This episode is sponsored by Brilliant We often say humanity's future is out among
the stars, but what if humanity's past was too?
Back in the 17th century Nicolas Steno, a Danish scientist with a heavy interest in
anatomy and geology, and later a Catholic Bishop, noted that there was a connection
between the age and depth of fossils.
This gave us the Law of Superposition, that the higher layers of geological strata are
younger than the layers they sit on, and the Principle of Original Horizontality, that
layers of strata form as thin horizontal sheets.
That seems incredibly obvious to us nowadays but was quite the conceptual revolution at
the time.
It sparked a bigger one, as the sheer number of layers began strongly implying the planet
we dwelt on was a lot older than we thought.
A world predating human history by eons makes sense now, but at the time it was beyond absurd.
Needless to say, this vastly older world and universe being unveiled set the stage for
Darwin, as we could start seeing connections between fossils and modern life, converging
back to some original but different critters.
This actually caused a bit of a problem since around this same period we were beginning
to get microscopes and realizing that there were a lot of very small organisms and that
many others were not appearing effectively by spontaneous generation or abiogenesis,
like bees arising from flowers, flies from rotted meat, and so on.
We were getting rid of the idea that life could arise spontaneously from inorganic matter
while at the same time showing a historical convergence back to some original and simple
life, that would have had to do so.
We could hypothesize that all life emerged from a single simple original lifeform in
the distant past.
But that raises the question as to where that life form came from?
What caused that original act of abiogenesis and where did it take place?
To this day we still don't know.
We figure it needs to be a place with a decent chemical soup of the right materials and an
energy flux to power it all.
Tidal pools were popular, and underwater thermal vents are too, and those two basic options,
on which there are lot of variations, vie for position as the lead candidate.
There are many other positions but perhaps the best known of these is Panspermia, the
notion that life did not originate on Earth at all.
Now, as this is our topic for the day, inside the wider umbrella of the Fermi Paradox, to
which it is inextricably intertwined, I should note Panspermia has quite a few variations
too, and that it is indeed an entirely solid basic theory and in fact very easy to prove;
some versions are not only valid models but demonstrably correct, which I'll explain
in a moment.
Panspermia has often been associated with some fairly fringe theories and some versions
of it are indeed rather fringe, but those give it an unjustified bad reputation.
I also want to note that while we're interested in how life arose on Earth, whatever happened
here is not necessarily what happened elsewhere.
The probabilities for each method producing original life are unknown and would vary from
planet to planet.
So if life arose here in tidal pools it wouldn't mean that a planet with a smaller moon but
more tectonics might not have had life arise by thermal vents.
Similarly, we have no idea what the odds of spontaneous generation are.
They really could be so small that the odds of it happening on any planet in the galaxy
in a billion years are next to zero, or so high that it happens almost everywhere.
One method might be whole orders of magnitude more likely than others, so that it's effectively
the only realistic way, or they might be close enough that, say, tidal pools were more likely
than oceanic thermal vents on Earth but the dice came up for thermal vents anyway.
We might end up concluding it was the former off a model showing it 100 times more likely,
only to later find out that even so the less likely one did happen first.
And first is all that matters, because once the ball starts rolling even though spontaneous
generation could occur again, that first life will have the advantage if it can migrate
to the various niches where the other form can spontaneously occur before it has and
taken root.
Once there it will out compete any simplistic life that might arise, on that planet anyway,
and again how it happens on one world might not be the same as on another.
And once more, only the first time mattters, Earth could easily have had life arise in
its most basic form via multiple methods many, many times.
Except with panspermia though, since that tends to assume planets get infected with
life that started out in the wider void, and I mentioned a moment ago we can confirm this
theory is at least partially correct.
We have a notion called Soft Panspermia, which is itself rather variable, and it's less
a question of if it's true as it is to what degree.
For planets to develop life for instance, they must have certain chemical elements,
and enough of them to produce a decent concentration for a primordial soup, and most of those did
not exist in the Early Universe.
Giant stars exploding and seeding later generations with heavier elements is an example of Soft
Panspermia.
Giant stars detonated and spread heavy elements over many existing and future star systems,
creating the soil in which the seed of life could originate.
