Asteroid fragments upend theory of how life on Earth bloomed
nature.com
nature.com
> On Earth, amino acids in living organisms predominantly have a ‘left-handed’ chemical structure. Bennu, however, contains nearly equal amounts of these structures and their ‘right-handed’, mirror-image forms, calling into question scientists’ hypothesis that asteroids similar to this one might have seeded life on Earth.
The fact that the components on the asteroid is racemic meant or heavily suggested that they were formed using non-biogenic means. And if so, it also means that Earth could have had the same thing happened a long time ago, leading to the seeds of life.
tl,dr: this discovery weaken the panspermia hypothesis.
Nonzero-weighted sum of something and nothing is something.
If we found say, a high pressure synthesis method which was heavily biased chirally, then that would be good evidence but none has been discovered.
AFAIK there isn't even a known way to start with "normal chemicals" and produce highly-chiral reactions reliably in the lab. We get all our chiral molecules by extracting them from biology.
I don't understand this. Aren't cells just spherical structures that would form naturally from hydrophobic molecules suspended in water/tide pools? That seems likely to me, but I have no background in chemistry/biology.
Nick Lane argues in "The Vital Question" [0] that simple cell membranes are not enough for complex life and complex membranes may have evolved in matrixes around hydrothermal vents.
Actually finding chirality in space would be extremely cool in that it would mean one of two things:
- the panspermia hypothisis is correct, or
- some non-biological process creates chiral molecules.
Physics itself has plenty of chiral processes, but they only show up in the weak interaction. As the name implies, the weak interaction is really really weak and essentially doesn't exist on the energy scales of molecular interactions. So chirality would be a bit of a smoking gun for panspermia.
On the other hand, not finding chirality just means we don't have a smoking gun. There might be another asteroid flying around that is 100% chiral, or maybe 50% of them and we were just unlucky.
The briefing joins the point about chirality with evidence against panspermia, but really that might miss the point. Chiral or not, abundant amino acids in space means that one of the many steps to create life is relatively simple. If we could show that every subsequent step is simple that would be a big blow against panspermia. But in that case ruling out panspermia would be pretty cool, since it would suggest that life exists everywhere.
What I don't understand is why would chirality and panspermia be so tightly linked.
The data right now still leaves every option on the table just because having any ratio of chiral molecules doesn't have to define how life evolves. It can't answer whether those molecules formed on Earth or hitched a ride on an asteroid, or life itself formed here or was brought here.
We can assume that in a soup with balanced proportions of each chirality, the left handed molecules created a self replicating mechanism (some definition of "life") first or faster than right handed molecules, either accidentally or because some yet undiscovered advantage. Whether this happened on Earth, or was brought to Earth by one or more of the millions of asteroids is hard to prove.
If we find one asteroid with chirality that doesn't match earth, it's good evidence that self replication just happens spontaneously rather than being seeded.
If we find only one asteroid with one chiral molicule that does match earth, that supports panspermia very weakly: it still might just be random chance. But multiple matching molecules, on multiple asteroids, starts to seem like evidence for a common source.
On the other hand: If we find multiple molecules with matching chirality, on multiple asteroids, and none of them match with earth... well, we should probably start preparing for the invasion.
The more asteroids we look and not find any asymmetry, the more evidence we have that life never existed on any of them.
In chemical synthesis, most (but not all) processes tend to preserve chirality of molecules: replacing a bunch of atoms in a molecule with another set will tend to not cause the molecule to flip to a mirror image. If you start from an achiral molecule (one where its mirror image can be rotated to the original), almost all processes tend to end up with a 50-50 mix of the two racemates of the product.
In biochemistry, you can derive all of the amino acids and sugars from a single chiral molecule: glyceral. It turns out that nearly all amino acids end up in the form derived from L-glyceral and nearly all sugars come from D-glyceral. The question of why this is the case is the question of homochirality.
There's as yet no full answer to the question of homochirality. We do know that a slight excess in one racemate tends to amplify into a situation where only that racemate occurs. But we don't know if the breakdown into L-amino acids and D-sugars (as opposed to D-amino acids and L-sugars) happened by pure chance or if there is some specific reason that L-amino acids/D-sugars is preferred.
