Earth is the place we call home. It’s where we work, eat, sleep and go about our daily lives. But
how well do we really know it? We like to think of Earth as the “Blue Marble,” a stable, temperate
world hospitable to life, but that’s really just a snapshot of a dynamic and evolving planet.
If we zoom out on the cosmic timeline, we discover that our
home would have been unrecognizable to us at most other times in its history.
So, let’s see if we can reconstruct what Earth might have looked like in the distant past.
Let’s imagine that alien scientists, who have never seen Earth as it is today,
visited at various stages in its development. Depending on when these visitors arrived,
they would have formed completely different ideas about the kind of planet Earth was.
What would they have seen 4.5 billion years ago? 2 billion years ago? Half a billion years ago?
I’m Alex McColgan, and you are watching Astrum. Join me today as we recreate Earth’s ancient past
and imagine what the planet might have looked like at various points in time.
Now, before we start, a little disclaimer: to answer these questions, we’ll need to draw on
some models that not all our scientists agree on. Travelling into the extremely distant past
always carries some error bars. Some of the science, we’re pretty sure about. Other things
are still being debated. So, as we indulge our imagination, let’s keep in mind that some of
these claims are still being developed, and rigorously questioned – as they should be!
But to the best of our current knowledge, this is what our planet could have been like.
As long as we’re going into the past, we might as well go way back. Let’s start 4.5 billion years
ago. It’s not the very beginning but pretty close to it. At this time, the Earth is basically a hot,
viscous ball of molten rock, and I doubt our alien visitors will want to hang around for very long.
The Earth is still young, not even 100 million years old yet, which in planetary terms is an
infant. Earth’s thin atmosphere is made mostly of hydrogen and helium, most of which is stripped by
the solar wind, since the Earth hasn’t yet formed a magnetic field. As a result, the young planet
has little protection from the Sun’s cosmic rays, and the planet is a hotbed of radiation. Earth is
also constantly being bombarded with asteroids and comets, which add to its mass – a process called
accretion. Just imagine a big ball of chewing gum that you keep adding to with fresh wads of gum
and you’ll get the idea. So, not only is the young Earth hot and gooey: it’s also growing! But these
are not the only impacts the planet has to contend with. In the recent past, there was a cataclysmic
event in which a protoplanet collided with the Earth, resulting in the formation of our Moon.
If our alien visitors were to see Earth in this state, would they see its potential? I’m unsure
I would, at this stage. Despite these ominous beginnings, the seeds of change are taking root.
Riding aboard these inbound asteroids is a special compound that will play a crucial
role in Earth’s history – water. While not all of Earth’s water comes from these meteors,
scientists believe much of it does. According to this theory, hydrogen ions in the solar
wind impact these meteors, ejecting oxygen atoms under their surfaces, thereby producing water.
These frequent impacts seed much of the young planet’s water.
Then, as the planet cools, heavier elements sink towards Earth’s centre, and lighter compounds,
such as water, rise to its surface. Volcanic eruptions spew other gases like carbon dioxide,
ammonia and additional water vapour into the atmosphere. Eventually, all of this accumulating
water vapour will lead to the formation of Earth’s oceans and its mature atmosphere.
So, even if Earth is a dangerous place to be at the moment, our alien visitors might see some
untapped potential and come back to Earth once it’s had a bit more time in the oven.
Let’s imagine that the scientists return to Earth 2.5 billion years ago, to see if anything
has changed. Two billion years have passed, and instead of a molten planet, they find what our
scientists call the Pale Orange Dot. Earth has developed a solid crust and a magnetic field,
which has retained a methane-rich atmosphere with a distinct pale orange haze – very much
like Saturn’s Titan. Earth also has vast oceans of liquid water formed from rainfall. Looking upward,
our alien scientists see a terrifyingly huge moon overhead; far closer than it is today.
(Here’s a fun fact: the Moon has slowly been drifting away from Earth for the past 2.5
billion years and is still moving away from us at the rate of 3.8 centimeters per year. This
drift will eventually stop, but not for another several billion years!) But 2.5 billion years ago,
the Moon is much, much closer, and this proximity leads to far stronger tidal forces in the oceans.
While this snapshot of Earth is probably more Earth-like than the previous snapshot, to me,
it almost feels more alien than before. Earth must have looked like a barren dreamworld,
a lot closer to a surreal Salvador Dali painting than the teeming planet we know today.
Being experienced in the field of astrobiology, our alien scientists take some blue-green water
samples and find the cause of this orange haze – photosynthesizing bacteria called cyanobacteria.
They are excited to find that Earth has developed life! These primitive bacteria are
living in communities in shallow sea water, which in turn have released oxygen into the atmosphere
in an event called the Great Oxidation. As part of this transformation, the Earth now
has a protective Ozone Layer, which shields life from the most harmful effects of solar radiation.
The bacteria are also producing unusual rock structures called stromatolites,
similar to these modern ones located in Western Australia’s Shark Bay.
Cyanobacteria form stromatolites by cementing grains of sediment together with biofilms,
or, to put it another way, microbial slime. Stromatolites are one of the biosignatures
that astrobiologists say we should be looking for on other planets. (Although, they can also have
nonbiological causes so you have to look at them on a microscopic scale to be sure.)
Without any competitors, the cyanobacteria reign supreme. Using solar energy, they are converting
carbon dioxide and water into nutrients, seeding the atmosphere with oxygen as a byproduct.
Bacteria are now the uncontested rulers of planet Earth, and they have permanently
altered their atmosphere in ways that make our current way of life possible.