This is not the full extent of Soft Panspermia though, because we noticed about half a century
back that interstellar dust contained a lot of organic molecules.
Now that can be a bit confusing to folks sometimes who are a bit vague on what organic molecules
are.
They're defined as molecules including carbon, though not all carbon-based molecules count.
Carbon monoxide and carbon dioxide, for instance, aren't considered organic, which is interesting
since carbon dioxide is so heavily wrapped up in biological processes, and carbon compounds
like diamond or graphite aren't considered organic either.
Needless to say, this all assumes carbon-based life.
Silicon-based life would technically be inorganic, and there's obviously a bit of tautology
in the definition.
Some are trivial too.
For instance the first we detected was way back in the 1930s and was simply carbine,
which can be nothing more than a single hydrogen and single carbon atom bonded together.
Such a molecule is highly reactive, so would not last long on a planet where everything
is stuffed together to react with, but in the interstellar medium it can last quite
a while with everything spread out and we believe now that it is created by ultraviolet
light from stars, which they're not shielded from like we are, with our thick atmosphere
blocking most UV from reaching us.
We've since detected a lot more elaborate molecules than that like formaldehyde, pyrene,
and even buckyballs, one of those inorganic carbon molecules and one that interests us
heavily for nanotechnology, but which some have been suggesting might be vital to forming
life.
Another thing common in space is also water.
Indeed it's incredibly plentiful, and we have good reason to think all of ours on Earth
is extraterrestrial in origin.
The current theory posits that Earth's atypical large molten core and moon both resulted from
an earlier, slightly smaller Earth being hit by another planet, Theia, jettisoning vast
amounts of materials into space, some of which coalesced into our moon.
But any atmosphere or oceans we had at the time would have been gone.
Indeed the Earth spun a lot faster then.
The day was perhaps 12 hours long, and the Moon was a good deal closer to us.
With a bigger core comes more tectonics.
With a big close moon and short days, more tides, enhancing both the tidal pool and thermal
vent theories of abiogenesis.
But there would have been little atmosphere or ocean back then, and we think it was regenerated
by comets and asteroids hitting us, in what is called the Late Heavy Bombardment, comets
that would have contained more than just water.
We'll come back to this in a moment but let's talk ice and water first.
In the early universe there wasn't much more than hydrogen and helium, though there
were still trace amounts of other matter.
In the very early universe though space was not this big cold empty place we think of
it as nowadays, about 14 billion years after the Big Bang.
About 14 million years after the Big Bang, the whole universe was smaller and warmer,
about the temperature of a warm bath, and again it also had water.
It's been suggested that life could have originated all the way back then.
Things were warm and tight, and you could have had immense spheres forming with trace
heavier elements scattered throughout or even clumping up.
This is sometimes called the Habitable or Bathtub Epoch, a period of several millions
years, and indeed likely a bit longer since any large clumps of matter would have had
gravitational heating and cooled more slowly.
There would have been little heavy elements, of the six primary organic elements, Carbon,
Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur, or CHNOPS, only hydrogen was common.
The CNO isotopes are thought to have been only about one quadrillionth as common as
hydrogen, and Phosphorus and Sulfur would have been even less common.
Of course that's not as small as it sounds, since all of these still only make up a trace
amount of our universe and places like Earth obviously exist, where hydrogen is not that
common and helium is quite rare.
While the universe cooled a lot before the first stars formed over a hundred million
years later, it's conceivable that some pockets of matter from that bathtub epoch
might have stayed warm enough to cuddle a kernel of early life till those first stars
went supernova, which would have been quite quick, and one can imagine them absorbing
materials and energy from such blasts and even being scattered in some cases.
Clumps of life frozen inside icy bodies wandering space to land on early planets.
For that matter, one version of Panspermia known as Radiopanspermia notes that very small
particles, those of the micrometer scale, can be pushed at high speeds between solar
systems by radiation pressure, and though we do have organisms in that size range, they
wouldn't have any protective sheath against radiation damage if they were that small.
This is one of the oldest Panspermia theories, dating back to 1903, before we had special
relativity, any clue how stars worked, or even what our galaxy was, and I have to say
that while the idea is neat, it's not terribly likely.
