what if the first genesis of life the abiogenesis is so unlikely that it only
happened once in the entire galaxy and that once was not on earth
what if primitive life arrived on earth after having traveled vast distances
across the Milky Way some scientists think this may be the case this is the
panspermia hypothesis there's something odd about the first appearance of life
There's something odd about the first appearance of life on earth.
The oldest fossils are now dated to only a few hundred million
years after the moment earth first became habitable is it really reasonable
to imagine that evolution turned an unliving chemical soup into the first
true living cells in that geological eyeblink
well maybe but it's also possible that life on Earth didn't start on earth at
all perhaps it started on a distant world somewhere in the Milky Way and
somehow survived a long journey through the void to colonize the early Earth the
plausibility of panspermia comes down to whether or not any living organism can
survive it's three deadly stages ejection from an origin world traveled
to the new world the entry to that world's biosphere let's start with the
exciting parts the beginning and the end of the journey one way for a budding
microbial astronaut to travel the Stars is via Luther panspermia basically be
attached to a rock that travels between planets now we know that debris from
planetary surfaces can be ejected into space during asteroid or comet impacts
hundreds of meteorites have been found on earth that have compositions
suggesting they originated from the Moon or Mars many chunks of Earth have also
been ejected into space most of them from the largest impacts like the one
that killed the dinosaurs some of that debris would have contained life but
could hitchhiking microbes have survived that ejection to escape Earth's
gravitational field a chunk of impact debris
has to be kicked to a minimum of eleven point two
kilometers per second that's the escape velocity at Earth's surface that
requires an acceleration of up to hundreds of thousands of GS and hundreds
of thousands of Earth atmospheres in shock pressure similar forces apply when
it smacks down at the other end of the journey temperatures in these rocks can
rise to several hundred Kelvin in the impacts and during reentry that all
sounds pretty unsurvivable but scientists have engaged in various forms
of microbe abuse to test this this includes shooting high-velocity
projectiles loaded with colonies spitting them up in an ultracentrifuge
attempting to splat colonies with extreme pressure in back plates and if
that's not realistic enough microbe laced rocks have been dropped from space
strapped to the outside of reentry vehicles many life forms have been
tested from corn bacteria that you find everywhere in the soil to primitive
cyanobacteria to heat resistant extremophiles and even fungus spores and
lichens while survival rates may be low at least a small fraction of bugs of any
of these types can survive the pummeling of high pressure acceleration exposure
to extreme temperature for example in atmospheric reentry is more problematic
some extremophiles can survive at temperatures above a hundred Celsius but
the surfaces of rocks during impact or reentry will be much much hotter than
this and this is where we identify our first microbial contender for panspermia
endoliths these are organisms that live deep within rocks deep enough that
they're protected from the extremes of temperature change as we'll see
endoliths are exceptional candidates as micro cosmonauts for a number of reasons
they may also be the only critters that can actually survive the journey between
planets but before we talk about the hazards of that journey let's look at a
gentler mechanism for getting life into space a wide variety of living
single-celled organisms are found floating around in the atmosphere as
high as the stratosphere some of this material may make it into space
just this year Russian cosmonauts reported the discovery of familiar
bacterial DNA on the outer surface of the International Space Station they
suggested rowed electrical currents from the atmosphere below or it came from
space probably the former though these levitating microbes have a gentler
journey up and they may also have a faster trip to their destination very
small individual bacteria can be accelerated by their star's own
radiation and be ejected from the solar system this is Radio panspermia stars
may be constantly spraying their germy life through the galaxies in some
respects it sounds like levitating into space and becoming your own solar sail
is way more efficient than Luthor panspermia
except for one thing the vicious environment of space
probably annihilates all microbes not surrounded by a nice big spaceship made
of rock getting up and down again from space is the fun part but for a budding
panspermia life-form the journey itself you simultaneously the most boring and
the most lethal our pilgrim microbes have to contend with near absolute zero
temperatures extreme dryness a hard vacuum and probably worst of all
some incredibly destructive radiation for up to millions of years so can they
do it the best way to simulate the conditions of space travel is to send
things to space the first experiment was way back in 1936 when fungal spores were
