- 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 is 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.

(upbeat techy music) 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 eye blink?

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, travel to the new world, and entry to that world's biosphere.

Let's start with the exciting parts, the beginning and the ends of the journey.

One way for a budding microbial astronaut to travel the stars is via lithopanspermia, basically be attached to a rock that travels between planets.

Now, we know the 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've 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 11.2 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 force is applied 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, spinning them up in an ultra centrifuge, attempting to splat colonies with extreme pressure impact plates, and if that's not realistic enough microbe-laced rocks have been dropped from space strapped to the outside of reentry vehicles.

Many lifeforms have been tested from common 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 100 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, endolyths.

These are organisms that live deep within rocks, deep enough that they're protected from the extremes of temperature change.

As we'll see, endolyths are exceptional candidates as microcosmonauts 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 a familiar bacterial DNA on the outer surface of the International Space Station.

They suggest it rode 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 radiopanspermia.

Stars may be constantly spraying their germy life through the galaxy.

In some respects it sounds like levitating into space and becoming your own solar sail is way more efficient than lithopanspermia 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 panspermial lifeform the journey itself is 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 panspermiating lifeform 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 translate 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 extremophilic multicellular teddy bear, can revive after exposure to space.

The key is cryptobiosis.

Many lifeforms can enter or produce a hibernating form that's incredibly resistant to adverse conditions in particular lifeforms that can survive extreme dryness and hydrobiotic lifeforms do especially well.

That includes certain reproductive spores, especially fungi, but also critters can enter an anhydrobiotic 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 of 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.

Lichen seem to do the best with cells surviving up to several months of exposure but probably not for years.

A star's intense UV radiation probably rules out radiopanspermia.

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 ejector and lithopanspermia.

Microbes buried deep enough in rock are entirely safe from UV and safe from cosmic rays if deep enough.

Endolyths, 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 endolythic 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 lifeforms can survive the journey into and out of space.

Endolyths can survive long space journeys buried in rock but 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 planet's gravitational field but its star's field also.

To get a microbe from the surface of the Earth to interstellar space requires a launch 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 kilometers per second 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 'Oumuamua 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 lifeforms 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 disks of dust and gas that precede the formation of planets, so called protoplanetary disks.

These could act like gigantic nets to capture, break apart, and disperse the seeds of life before a solar system even forms.

Starfaring microbes could then remain in hibernation until 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 40 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.

That 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 spacetime.

(musical tones)