Did Life on Earth Come from Space? - Script (Patreon)

What if the first genesis of life – 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 is the case. This is the Panspermia hypothesis.

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 its the three deadly stages: ejection from an origin world; travel to a new world, and entry to that world’s biosphere. Let’s start with the exciting parts – the beginning and 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. 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 11.2 km/s – that’s the escape velocity at Earth’s surface. That requires an acceleration of up to 100s of thousands of g’s and 100s 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, spinning them up in an ultracentrifuge, 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: endoliths. These are organisms that live deep within rocks – deep enough that they are 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 suggest it rode electrical currents from the atmosphere below. Or came from space. Probably the former though.

These levitating microbes have a gentler journey up, and they also may have a faster trip to their destination. Very small individual bacteria can be accelerated by their own star’s own radiation and 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 space ship 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 into 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. Mostly single-celled organisms and spores, but also tardigrades and nemotodes. 

Some of these tests were actually to test survivability of Mars - but the results translate, and in sum 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 archaia, and viruses have been revived after months of exposure to the cold, dry, microgravity vacuum. In some cases after 6 years. Even the tardigrade – everyone’s favorite extremophilic multicellular teddybear 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 – anhydrobiotic lifeforms – do especially well. That includes certain reproductive spores – especially of fungi. But some critters can also 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 can enter when deprived 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 6 years exposed 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 at 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. Lichens seem to do the best, with cells surviving up to several months of exposure, but probably not years.

The Sun’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. 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’s a limit.

The only true protection is a thick wall of solid material. Once again, we’re back to impact ejecta and lithopanspermia. 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’re 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 endolithic bacteria, viruses and fungi dug out from deep beneath the ocean floor that appear to be a couple 100 million years old. If anything can travel between the stars, it’s these guys.

OK, to review: many microbial lifeforms can survive the journey into and out of space. Endoliths can survive long space journeys buried in rock. But can they really make it to another planet? And is it ever likely to happen, like, statistically? Let’s start with the easy – journey within a solar system. Mars to Earth for example. The minimum travel time for a Martian impact ejecta to get to Earth is something like 6 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 hardy 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 4 times what it 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 are likely to be 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 ‘Oumoumou 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’s had for hundreds of millions of years. Some lifeforms may be able to hibernate that long, but at this point the issue is just the 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? 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 proto-planetary disks. These could act like gigantic nets to capture, break apart, and disperse the seeds of life before a solar system even forms. Star-faring 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? 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 galactic reaches of space time.