through the Phanerozoic Eon. We made it through the Paleozoic and Mesozoic Eras,
which brings us to the third and final era of this eon, which is called the Cenozoic Era.
The Cenozoic Era is the period in Earth’s history spanning from 66 million years ago to the present.
The Cenozoic Era is called the “Age of Mammals” due to their rapid and intense
radiation, thanks to the end-Cretaceous mass extinction which killed off the dinosaurs.
The early mammals were small creatures, ranging from the size of small rodents to medium-sized
dogs. The first primate, Altiatlasius, evolved around 57 million years ago in North Africa.
Primates, like other mammals, experienced rapid radiation in the Cenozoic Era.
Types of primates include lemurs, gorillas, chimpanzees, and humans. The first member of
the human genus, Homo, evolved around 2.4 million years ago from Australopithecus. Homo sapiens,
or humans, emerged between 200,000 and 300,000 years ago in Africa.
But this fascinating story will have to be saved for the upcoming anthropology series.
By the Cenozoic Era, most of Earth’s continents were near their current locations,
except for India. The Indian continent was making a beeline toward Asia during the early Cenozoic,
with collision and orogeny occurring around 50 million years ago.
This event initiated uplift of the Himalayan Mountains and is still occurring to this day.
Additional smaller changes in geography greatly impacted the Cenozoic climate. For example,
Antarctica became an isolated continent around 34 million years ago as Australia and South America
broke away, moving northward. This led to the development of a cold ocean current around
Antarctica, helping trap colder air over the continent. The development of the so-called
circumpolar current led to global cooling and the development of the first permanent Antarctic
ice sheet, marking the beginning of the modern ice age. The global cooling was caused by the
development of a robust oceanic circulation, where the cold, dense water of the circumpolar current
sinks down and spreads out. This led to the cooling of the lower ocean, which set the stage
for more atmospheric CO2 to be stored as sediment on the ocean floor, reducing greenhouse warming.
The next major change was the development of glaciers in the Northern Hemisphere,
which existed by around 3.2 million years ago. The cause of this is still debated,
but it was probably related to the closing of the Isthmus of Panama,
which occurred around the same time. The Pacific and Atlantic Oceans used to be connected where
Panama is today, which allowed their waters to mix. Since the closing of the isthmus,
the Atlantic Ocean has become much saltier, which initiated a reorganization of ocean circulation.
Gulf stream waters stopped circulating through the Arctic Ocean, instead sinking to the seafloor
around Greenland and spreading out. This shut off the transport of warm water to the Arctic Ocean,
causing the surrounding regions of Greenland, Canada, and Northern Europe to cool and develop
permanent ice sheets. We have been living in this northern hemisphere ice age ever since.
Within an ice age, there are periods of time called glacial periods where glaciers expand
and interglacial periods where glaciers retreat. Fluctuations in climate due to
changes in Earth’s orbit are responsible for glacial cycles. These changes in Earth’s orbit,
called Milankovitch Cycles, include precession, or the change in the direction that Earth’s
north pole points, obliquity, the change in the angle of Earth’s axis,
and eccentricity, the change in the ellipticity of Earth’s orbit.
They operate on timescales from tens to hundreds of thousands of years.
We are currently in an interglacial period with the prospect of expanding glaciers in sight,
at least according to the Milankovitch Cycles. We are instead barreling toward a subtropical
arctic with no ice sheets, due to the global warming caused by burning fossil fuels.
More on glaciers later in the series. So with that, we are now roughly familiar
with the timeline describing the history of the Earth. This will be useful moving forward,
as we will be referencing structures that originated very far back on this timeline,
so with this understood, let’s move forward and start learning about these structures.