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 what’s a fact and what’s opinion. That’s why I’ve
found Ground News, the sponsor of today’s video, to be so useful. Ground News is the world’s
first news comparison platform. Sadly, most news outlets don’t present you with a
nice rating score about how factual they are, or their political leanings. But when
I open a story on the Ground News app, I’m met with a factuality rating and political
spectrum breakdown for articles collated on their platform. If you’re interested in erasing your
news blind-spots, and understanding the context of your news, why not click my link in the
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you’ve seen here today? Go give it a try! Thanks for watching. Want more about
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All the best and see you next time.