sent on a stratospheric balloon ride since then every potential Pants
permeating life-form has taken rides on various satellites most notably the
International Space Station it's mostly single-celled organisms and spores but
also tardigrades and nematodes some of these tests were actually to test
survivability on Mars but the results Trane's
and in some the results are clear many bugs can survive the vacuum freezing
cold microgravity and absolute dryness of space but radiation is a problem
we'll come back to that when protected from radiation various bacteria fungi
lichens archaea and viruses have been revived after months of exposure to the
cold dry microgravity of the vacuum in some cases it's after six years even the
tardigrade everyone's favorite extremely philic multicellular teddy bear can
revive after exposure to space the key is cryptobiosis many life-forms
can enter or produce a hibernating form that's incredibly resistant to adverse
conditions in particular life-forms that could survive extreme dryness and hydro
biotic life-forms do especially well that includes certain reproductive
spores especially of fungi but also critters can enter an N hydro biotic
state they dry out shrivel up and wait out the bad conditions
tardigrades are an amazing example of this but the real champions are
endospores this is a hibernation state that many bacteria enter when deprived
of the needful conditions for life they generate a protective wall shrink down
stabilize their DNA and essentially shut off all metabolism they don't need water
air or nutrients and are remarkably resistant to radiation damage or DNA
decay bacterial endospores have been rejuvenated after up to six years
exposure to the cold and vacuum of space there are reports of viable endospores
found on earth and dated to millions of years perhaps even a couple of hundred
million years plenty long enough to make an interstellar journey the real
challenge is radiation solar ultraviolet radiation in space can reach a hundred
million times the UV intensity of sea level this will typically completely
destroy any unprotected microbe in a fraction
a second DNA molecular machinery and cell walls are shredded by ultraviolet
light tardigrades and bacterial spores are somewhat resistant the latter if
they are in layered colonies photosynthetic cells like cyanobacteria
and lichens can also survive direct sunlight in low Earth orbit briefly
mortality is still high in all of these like and seem to do the best with cell
surviving up to several months of exposure but probably not for years
a star's intense UV radiation probably rules out radio panspermia only the
tiniest microbes probably bacterial spores are light enough to be
accelerated by a star's light and those things are likely destroyed before they
can get far enough from their home star anyway even far from the intense UV
radiation of a star interstellar space is thick with energetic cosmic rays near
light speed atomic nuclei as well as x-rays and gamma rays endospores are
somewhat resistant to cosmic rays but there is a limit the only true
protection is a thick wall of solid material once again we're back to impact
ejecta and litho panspermia microbes buried deep enough in rock are entirely
safe from UV and safe from cosmic rays if deep enough endoliths natural rock
dwelling organisms have representatives in all domains of The Tree of Life
they are often extremophiles and their resilience often translates to the
conditions of space they also have exceptionally low metabolisms with some
having reproduction rates of decades to centuries or perhaps even millions of
years there are endo lytic bacteria viruses and fungi dug out from deep
beneath the ocean floor that appear to be a couple hundred million years old
if anything can travel between the stars it's these guys okay to review many
microbial life forms can survive the journey into and out of space endoliths
can survive long space journeys buried in rock
can they really make it to another planet and is it ever likely to happen I
mean statistically let's start with the easy journey within a solar system like
Mars to earth the minimum travel time for a Martian impact ejector to get to
earth is something like six months although it's more likely to take years
or centuries given the random nature of the journey
those timescales are totally reasonable for the more hearty earth microbes
traveling between solar systems is a whole different game first the rock has
to escape not just from the planets gravitational field but its stars field
also to get a microbe from the surface of the earth to interstellar space
requires a large velocity around four times what is needed to just escape the
earth that rock would endure a commensurately higher acceleration to
attain that speed with all the pressure and temperature pain that went with it
once an infested Rock makes it out of the solar system it has a very long very
boring journey ahead to get to the nearest stars a rock traveling at the
sun's galactic orbital speed of 30 km/s would take several tens of thousands of
years but real panspermia expeditions likely take much longer there's only a
tiny chance that an interstellar rock will be caught in the gravitational
field of any given star that it passes take uma or more for example the
asteroid that zipped through our solar system in 2017 it's going to escape our
