How about this idea. Orc's and all their lesser kin are a self replicating self reinforcing and adaptive genetically programmed Bio weapon. Designed to destabilize a world before invasion or conquest.

1) Designed by humans during the dark age of technology and gone wild.

2) Or far older and created by the Old Ones perhaps, and now gone wild.

Life cycle: Spores seeded by covert means,

Fungus grows from spores,

When enough fungus or other organics available, Snottlings begin to emerge.

When enough Snottlings to serve as food, Squigs emerge.

As Squig and nuisance Snottling population hit critical mass, Grots emerge.

When a large enough Grot population exists to serve as slave labour force

Ork's emerge.

Infestation complete.

Once this point is reached your world is toast!

Addendum, Orcs are green so as to be photosynthetic. They can survive without food, for long periods or even go dormant for a longer time subsisting off sunlight alone in a, should I say it. "Vegetative state."

The first Orks were apparently created by The Old Ones (like a lot of other races), to fight the Necrons and Enslavers.
The Orks are known to have fought Humans during the Dark Age of Technology.

http://wh40k.lexicanum.com/wiki/Ork#History

In-universe, nobody normally knows this stuff though. Other than the Necrons themselves. And some daemons if they care.

In-universe, nobody normally knows this stuff though. Other than the Necrons themselves. And some daemons if they care.

How about the Eldar? Wouldn't they know about it to?

In-universe, nobody normally knows this stuff though. Other than the Necrons themselves. And some daemons if they care.

How about the Eldar? Wouldn't they know about it to?

It's been 60 million years. I thought that their knowledge of the War in Heaven was all dim legend.

In-universe, nobody normally knows this stuff though. Other than the Necrons themselves. And some daemons if they care.

How about the Eldar? Wouldn't they know about it to?

It's been 60 million years. I thought that their knowledge of the War in Heaven was all dim legend.

The War in Heaven predates not just the Fall of the Eldar and the Birth of Slaanesh, but it does so by several million years. For all we know, even from an outside observer's, objective standpoint, the eldar empire have risen and fallen a thousand times in that timespan.

Harlequins probably keep track.