History of the Earth Part 4: Phanerozoic Eon – Cenozoic Era

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.