LV223 or LV426, which came first?

Aramaki
MemberOvomorphJuly 01, 20124234 Views59 RepliesAfter reading many of the posts here it seems a lot of people think that the crash on LV426 happened before the events of LV223. That after the failed delivery of the eggs the Engineers reworked the delivery system. Transporting the facehuggers in a more basic form (all the materials needed to grow them separated). But how do we know LV426 happened before?
I would argue that LV426 occurred after. This is why.
Based on what i saw from the film, that the facehuggers are created by taking the oil (xeno) combining it with the seed and a third organic specimen. (This third organism is so the hugger is imprinted with the future host's DNA, making the hugger genetically compatible.) This is why the 'worm hugger' attacked the biologist, went down his throat (huggers do this to impregnate) yet no xeno was born (or seen to be born).
Could this be because the hugger didn't contain any human DNA and therefore was incompatible. This would explain why the xeno/hugger was being transported in the urns. The engineers didn't have any human dna yet. This was to be acquired on site (earth).
Once the events of LV 233 happened, if the engineers returned to the planet (learning of the events through shaw?) they would have access to human DNA. Or at lest a live human/xeno hybrid alien to lay the compatible eggs for them. This would allow them to create the huggers prior to getting to earth.
You could argue that transporting the huggers this way would be safer. The oil is clearly very adaptable and dangerous. A human sized hugger would be a lot easier to defend against. (8 foot man vs a 1 foot hugger?) The fact the the SJ on LV426 was still in it's seat might suggest he was taken by surprise.
My question would be, where did they get the human dna to make the huggers. Would it not make more sense that LV233 is where they got it from?
Though this would mean that the crash on LV426 happened after 'Prometheus' but before 'Alien'. Is that time frame too short?
OR
The ship on LV426 was returning after collecting eggs from an all ready infested planet. (A planet that had the urns dropped on it's surface).
But I'm probably wrong