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Did you know that many of us have up to 4% of our DNA from neanderthals? And that 100% of us
may have DNA from outer space? No joke. At least the biochemistry that defined the coding system
of your DNA may have happened off-world, and perhaps even long before Earth existed.
Before we get started, just wanted to let you know we have some new merch at
the merch store celebrating 10 years of Space Time. We’ve got a limited edition
10 year anniversary design as well as some classic logo merch. Having his gear doesn't
just make you incredibly cool, you also helps us keep going for another decade.
Life is the coolest thing to have happened in our universe. It would be nice to know if it
happened anywhere else besides Earth—if nothing else to know just how badly we’re screwing up
as we flirt with self-extinction. But not only have we never found credible
evidence of life on or from other worlds, we only have the sketchiest of ideas of how it
began on Earth. Makes it pretty difficult to say much about what’s happening out there. But one
encouraging detail is just how quickly life got started on Earth. The earliest fossils are dated
to within a few hundred million years of the Earth first cooling from its early molten hell-ball
phase. And if you believe some of the more tentative biosignatures, it could be much earlier.
In fact, life got started so quickly on Earth that some have argued that natural selection
just didn’t have the time to take raw elements all the way to the first single-celled lifeform.
This difficulty led some scientists to propose panspermia— the idea that life did not start
on Earth at all, but rather traveled here in the form of extremely simple and presumably
extremely robust simple organisms. It’s a fun story, and we’ve talked about it before,
but it’s not supported by evidence and generally not considered particularly likely.
But there is a middle road. Perhaps life didn’t start in space, but maybe its building blocks did.
Pseudo-panspermia is the idea that many of the complex molecules critical for abiogenesis—the
formation of life—were not formed on Earth, but rather in the depths of space, in some cases
long before the formation of itself. If these rained down on our planet in its early years,
that would allow our molecular ancestors to skip many steps in their path to the first cell.
The evidence for pseudo-panspermia is rather strong. And the most compelling of that evidence
is just in, with the safe return of the OSIRIS-REx mission. But before we get to those results,
let's review what little we know about the first formation of life.
Life as we know it is based on a complex interplay of organic molecules, primarily nucleic acids,
proteins, lipids, and carbohydrates. In your body, these work together to form
the intricate machinery of the modern eukaryotic cell. Of course, the first life was much simpler,
and life’s precursor simpler still—for example, self-replicating RNA molecules of the RNA-world
hypothesis. But how did a bunch of non-living chemicals even get to something as complex as
RNA? Extremely broadly, assume that in some energy and chemical-rich primordial soup,
where simple elements combined into organic molecules, when then combined
and recombined in a chain of trial and error until this natural laboratory first
stumbled on self-catalyzing and then self-replicating molecule.
The step from organic molecules to complex, self-replicating RNA is still pretty mysterious,
even if there have been recent advances. But the pre-RNA part of the story? We’ve know that’s
possible for decades. Back in 1952, a pair of scientists attempted to replicate the conditions
of the early Earth, with liquid water “ocean” evaporating into an atmosphere of hydrogen,
methane and ammonia. An electrical spark simulated primordial lightning powering
a chain or reactions that ultimately led to a variety of organic compounds,
including five amino acids—the building blocks of proteins. This was the famous
Miller-Urey experiment, and it was our first hint at how the seeds of life may have formed.
That experiment has now been repeated in a variety of ways, and we now know that
amino acids can spontaneously form given the right raw materials, an aqueous environment,
and an energy gradient. The apparent ease with which amino acids form led scientists to wonder
just how ubiquitous these organic molecules might be, not just on Earth, but through the
universe. For example, in the chunks of rock and ice that rained down on the early Earth,
first building our planet and then perhaps seeding it with the precursors to life.
One of the first pieces of evidence that space
rocks are packed with organic compounds was the Murchison meteorite, which fell
in Australia in 1969. This is a carbonaceous chondrite meteorite, so, made mostly of carbon,
clay minerals, and water. But it was also found to contain over 90 different amino acids,
including some that are not commonly found on Earth, as well as the nucleobases purine
and pyrimidine which are critical components of DNA and RNA, and other organic molecules.
Carbon dating estimates that Murchison is around 7 billion years old,
so a couple of billion years older than even the Sun, meaning that complex organic
chemistry would have been occurring in space long before life emerged on Earth.
It’s hard to be 100% certain about the origin of the molecules found in the Murchison meteorite
because the rock was in contact with Earth, which we already know is infested with life. However,
the amino acids in the meteorite had one distinct difference to those typically
found on Earth—they came in both left- and right-handed varieties—both chiralities, both
mirror reflections. But all amino acids produced by Earth life are left-handed—a chirality that
has been locked in since the first common DNA or RNA ancestor of all life won that great molecular
war 3.5 to 4 billion years ago. The presence of both chiralities in the Murchison meteor
tells us that these amino acids had an abiotic origin—supporting their pre-life formation.
Pretty convincing, but the best way to be sure there’s no cross
contamination from Earthly life is to get samples directly from space.
And there have been some incredible efforts to do this. NASA’s stardust mission collected samples
from the coma of comet Wild 2 and returned them to Earth. The Japan Space Agency was the first to
land on an asteroid and return samples—twice, actually with Hayabusa and Hayabusa2. And
ESA’s Rosetta mission was the first to send a lander to a comet where it analyzed samples on
location. These and other missions have shown that organic compounds are abundant on space rocks.
But how far along can this pre-biotic biochemistry really advance in space? How much of a head start
might pseudo-panspermia have given to life on Earth? Some of that has now been answered by
NASA’s OSIRIS-REx mission. In our long tradition of unwieldy science acronyms, this stands for
Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer.