Enthusiastic for Earth’s future, our alien scientists decide to come back to Earth 650
million years ago. And boy, what a difference 1.9 billion years makes! The planet is now
in its Cryogenian Period, at a time scientists call Snowball Earth. According to these models,
Earth is completely frozen over. Think of the last Pleistocene ice age, only a lot colder. In fact,
all of the Earth’s surface and half of its total ocean water are frozen solid. The Earth has become
so cold that temperatures at the equator are similar to those in modern-day Antarctica.
While the scientists aren’t sure what triggered this downshift in global temperatures,
some theories include a major volcanic eruption that spewed ash into the atmosphere,
a vast reduction in greenhouse gases due to photosynthesizing lifeforms and Milankovitch
Cycles (which we’ve previously covered on this channel). Of course, it was quite likely a
combination of factors that amplified this global cooling. Interestingly enough, underneath all this
glacial ice is a single supercontinent called Pannotia, centered on the South Pole. However,
given the vast glacial cover, our alien visitors are having trouble discerning land from ocean.
(As a side note: don’t confuse this supercontinent with Pangaea. Plate tectonics will eventually
break apart Pannotia and reform that later supercontinent in a few hundred million
years. But let’s not get ahead of ourselves!) Our space visitors land and wander around the vast,
endless glaciers, amazed that this frozen landscape was once covered in molten rock.
At first, they think the harsh climate might have wiped out Earth’s fledgling life, but that life
has proven remarkably resilient. Cyanobacteria remain in the ocean, perhaps clustered around hot
hydrothermal vents, much like the black smokers that are currently at the Mid-Atlantic Ridge.
There are also newly evolved microbial lifeforms, such as red algae.
Complex multicellular life remains elusive, but the fact that life has found a way to
survive the planet’s harshest freezing event so far is a testament to its staying power.
At this point, I’d like to imagine our alien visitors having an argument amongst themselves.
The youngest and most temperamental of the group is having a tantrum.
“First a lava ball, then a barren bacterial playground, and now a ball of ice!” it exclaims.
“Why do we keep coming back here?” But the eldest of the group puts an arm, tentacle or
articulated limb of our preference around its younger comrade. “Patience,” this wise alien
tourist says to its impertinent partner. “I have a feeling this planet is in for some major shakeups.
Let’s come back in another few hundred million Earth-years. I think it will be worth the trip!”
So, giving the planet time to change, our alien scientists decide to come back
during the Devonian Period 360 million years ago, and they are shocked by what they see.
The planet has thawed completely, with temperatures generally warmer than those
of the present day. Scientists estimate that tropical sea surface temperatures range from
30 degrees Celsius to 21 degrees Celsius later in the Devonian – a temperature drop that coincides
with diminishing CO-2 levels due to increasing plant growth. The supercontinent has broken up
and formed multiple continents, the largest of which is Gondwana, covering the South Pole
and much of the Southern Hemisphere. Gondwana incorporates much of what is modern-day Africa,
South America, Antarctica, Australia and India, so it’s pretty big! But the most dramatic change our
visitors notice is that life has already undergone the Cambrian explosion – a rapid expansion of
biological diversity that filled the seas with all sorts of complex life. The oceans are now
teeming with trilobites, clam-like brachiopods and complex marine vertebrates, such as fish. Among
the more fearsome specimens are Dunkleosteus, a massive, armoured fish 10 meters in length,
and Titanychthus, another giant with a taste for smaller prey, like krill-like zooplankton.
Given the presence of these fish that look like great white sharks crossed with armoured
submarines, our alien scientists decide not to go for a swim despite the invitingly warm waters.
Meanwhile, plants have completely transformed what was once barren continental crust. With no
large herbivores in existence, vegetation grows unchecked into dense and sprawling forests, which
have produced a layer of stable and nutrient-rich topsoil. But these trees would look very unusual
to us. They are vascular plants, related to today’s ferns and some connifers. There is also
an enormous treelike fungus, called prototaxites, which stands some 8 meters tall. Very likely,
these mysterious tree-like structures are fruiting bodies of far larger subterranean organisms,
which haven’t been preserved. A few marine species have even evolved limbs and are beginning to walk
on land, such as ichthyostega, a rather charming four-legged vertebrate that looks a bit like a
modern amphibian, whose stout limbs and lungs allow it to navigate swamp-like habitats.
As our alien visitors leave the Devonian Period Earth, they finally have a sense of the ecological
diversity and temperate climate that will follow. The once-molten-planet-turned-Pale-Orange-
Dot-turned-giant-snowball now somewhat resembles the Blue Marble we know today.
Of course, other big changes are yet to happen. A mass extinction event will soon decimate
marine life and eventually lead to greater complexity among the land animals that adapt.
Temperatures will again plunge, setting off the Late Paleozoic Ice Age, before rebounding to
warmer temperatures again. And complex life will continue evolving in remarkable ways.
Among the new species will be amphibians and some giant reptiles you might have heard about
called dinosaurs. I wonder when our alien scientists will visit next, how far in the
future will it be? Will they meet us? Will they exchange some intergalactic travel tips with our
future descendants? I guess we’ll have to leave that chapter of Earth’s saga for another day.
I hope you enjoyed this journey of planetary evolution. Are you interested in hearing more
about Earth’s past in future episodes? If so, let me know in the comments.
When we’re learning about things that happened millions of years ago, it is inevitable that we
are confronted with a mixture of facts and theories – scientists’ best guesses about
what conditions were like. But when I’m learning about the world today, I don’t want confusion over
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Thanks for watching. Want more about Earth? Check this playlist here. A
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check the links in the description. All the best and see you next time.