Right, but the problem is evolution permits the existence of mutually exclusive biomes. Chirality is a factor in abiogenesis, and solving the chirality problem is one of the areas of active research. One of the things you will learn as you progress your education in science is that sometimes the answer is "I don't know, but here are some likely possibilities consistent with the evidence."
How is chirality a factor in abiogenesis? Both types of organisms can start to assemble under the same external conditions regardless of what is the ratio between L and D proteins in the environment, isn't that right? And didn't we already explain with evolution why we would only see left-handed amino acids in proteins of today?
It's not just that one population dies off more likely due to some initially small imbalance, it's that larger population produces more of their own chirality molecules as byproduct of their existence, thus magnifying the effect. The genocide of right-handed amino acids would have happened at exponential rate, pretty quickly in terms of thousands of years I'd guess.
There are two ways that earth could have gained amino acids. The first one is they were formed locally, the second one is that they were formed remotely and delivered to earth in its remote past.
Billions of tons of protein arrived to Earth riding on meteorites? C'mon, the Earth had everything it could ever wish for to create you and me having this conversation, and that's exactly what she intended to do, apparently. We are both just puppets, slaves to our cells, molecules and atoms. These little vampires are riding us from the inside, controlling us like a Nintendo.
The actual truth of the matter, in my opinion anyway, is that the truth is probably that earth was manufacturing its own amino acids, and there was a load delivered to the earth by meteorites and comets at the same time Earths supply of water was delivered. The evidence suggests that this mixture was heterochiral. Whether or not it was heterochiral enough to give rise to the homochiral biome we see today is a seperate question that needs to be addressed. We may yet find that there was a chiral substrate of some kind involved - quartz being an example of this.
Is there any question that abiogenesis can not with equal success assemble organisms of both chiralities? Is there any question that it could not have easily been the other way around so that right-handed amino acids won the battle over the Earth? In other words, do we expect that alien lifeforms on other planets similar to our planet would also use left-handed amino acids or do we believe the chance is really 50-50% for either one. Isn't there also a chance there could be mixed ratio of both chiralities on a single planet?
The issue of chirality means they don't eat the same food because digestion involves enzymes and enzymes have a handedness, and even if they could the amino acids they extracted would be unusable.
Can you really say there can not exist such enzyme that could break apart either chirality molecules and still continue to function as usual? What if there was an organism that utilized both chiralities in its design and functionality so it could live in either type of environment equally well? Or how about two organisms of different chirality forming a symbiotic relation and eventually merging into a single organism utilizing both chiralities?
Digestion does not racemise amino acids. If it did we'd need to eat twice as much. Converting one enantiomer into another is a chemical rearrangement, not a physical one.
But new left-handed amino acids are created from simpler molecules or atoms somewhere in consumption/digestion process, right?
Do you think that right handed DNA can combine with left handed DNA? And I suspect that if you injected yourself with blood based on right handed amino acids that your body would think it was foreign material and your immune system would react to it as such.
I didn't know about it before you told me, and I couldn't imagine the effect would be so drastic, so I find it very interesting.
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