While this Bathtub Epoch is a more modern concept that would potentially put panspermia
far sooner in the age of the Universe, it's also probably not very likely.
However, it does take us into one of the primary panspermia concepts, which states that very
simple life formed on comets and those comets transported such life to the Earth by crashing
into the Earth's surface.
The primary panspermia concept asserts that actual life, not merely organic precursors
of life, formed on these comets.
Such comets, carrying both water and these life forms, would have gradually built an
ocean on the Earth and also would have seeded them with life.
Thus, this concept lets us kill two birds with one stone by explaining, both, where
the Earth's oceans came from and also how life got here.
However, the Bathtub or Habitable Epoch in the early Universe does raise one key problem
often expressed with Panspermia, which is that it isn't so much a first cause as punting
it back from primordial Earth to a more distant time and place.
That's not really a valid objection, and there are mechanisms for life forming on comets
we'll get to momentarily, but I suspect it is part of the reason the idea is disliked,
and it is a valid objection when we discuss Panspermia variations like Earth being a colony
of an older alien civilization.
We discussed the problems with that in the Ancient Aliens episode, but they don't apply
to simple organisms being birthed in cosmic dust and landing on Earth, moving the location
where life first emerged to comets isn't any stranger than moving it to deep sea thermal
vents that migrated to higher regions of land and sea, in that regard.
And yet we have a very obvious way to get a chemical soup and power source on a thermal
vent, and in the mixing and draining and refilling of tidals pool.
What about comets?
First, we need to consider volume, or for comets, more surface area.
Comets were much more common in the past when the solar system was younger and more cluttered
with objects that hadn't merged with each other yet, and the Kuiper Belt, a vast body
of icy rocks out past Neptune, vastly dwarfs our Asteroid Belt in size and mass.
Though both are small compared to Earth's mass, we believe the Oort Cloud even further
out may dwarf Earth in mass, and more importantly, Earth's mass is irrelevant to the equation.
You can't calculate probabilities for life off the volume of the Earth or even its oceans,
rather you're talking about that tiny volume in tidal pools on the coast or surrounding
a thermal vent before growing too dilute in the ocean around it.
There's not that much water around a vent or in a tidal pool compared to the ocean,
10 quadrillion liters is the higher end figure I've seen for this possible soup volume
and I suspect that's a few order of magnitude too generous, but is still only the volume
of a single fairly large comet, and we guess at a total cometary volume about a trillion
times higher than that.
A typical cometary body is quite rich in organic compounds and minerals, not just water ice.
As to energy, they might have radioisotopes in them, those were vastly more common in
the early solar system, too.
Pockets of radiation warmed soups could easily pop up inside such things, and they'd get
sunlight, albeit less than Earth.
However it's actually their surface that interests us, and you get a lot more surface
area out of billion small spheres than a single one of equal mass.
For instance a single comet of 2 kilometers radius has the same volume as eight comets
of 1 kilometer radius, but only has 4 times the surface area of any one of them, they've
got twice the total surface area.
One 3 kilometers in radius has only a third the surface area of the 27 single kilometer
radius comets it would take to match its volume, and so on, so that a 100 kilometer wide icy
moon has only a hundredth the surface area of the million one-kilometer comets that would
match its mass.
And the nature of small body formation is that the small ones are a lot more frequent
than the large ones.
On those surfaces, icy though they are, you get some interesting chemistry.
Liquid water can't exist in a vacuum, it goes straight from ice to vapor, producing
beautiful comet tails, but on that edge you can get quite a lot of reactions going on,
and of course here on Earth we have plenty of examples of bacteria living just under
the surface of ice.
Of course you'd expect that surface to burn off as the comet approaches the inner solar
system and to be vaporized as it approached Earth, two quick caveats though.
First, while comets often lose several tons of matter per second when they get as close
to the Sun as the Earth is, this is actually only in the range of millimeters of depth
per day, and a quantity no harder for life to adapt to, by growing deeper as evaporation
occurs, than it is as water evaporates here on Earth.
Algae and pond scum aren't troubled by the water they float on evaporating away a little.
