Bits of Precursor research data have been accumulated over time, and have come to describe an unexpected origin for the ever-present raiders: bio-weapon. The orcs were created by the Precursors as a means to grow an army in a short time, with little need for supply lines. The following facts have been extrapolated from research data and confirmed via brave observers:
Orcs grow very quickly, reaching adulthood in just a couple of years.
Orcs sustain themselves with simple sunlight, filling in gaps in their diet with anything edible they can get their hands on. This allows large tribes of orcs to consume far less food than expected.
Orcs often brawl, but never maim or kill each other. Their hierarchy appears based upon strength, and the only time any of them do not follow orders from their superiors is when they are challenging them for leadership.
Orcs are as coordinated as any military unit, often without any vocal communication beyond barely-audible grunts. None of them have been observed disobeying orders in a combat situation.
Reproduction appears to be much faster after a raid, perhaps replacing losses or owing to the increased resource availability. Additionally, orcs produce many more twins and triplets than any other race, allowing them to grow their population very rapidly.
Half-orcs breeding with full orcs produce full-blooded orcs.
Orc and non-orc pairings always result in half-orcs, and this genetic dominance persists even through several generations, as there have been observed instances of up to 5th-generation part-orcs coupling with non-orcs and still producing half-orcs.
Half-orcs seem to lose some of the more mystical traits, such as rapid growth and ability to subsist off sunlight, but retain the power and toughness of full orcs. They also seem to gain some of the mental independence of their non-orc side and lose the ability to follow non-verbal orders or work in perfect sync like full orcs.
In short, the Precursors created a perfect army. The question that has remained unanswered, however, is why the Precursors needed such an army, dominating Sarella the way that they did. Current theories point to the Precursors being physically weaker, needing a fast-growing shock troop to deploy on the front lines.