The spacecraft launched in 2016, with its sights on an asteroid by the name of Bennu.
Bennu is a pretty ordinary carbonaceous asteroid of middling size,
at around 500 m in diameter. It’s part of the Apollo group—a collection of more than 20,000
asteroids with orbits that cross Earth’s. That means they do sometimes slam into our
planet—like the one that exploded over Chelyabinsk in 2013. Or as in 2024 YR4, which has a miniscule
chance of hitting Earth in 2032. Bennu has no immediate chance of hitting us, but it
does make a close approach to Earth of around 300,000 km or 24 Earth diameters every 6 years.
The upside of these potentially cataclysmic neighbors is that they’re quite easy to visit.
Which OSIRIS-REx did. It started that journey in orbiting around the Earth, followed by a
mad dash to catch up to and orbit the asteroid. Then after a few years of observation and analysis,
a suitable landing site was identified and the spacecraft touched down. Samples were collected
before an easy escape from Bennu’s frankly pathetic excuse for a gravitational field.
OSIRIS-REx rejoined Earth’s solar orbit and dropped its sample capsule into our
atmosphere in September of 2023. And after all of that, the capsule’s parachute failed.
Its first parachute failed. The second opened just fine and the capsule landed right
on target in Utah. Since then, scientists have been hard at work analyzing the sample,
and the first results of the findings were finally published in January this year.
So, what did we find? Among the most exciting discoveries in the sample are all five of the
nucleobases that serve as the code for DNA and RNA. This is the first time we’ve found
five on the same space rock. The samples also contained 14 of the 20 amino acids
that life on Earth harnesses to make proteins, These amino acids have both left- and right-handed
chirality, just like the Murchison molecules. Bennu sample has various other interesting
compounds, including a surprising richness of ammonia—12 times higher than in the
Murchinson meteorite. Curiously, Bennu orbits too close to the sun to preserve pure ammonia,
so its richness hints at a colder, more distant origin. But more on that later.
One of the most exciting finds were 11 different minerals that we know form when brines
slowly evaporate. That means Bennu, or the stuff that formed it, was once in an aqueous,
salty environment. In fact, there were many signs of the role of water in Bennu’s formation. Its
minerals match most water-altered meteorites ever found on Earth, as well as the samples collected
from asteroid by JAXA. But, unlike most meteorite samples, Bennu contains sodium rich salts,
which are rare in meteorites. Similar salts are found here on Earth, like in Searles Lake,
California, confirming that Bennu once contained pockets of sodium rich water.
But normally we don’t find sodium rich salts in meteorites because they react
strongly with our atmosphere. But because Bennu’s samples were stored in pure nitrogen,
they maintained a pristine suite of salts for analysis.
With this analysis in, we’re able to reconstruct a likely origin story for
this asteroid. And that origin is probably a water-rich world that
existed for a time in the early solar system, but has since been destroyed.
Let me take you back 4.5 billion years ago, on the icy fringes of the early solar
system beyond Jupiter’s orbit, a distant protoplanet formed from a mixture of rock,
metal, and frozen water. As radioactive elements created in an ancient supernova decayed within,
heat melted some of the ice, creating mineral-rich reservoirs. These ancient
waters interact with ammonia and formaldehyde, sparking the formation of complex organic
molecules. When the water eventually evaporates, it leaves behind veins of brine-sourced minerals.
This protoplanet, however, is doomed. Before it can grow to full planet-hood, a catastrophic
collision—perhaps with another similar body—scattering its fragments into space. And
Bennu and many other asteroids pulled themselves together from this debris. In the unsettled
early solar system with its wandering gas giants, Bennu’s orbit was also fated to wander. Eventually
it settles into an orbit perilously close to an inner-solar system planet. A planet that
would slowly turn blue-green and then twinkle with night lights and then send a tiny visitor
of metal and silicon named OSIRIS-REx. Or at least, it seems like this is the most
likely story based on the bits of Bennu that we brought back home. And if Bennu has the variety
of organic compounds that we discovered, so do many asteroids, and probably comets. Many of these
peppered the early Earth—far more than today. In fact, they are kind of what Earth is made of.
Like I said, pseudo-panspermia—the idea that the chemistry of life got a head start from space
molecules—is pretty well supported. But does this tell us anything about the
likelihood of life elsewhere? We can be pretty sure that there are watery worlds out there that
started out with an abundance of organic compounds, all the way up to amino acids
and even nucleotides. It’s becoming harder to imagine that simple life doesn’t form in
lots of places. But the observation of DNA and RNA coding nucleobases could tell us something
pretty profound. If all planets start with a similar chemical cocktail, maybe there’s
a pretty narrow path to life everywhere—one that involves DNA-like molecules. Maybe the
system that codes Earth life—including your own DNA—was set by extraterrestrial biochemistry.
And what about OSIRIS-REx? It delivered its sample, but it stayed out there to continue
its adventure. NASA has renamed in OSIRIS-APEX, and it’s now preparing to intercept the asteroid
Apophis. This time the mission isn’t primarily to discover the origin of life,
but to help prevent its annihilation. Apophis is one of the most hazardous Apollo asteroids.
It has a miniscule chance of hitting Earth in 2036. OSIRIS-APEX will study any changes
to the asteroid that occur during its close encounter with Earth in 2029. This will help us
track Apophis more precisely. It’ll also teach us more about these close encounters, perhaps
allowing us to avert a future giant impact. Well, thanks for looking out for us, little buddy,
and for helping us seek our chemical origins in a protoplanetary, perhaps pre-solar spacetime.
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