Thanks - interesting article.
It says "However, the first animals were not bilaterians." This article only applies to bilateral modern animals = bilaterians. No surprise then that they are bilateral

.
Also what about disabled people, why are they are also ignored as one of Allahs creatures in the quran?
Its interesting though that most organs are repeated on the other half, I cant see any practical reason why this should be so - I wonder if this was just 'laziness' on evolutions part.
Also I did not see a reason given for liver not having its mirror image on the left hand side?
I think the unpaired liver clue is in the second to last para. And isn't it strange that the liver is just about the only organ that can regenerate itself after certain types of damage: for instance, the amount of warfarin damage repair and limitation that some popns. of rats have evolved. They can grow a brand new liver in a few weeks; faster than Warfarin can destroy it.
As for evolution: It's survival of the fittest remember, so whatever trait began to dominate at a certain point in Earth history (ie increasing tendency to pair organs) is still fitted to be most viable today.
Perhaps there's never been any evolutionary value in paired livers over single versions. Or maybe we just can't detect the join yet.