Life has to be monochiral because biochemical reactions are catalyzed by physical machines- enzymes, where an entirely different physical shape of the enzyme active site would be required for it to work with the opposite isomer. Any particular DNA sequence that evolves to make a protein will only be able to work with one orientation- unless the enzyme was so floppy and flexible that it reacts with everything, which would be inherently dangerous to a living cell.
Think about trying to “evolve” a glove that fits perfectly on both hands yet is also specific and does not accidentally fit onto non-hands… it would be much harder and less likely than evolving one that only fits left or only right hands.
Spontaneous chemical reactions that make the things we find in space never had to physically fit into a machine like a key into a lock, so both chiral isomers are equally likely to form.
"Both" don't have to be represented in the same place at the same time. And even then, I could imagine them competing. I mean if both can work, why should only one exist?
PS: I mean both as in separate ecosystems. Not arguing that one organism could contain both.
I agree, but as far as we know life evolved only once, and particular chiral orientations in central carbon metabolism were then locked forever, because they can't be changed without breaking everything else. The same enzymes won't generally work with both steroisomers- every enzyme they interact with needs radical structural changes.
Less critical things outside of central carbon metabolism often do evolve different stereoisomers in different species, or even the same species. For example, the large macrocyclic antibiotic molecules sometimes evolve to flip sterochemistry, creating a new antibiotic.
Why should we talk about "same enzymes"? There's a 50% chance to have "other enzymes" working for the other orientation, whenever we find life similar to ours (which it doesn't have to be). Or is there any reason why other orientation cannot exist?
For any multistep reaction there is a strong likelihood that "other enzymes" would make something that does not work for the next step (depends on the 3d shape not fitting in the next enzyme). So at best some of the intermediary products for each organism with opposite chirality enzymes may not work in the other, at worst, it may be actively poisonous. Since time + competition breeds monopoly one would expect to see only one per competition zone (i.e. planet) given sufficient time. You could see both if there was an active breakdown mechanism developed in each that allowed minimally inefficient usage of the others resources (i.e. we see this on a macro level with things like lobster blood using copper instead of iron and being blue. We can still eat each other no problem without getting sick so there's no pressure to get rid of the one system)
If you’re talking about hypothetical extraterrestrial life that evolved separately- sure. But as far as we know all life, at least on earth has a common ancestor and is heavily locked into the existing enzymes and substrates- basic central carbon metabolism is virtually identical in all living things.
there is a good argument to be made that there is exactly one life form on this planet,with various
sub types speaking different dna dialects, and there exists a kind of universal mutual comprehension of the base language in between all life forms
the explanation of chirality is very good
Think about trying to “evolve” a glove that fits perfectly on both hands yet is also specific and does not accidentally fit onto non-hands… it would be much harder and less likely than evolving one that only fits left or only right hands.
Spontaneous chemical reactions that make the things we find in space never had to physically fit into a machine like a key into a lock, so both chiral isomers are equally likely to form.