solar system and this was probably the closest encounter it had for hundreds of
millions of years some life-forms may be able to hibernate that long but at this
point the issue is just frequency of impacts is it at all likely that a
microbe bearing rock from another star system hit the earth in the short time
it took life to take hold well it depends on the abundance of life in the
galaxy but it seems dubious one interesting alternative possibility is
that life bearing rocks aren't so often captured by planetary systems but rather
by the giant deceived and gas that precede the formation of
planets circled protoplanetary disks these could act like gigantic nets to
capture break apart and disperse the seeds of life before a solar system even
forms star fairy microbes could then remain in hibernation into a planets
coalesce from the protoplanetary disk and one of these life bearing rocks
slams into a newly habitable world perhaps the most convincing argument
against panspermia is that if the dormant seeds of life are so common
throughout the galaxy why haven't many of the forty billion earth-like planets
produced technological civilizations far in advance of us and speaking of aliens
a fun version of this idea is directed panspermia in which the seeds of life
are sent out deliberately to engineer life across the galaxy they would
actually be pretty easy for a moderately advanced civilization but aliens aside
panspermia is a plausible but by no means accepted explanation for the
origin of life on Earth if it's true then you me and everyone you know is an
alien a genetic immigrant from another world
perhaps with many cousins scattered across the galactic reaches of space
time before we get to comments I am very pleased to announce today's episode is
brought to you by our brand new space-time merch store we have all new
nerd tastic t-shirts mugs hats posters and even a baby onesie we'll be updating
the store with even more merchants coming weeks so make sure you check out
PBS space time comm in the last episode we talked about the interstellar space
rock boom or more and the proposition by some astronomers that it may be an alien
light sail I laid out the case why this is very unlikely and I asked you guys
what you thought about the out waited attention the media gives to these sorts
of Fringe ideas regardless of their connection to reality is it harmful or
is any public exposure to science good it seems that you did think about it and
quite hard justin o'brien places the responsibility
on the media stating the problem is the rampant lack of journalistic integrity
that problem exists wherever a journalist is looking for fame and its
trappings and when a media is slave to advertiser demands well this is
definitely a factor when both the media outlet and the writer live or die by
that month's clicks it's easier to lose sight of old-fashioned journalism or to
make the necessary sacrifices to pay the rent hmm
hard to say whose fault that is advertisers editors journalists mindless
market forces or the viewer for insisting on free content which brings
us back to advertisers guest who puts it well and I quote when the media says
Harvard scientist says X this is received effectively as a pronouncement
of truth from scientists when it is inevitably proven false or the predicted
thing doesn't eventuate the public naturally learns not to trust in science
as a whole even casting doubt on all established science as well Ian Oksana
made similar points also adding that it's harmful of scientists or
communicators do not qualify their fringe ideas with notes of caution
otherwise it gives the public a distorted view of how science works and
a related point Steve Zara says that exaggerations and hype are harmful
because they encourage excitement about the wrong things
Sam Winfield agrees saying that hype over nothing leads directly to
disinterest they are crying wolf all good points if the press only emphasizes
the most outlandish interpretations do we eventually get bored by the real
universe give kids too much candy and they won't eat their vegetables in this
case umemura is totally amazing as our first natural interstellar visitor and
the glory belongs to those who discovered and unlocked its real
properties seems a bit cheap for someone to come swooping in and shout aliens
every time something inexplicable is observed and we have at least one
counterpoint mark gibble says that no it's probably not harmful at least in
this case as long as we have people to bring rationality to the dreamers and
the skeptics alike it's a proud Duty we have here at PBS space time squashing
dreams and killing the buzz since 2015 heartbreak one changes the subject
pointing out that if aliens were to land on PBS studios we would do a lot of
biological testing and determine that most probably it isn't aliens wait are
you disparaging our ability to biologically test aliens or are you
saying we'd lie about it if we found out that there were aliens I assure you I
will be the first on the alien bandwagon and when the evidence is clear so you
know how to be sure Italians no fun buzzkill skeptics like us will be saying
it too although a few of you point out that denying that the murmur is aliens
is just what aliens would say, or go we are aliens well to deny that would make me
even more of an alien so I guess you got me