Do any of these affect the fields that would have selected for molecules on Earth? The Sun's rotation, Earth's rotation, the direction of revolution in our by now almost coplanar solar system, Galactic rotation
I guess the reason it makes things more likely is because homochirality happens so aggressively that finding right handed molecules at all drastically reduces the odds of left.
– enantiomer refers to each mirror image of the molecule. https://en.wikipedia.org/wiki/Enantiomer
– racemate refers to a 50-50 mix of the two enantiomers. https://en.wikipedia.org/wiki/Racemic_mixture
Since they are a mixture of right and left-handed they definitely come from space.
Chirality of life is not a consequence of panspermia, just a consequence of incompatibility so one had to win eventually.
If these truly were the sites of OoL in our solar system, then that means life either originated early, before these asteroids froze up or dried out, or it wouldn't arise at all. This would vitiate the argument that because life appeared early on Earth, OoL must occur with high probability.
The water flow has the motivation driven by gravitational force. What is driving the motivation behind the growth the reproduction of an organism? In fact, life, growth and build go against the principle of least action that governs the physical universe.
And I'm not sure why someone has evolved the intention of downvoting me.
I understand it doesn’t seem intuitive at first, but evolution is actually quite similar to that.
> Trees produce thorns to keep away animals.
It's not that trees consciously produce thorns to keep animals away. Rather, trees with thorns (due to random mutations) are more likely to survive because they are less likely to be disturbed by animals. Over time, those trees are more likely to reproduce and pass on those traits.
Just like water flows down the path of least resistance, evolution naturally selects for traits that improve survival. The "selecting" we are referring to is used in the same sense that water "selects" which route it takes. It doesn't actually select anything, it's just following the laws of physics.
Mutations in DNA can have a positive or negative impact on the ability to reproduce. Because of natural selection, positive mutations tend to be preserved while negative mutations tend to disappear. There is no intent behind this process; it is simply a consequence of how life works.
While we can't yet say for certain how life got started, it is likely that the first "life" was made up of self-replicating RNA. Because this RNA self-replicated and produced imperfect copies, it was already undergoing evolution at that point, where the RNA strands which were better at copying themselves (due to mutations) were more plentiful.
The example that I heard in school was that of a giraffe's neck. It seems reasonable enough to say "giraffes evolved long necks in order to reach food sources in taller trees", but this is usually a kind of misleading shorthand for the much more accurate "some proto-giraffe acquired a mutation that increased the length of its neck, that change increased its likelihood of procreation (possibly by allowing it to reach food sources shorter-necked proto-giraffes could not, therefore allowing it to survive in harsher conditions), and that mutation was passed down to its offspring".
It's easy to make the mistake in thinking "organisms change their characteristics to survive", but that's applying intent where there is none. Organisms' characteristics change naturally through mutation. Those organisms either live to reproduce or they don't. Mutations that increase the chance of reproduction are more likely to get passed down to future generations. Over a long enough time period you end up with a lot of distinct species that evolved naturally and entirely accidentally.
It is kind of weird how defensive is that crew. It is very much possible that life on Earth has extraterrestrial origins. I do not see why somebody would try to discard this idea.
We have actual evidence of evolution as a real and active process and can (and have) studied and mapped that process across species and across time - including in humans - and we find absolutely no evidence of nor the necessity for extraterrestrial influence anywhere.
And even if some flavor of is assumed true for the sake of argument, that still wouldn't somehow negate evolution. It's entirely possible for life to have an extraterrestrial origin and to have evolved on Earth after that origin, having first evolved somewhere else.
Tldr the finding is that abiogenesis may be easier to get to than previously thought.
It doesn't really mean that, though. We already knew synthesis of simple amino acids was pretty easy. Urey-Miller did that decades ago. Making the easy part of a multistep process easier doesn't make the whole process much easier.
IMO, the rate-limiting step for OoL is later, when by some means the enormous complexity barrier is reached between abiotic stuff like this and the simplest self-replicating system capable of evolution. Of the latter, the simplest we know (the most stripped down cell) still contains billions of atoms.
https://evolution-outreach.biomedcentral.com/articles/10.100...