Second, while we have been able to determine that some paths would allow a body to enter
Earth's atmosphere and allow life to survive the trip, it's worth noting that those comets
wouldn't initially have been falling through as thick of an atmosphere, again it was stripped
off in the event which formed the Moon and was regenerated by the comets, so the early
ones wouldn't have burned as much.
This of course only applies to things on the outer skin, on larger objects the deep interior
would survive fine.
In terms of atmospheric entry, we have quite a collection of meteorites that not only survived
to hit the ground but survived being expelled from another planet such as Mars to impact
here.
Which raises the idea again that life, even if it didn't evolve in space and land here,
could have evolved on some place like Mars, Venus, or Europa, all of which have may have
held life in the past or even may now.
Indeed for icy moons with low gravity and subsurface oceans, it would be fairly easy
for a collision to send ice cubes laden with bacteria our way.
It wouldn't even necessarily require a collision event either, we've found microorganisms
on the outside of the space station that floated up there.
This approach, transplanting from another planet, is typically called lithopanspermia.
Again, on first glance this seems like just moving the goalposts, since life had to originate
there, it hardly solves the abiogenesis issue, but we don't know what the conditions for
abiogenesis were or that they might not have been easier on another planet.
We should note that such trips are hardly easy, and realistically are limited to very
simple life.
When I was younger everyone always talked about how tough cockroaches were and how they
might be the only life to survive an atomic conflict, which exaggerates both the sturdiness
of cockroaches compared to other life and the destructive capacity of nukes, and it
seems like something similar has happened in recent years with tardigrades, which are
tough little critters against things like radiation but hardly indestructible.
It wouldn't be very likely anything that big – and they're quite tiny but still
big compared to bacteria, could survive a multi-century voyage on ice, let alone the
tens of thousands of years involved in interstellar panspermia scenarios, but we can't rule
out life forms of that complexity making voyages.
We can for Earth though, since we can track our fossil record back to microorganisms,
so any Panspermia scenario for Earth doesn't involve anything more complex than that landing,
and likely simpler.
With one exception, there's many ways stuff can land on Earth and one is obviously in
a spaceship.
We clearly weren't colonized by aliens, again see the Ancient Alien episode for the
problems with that, but if travelers visited an early Earth and sneezed or dumped their
garbage or empties their sewage tanks, those various microorganisms contained inside, while
unable to make the trip on their own, nonetheless made the trip.
Similarly, a larger organism able to survive for long periods in space against radiation,
might die landing on Earth, but shield the microorganisms in its own guts, which could
then seed the planet.
While either is unlikely as our origin, this is sometimes suggested as a means by which
we might colonize other stars, except it has the big problem of taking billions of years
and producing nothing even as closely related to us as squid are.
There's also that conceptual issue we addressed last month in Seeding the Stars, people get
this image of firing a missile off that can impact on a distant world, but they have to
be able to slow down.
It's one thing to contemplate some bacteria surviving atmospheric entry and impact occurring
at interplanetary speeds, tens of kilometers per second, but interstellar speeds are tens
of thousands of kilometers a second and millions of times more energetic.
Nothing biological is surviving that, so you still need to slow down and it would seem
like you'd then proceed via more typical colonization methods.
Even if you could come up with some trick for accelerated and directed evolution, which
is arguably an oxymoron, you're still in for a very long wait, and you'd expect follow
up colony missions to arrive long before much had happened.
One interesting caveat to that though, before we close out by discussing Panspermia's
implications to the Fermi Paradox, is abandoned worlds.
It's quite likely a lot of planets that are marginally in the habitable zone of their
system have frozen over at some point, indeed it's likely Earth has at least once too,
the Snowball Earth Hypothesis, where plate tectonics and volcanism came to our rescue.
If you were planning to colonize such a planet, using some of the methods we've discussed
in the Generation Ships series, one of the more likely scenarios is that you'd be sending
in a small vanguard of probably automated machines to build mirrors to make Stellasers
to slow down your main fleet without it needing to use a lot of fuel, allowing more cargo
and faster ships.
The secondary use of those, in any system with a cold but viable planet in it, would
be to start thawing that planet out and try to search for life below the ice.
