This is assuming independent genesis. Panspermia has a lot of merit.
The preferred chirality of organic molecules could absolutely have arisen by chance, but it's an interesting to see this in meteorites.
On the unrelated subject of handedness, I saw an interesting thread on Twitter today [1] speaking about how we're starting to synthesize reverse chirality polymers and enzymes, most notably DNA and replication enzymes.
There are a lot of interesting implications.
You can't get rid of L-DNA without reverse DNase, leading to an accumulation of information and transcription. So they need to remake all the enzyme steroisomers.
That alone is interesting, but you can take it further to the limit and produce reverse biology that synthesizes reverse sugars that can't be metabolized by much of extant life [2]. Suddenly a lab-escaped reverse autotroph can out-compete all of us right-handed lifeforms because nothing can eat them. Bacteria, plankton, the entire food web collapses. When we have nothing left to fish or farm, we die too.
Never thought nanotech's grey goo was plausible. Now I see something that rhymes with it, and I could see it happening within our lifetimes.
It'd make a crazy MAD bioweapon on par with or potentially worse than nukes.
Wild tangent, sorry.
[1] https://twitter.com/eigenrobot/status/1420952351968432130
But I assume they're not immune-invulnerable.
To be clear, evolution of viruses is probably inevitable. But on what time scale does it take for them to arrive ex nihilo? Millennia? Longer?
> Such negative values occurring shortly after the GOE require a rapid reduction in primary productivity of >80%, although even larger reductions are plausible. Given that these data imply a collapse in primary productivity rather than export efficiency, the trigger for this shift in the Earth system must reflect a change in the availability of nutrients, such as phosphorus. Cumulatively, these data highlight that Earth’s GOE is a tale of feast and famine: A geologically unprecedented reduction in the size of the biosphere occurred across the end-GOE transition.
https://en.wikipedia.org/wiki/Microbial_metabolism
Water, sunlight, carbon sources, phosphorous sources, nitrogen sources. And any additional think they have enzymes to break down and process. Advanced organisms could feed off complex organic molecules, but this isn't necessary.
The point is that with reverse chiral sugars, polymers, etc., they wouldn't be a meal for anything in the wild. (At least not one with nutritional value.) No predation and no competition leads to massive expansion, and they could exhaust resources that other necessary parts of the food web require.
This would make a fantastic "phase one" alien bioweapon to aid in wiping out a planet's existing lifeforms and replace ecosystems with one better matching their own. Assuming post-biology aliens care about biology.
The first type is incidental: we've already encountered meteorites from Mars and the Moon, for example, so it's not hard to imagine life bearing material arriving that way.
The second would be intentional: either ET seeds planets or merely visits them with contaminated boots or probes.
Either way, we would pick up some DNA from offworld.
There’s some speculation that a large amount of their genetics were transferred from other animals.[1] I wonder how much this muddles the tracing through the evolutionary tree.
If I recall correctly, the tardigrades (and extremophiles like D. radiodurans) have evolved to handle damage brought on by desiccation. As a fortunate side-effect, this general robustness also protects against radiation.
https://www.onezoom.org/life.html/@=111445?img=best_any&anim...
(hopefully that links to the right place)