Ok, so I've been thinking hard on a theory to explain everything that happens in gears, not just all derived from one thing, but a theory on what everything is in gears and why it happens, I collected all the collectables in the game as to get the full picture before i even started writing it, so here goes.

This is my theory, on everything:

Its all about imulsion. Everything is about imulsion. Why? Cos Imulsion is BAD. It is some sort mutagen and it should be avoided and not touched or played with.

Ok, from the begginning.

At the discovery of imulsion, Serans mined it and pumped it out of the ground like there was no tomorow, but if they were digging for imulsion, surely they would have found the locust? No, because Locust weren't there. The locust are humans. In various sources from the game, patients that suffer from rustlung typically suffer from raspy breathing, loss of hair/hair stopped growing and exponential growth in nails. All they need to do now is grow scales and theyll sound a helluva lot like what weve been fighting this whole time wouldnt they? Back before the pendulum wars, Imulsion miners would suffer from 'rust lung' and would suffer these symptops. Theyd be given antibiotics and let back out to carry on with what they were doing. Then they began mutating more and more till they became monsters. 

Some were captured (the sires), some got away, those that got away escaped underground, away from those that were doing horrific things to their brethrin. Over the 60 years of the pendulum wars, the locust evolved and grew, they knew that what had happened to them had happened because of imulsion, and they had to stop imulsion from being overused. Sounds stupid, doesnt it? Well it would, until you consider the lambent. And the fact that the locust do NOT use imulsion for ANYTHING. We know rustlung only reappeared after the lightmass bombing, when imulsion fumes resulting from the explosion were inhaled, this is what mutates humans into locust. Well what are the lambent? The lambent are those exposed to LIQUID imulsion, as opposed to the fumes. These creatures were locust, but stronger, more violent and generally very aggressive, mainly because they had a tendency to explode. The locust saw the lambent and their destructive ways and shunned them, banished them from the rest of the world, knowing that if they were allowed to spread that they would mean the end for the locust and the humans. So the lambent went EVEN FURTHER DOWN. I think this because they attacked the nexus from below. And whats below the nexus? Imulsion, the lambent live under what we would call the 'sea level' for imulsion.

So the locust grew, they prospered, they evolved, they found indigienous life forms that had mutated and were able to be used as weapons (corpsers, brumaks, reavers, I believe the riftworm is a mutated rockworm). But why? Imulsion is bad, they need to stop it from being used, whose using it? Humans, will humans part with it peacefully? No. That is why they fight, they fight us to stop imulsion. Stop us from using imulsion. Baird said that he found locust... stuff saying that they fight on the surface to escape whats below. Which is the lambent.

That is my theory on the locust, they attacked humanity to stop the use of imulsion. Because all that imulsion has brought us is war and destruction. And the lambent show that the imulsion actuallly does cause war and violence. The lambent are locust directly mutated from imulsion and are now super powerful and extremely violent. The Sires are early locust that were studied by humanity. The Queen is (what i believe) to be something of the Wesker of gears of war. Rather then be mutated by the imulsion, it somehow worked with her body or some science fiction **** like that.  

So thats what happened before gears. Now for what happened during Gears of War 2. The lightmass bombing killed most of the locust. They knew the lambent were going to return some day, but they were weak, they were desperate, they turned to the most viable way of getting large numbers quicker then you can. Conversion. They captured humans, and turned them into locust. This helped them grow, endure and prosper. They carried on with their war against humanity. The lightmass bomb had awaked a giant worm, but with their chants, the kantus (who are the converters) were able to tame it, use it to sink whole cities so that they could capture entire populations to convert. Things were going well. Then the Lambent attacked. They attacked the nexus from beneath, overwhelmed locust forces and were on the brink of wiping them out, so the Queen instigated the fail safe. If ever the locust were invaded by the lambent, they would flood the hollow by sinking Jacinto and wipe out the Lambent. But they couldnt sink Jacinto, because WE did. The locust didnt have time to evacuate, and the lambent werent in full force in the hollow. We flooded the hollow and killed the Locust, the only thing between usand the horrors of the lambent. 

THAT, is what we have done.

I am fully aware that there is holes in this theory, and I tried to include just about everything in it, so if anyone would like to critisize/correct/add to it, please do, one person can't work everything out on there own, so corrections are not only accepted, but encouraged. Aslong as they are constructive