Such planets, if it had life, would probably have kept it, albeit but a remnant of what
it was when warmer, deep down under the sea, and now you're melting that ice covering.
Life, especially simple life, doesn't need a ton of time to spread back over a planet
and regardless, you've got an issue if you arrive at a system you thought was empty and
which has life on it.
You've also got that giant pushing laser, so you can redirect your fleet to another
system they can colonize, and since that colony ship probably has legal claims on that planet,
I could imagine it being abandoned for quite some time as people tried to deal with the
legal issues, usually a slow process even when there aren't centuries of light lag
involved on any communication, not to mention travel times.
This is an interesting in-between case for Panspermia, bordering on Uplifting, since
you discovered that life by radically altering the environment, and it could be quite complex
life too, not just simple organisms.
It also wouldn't be hard to imagine that samples might be taken, studied, and planted
on new worlds, indeed that might be fairly normal for colonial approaches like the Gardener
Ship, which we'll discuss next week.
Now, how does all this affect the Fermi Paradox?
The apparent contradiction between just how old and big the Universe is and its absence
of intelligent life?
This is one of our major points for today but also fairly quick to address.
In a certain perspective, the answer is not at all, even in versions like Radiopanspermia
which might permit vast clouds of life to seed a whole galaxy, there's no real change
in our basic assumption that intelligent, technological civilizations are rare.
On the other hand, it makes it a lot worse, especially if we are assuming interstellar
Panspermia or that life originated way back in the Bathtub Epoch, because it implies early
life is very robust against hazardous environments and will have landed everywhere by now.
This is fine under the versions of the Rare Earth theory that focus on evolutionary Great
Filters, and assume life is probably rather common but that complex, let alone intelligent,
life is not.
It's murderous though, to versions which rely heavily on initial planetary conditions
or abiogenesis being the big filters.
Remember, fundamentally the Fermi Paradox derives from Panspermia in the first place,
albeit soft Panspermia, and assumes the planets of the Universe derive their makeup from a
fairly uniform process of stellar evolution and supernovae leading us to think that Earth-like
planets, in terms of composition, age, and location relative to their own sun, are fairly
common.
This adds to that, since it means basic life or biological precursors are also pretty uniform
and spread all over the place and landed everywhere and likely regularly enough to seed any planet
as soon as it was even vaguely habitable to them.
It is also taken as sometimes meaning everyone would all be related, but that's probably
wrong.
Even if the theory were correct, more likely you'd have had a lot of separate abiogenesis
events, unlike what happened on Earth, the distance and timelines involved don't rule
out multiple origins, and regardless, the products of anything that simple would be
hugely divergent, at least as much as life on Earth is.
It's poetic, but if one just wants a common origin with alien life, we've always got
that in the Big Bang or the very soft Panspermia case of sharing supernovae.
So how likely is Panspermia?
We don't know, personally I rate it a distant third to Deep Sea Thermal Vents and Tidal
Pools but as I hope we've demonstrated today, distant third or not, it's an entirely valid
hypothesis.
To test things like that we're either going to need to get out in space and start taking
samples of the interstellar medium and other planets, or get way better with our biological
modeling.
That is a developing field and making a lot of strides, and if you're interested in
learning more about it, you might want to check out Brilliant's courses on Computational
Biology.
It's very well designed to explain biological concepts with visual presentations and analogies
tailored more to those of us with that background in math, computation, and physics.
If you'd like to learn more about that topic and others, and do so at your own pace, go
to brilliant.org/IsaacArthur and sign up for free.
And also, the first 200 people that go to that link will get 20% off the annual Premium
subscription.
So as mentioned, next week we'll be returning to the Generation Ships series to look at
what effect life extension technology might have on such colonial expeditions, often known
as Methuselah Ships, and a different approach to colonizing called Gardener Ships, where
one sends out fleets to colonize world after world, rather than a single destination, in
Galactic Gardeners.
For alerts when those and other episodes come out, make sure to subscribe to the channel
and hit the notifications bell.
And if you enjoyed this episode, hit the like button and share it with others.
Until next time, thanks for watching, and have a Great Week!
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