If we are going to learn about the Earth, the best way to do it is to start the story
from the beginning.
We want to know about the history of the Earth, from its formation until present day, so that
we have the proper context as we learn about all kinds of geological structures throughout
this series.
As for the origin of the solar system and planetary formation in general, that is covered
over in the astronomy series, so we won’t review that information here.
Instead, let’s just start at the precise time period that Earth first formed, which
by our best estimates is around 4.57 billion years ago.
How do we get from then to now?
First, some terminology.
In order to understand the timescales that Earth operates on, geologists came up with
a way to break down the 4.57 billion years of Earth’s existence into smaller chunks
called eons.
There are four eons.
These are the Hadean, Archean, Proterozoic, and Phanerozoic.
In the earliest days of Earth’s history, from 4.54 billion to about 4 billion years
ago, Earth would have been unrecognizable to us.
It was a hostile place with no continents, ubiquitous active volcanism, and frequent
asteroid impacts.
Given this description, it seems appropriate that this eon, the Hadean, is derived from
the Greek word, Hades, meaning the underworld, which is a fitting comparison to this hellish
landscape.
The frequent asteroid impacts were a result of the solar system being so very young, with
huge numbers of large rocky bodies that had not yet been incorporated into planets or
ejected from the system.
A particularly notable event is believed to have occurred 4.4 billion years ago, when
a Mars-sized protoplanet named Theia slammed into Earth, creating a worldwide magma ocean,
and producing the tilt in Earth’s rotational axis in the process.
Though Earth and Theia mostly coalesced, the impact caused parts of the crust and mantle
of both planets to be ejected into space, generating a ring of debris which collected
to form our Moon.
So interestingly, the moon is partly made of the Earth.
After the collision, Earth began a slow journey toward more calm conditions as proto-continents
and oceans began to form.
However, conditions were still extremely harsh due to constant asteroid bombardment, which
would continue until around 3.8 billion years ago.
This likely hindered any development of life during this time.
No rocks of Hadean age exist in the rock record, simply because older rocks are more likely
to be destroyed or altered by geologic processes.
The only surviving terrestrial material from this time are zircon crystals, which are durable
and can survive processes like subduction and weathering.
This makes the Hadean eon the most mysterious time in Earth’s history.
The next eon, the Archean, lasted from 4 billion to 2.5 billion years ago.
One of the most important events in the Archean was the tapering-off of heavy asteroid bombardment
and the subsequent emergence of life.
The oldest terrestrial rocks, as well as fossils, come from this eon.
The oldest fossils are 3.48-billion-year-old stromatolites, which are the rocky remnants
of microbial mats formed by cyanobacteria living in tidal flats.
The abundance of these fossils suggests that by the Archean, single-celled life was already
flourishing on Earth.
Another main event of the Archean was the formation of large continents.
Remnants of these ancient continents are called cratons and are found mainly in Canada, Africa,
and Australia.
The cratons formed via accretion of many smaller microcontinents as they collided along ancient
subduction zones.
This is essentially the same process that caused India to collide with Asia to form
the Himalayas, just on a much larger scale, and which will eventually cause North America
to collide with Asia, hundreds of millions of years in the future.
This is a good time to mention that Earth’s crust, or outer layer, is broken into rigid
chunks and pushed around by the convection of the layer beneath it, the mantle, which,
even though it is made of solid rock, can flow like a viscous liquid under the intense
pressures and temperatures found deep within the Earth.
We will talk more about continental drift and plate tectonics later in the series.
By the end of the Archean, Earth looked more or less like it does today, having oceans
and large continents with rivers and lakes, but in other ways it was still very different
from the Earth we know.
The Earth’s atmosphere, for instance, had 100,000 times less oxygen than the air we’re
breathing right now, instead containing large amounts of carbon dioxide and methane.
Life was also exclusively unicellular and would continue to be so for at least an additional
billion years.
The next eon, the Proterozoic, lasted from 2.5 billion years ago to 541 million years
ago, and began with a gradual increase in atmospheric oxygen, which was generated by
newly evolved cyanobacteria with photosynthetic capabilities.
This oxygen was absorbed by the oceans at first, which acted as a buffer preventing
a rapid increase in atmospheric oxygen, but the oceans eventually became saturated with
oxygen, allowing atmospheric oxygen levels to rapidly soar, increasing by a factor of
100.
Banded Iron Formations, or BIFs, are marine deposits from this period and are characterized
by thick deposits of iron oxides which precipitated from ocean water as oxygen concentrations
increased.
BIFs were not formed during any other period of Earth’s history.
The rapid increase in oxygen in both the atmosphere and oceans caused a mass extinction of obligate
anaerobes, which are organisms that are poisoned by oxygen.
This event is called The Great Oxidation Event, or perhaps from the perspective of the anaerobic
organisms that abruptly died, the Great Oxygen Catastrophe.
Additionally, the skies and oceans shifted from orange and green-tinted to the blue we
see today.
The evolution of unicellular eukaryotes occurred during the Proterozoic.
As we learned in the biology series, these are single-celled organisms containing nuclei
and other highly specialized organelles like mitochondria and chloroplasts, through a process
called endosymbiosis, where certain prokaryotic organisms were incorporated into others, with
organelles like mitochondria and chloroplasts being derived from the “eaten” prokaryotes,
so to speak.
The first multicellular life, and even animals, also evolved during the Proterozoic.
The oldest indisputable animal fossils are from 570 million years ago and are called
the Ediacara Fauna.
Some examples of these primordial fauna are Charnia, a frond-like animal, Spriggina, a
worm-like animal, and Kimberella, a mollusk-like animal.
The large diversity of animal fossils present in these rocks indicates that animals were
rapidly evolving as nature experimented with different body designs.
The Proterozoic was also a time of supercontinents.
Converging continents collided around 1.8 billion years ago to form the first supercontinent,
called Nuna.
However, around 1.5 billion years ago, convection currents in the mantle changed direction,
causing Nuna to break apart, or rift.
The assembly and later break-up of supercontinents is a cycle that was to repeat an additional
two times in Earth’s history with another cycle occurring later in the Proterozoic with
the formation and breakup of Rodinia between 1.1 billion and 750 million years ago.
During rifting, a large chunk of Rodinia called Pannotia, broke off and moved over the south
pole, which greatly affected Earth’s climate, triggering a massive ice age as glaciers formed
over the south pole and expanded northward.
The glaciation peaked 100 million years later in a “Snowball Earth”, where most of the
planet’s surface was covered with ice.
Average global temperatures dropped to negative fifty degrees Celsius, only about 10 degrees
warmer than the average temperature of modern-day Antarctica.
These first three eons we just discussed, the Hadean, Archean, and Proterozoic, are
commonly grouped into one super eon called the Precambrian.
The end of the Precambrian, which was 541 million years ago, was marked by an incredible
diversification of life called the Cambrian explosion.
This explosive event was a continuation and acceleration of the rapid evolution that was
occurring during the Neoproterozoic.
Every major animal phylum that we know of today, and are learning about in the zoology
series, can be traced back to these early, intricate life forms.
Bright green algae flourished, armored arthropods like trilobites roamed the ocean floors and
jawless fish resembling giant, shiny worms swam through the water.
Then the Phanerozoic eon began.
As this is the eon we are still living in today, let’s move forward and take a closer
look at this next.