Nobody said that here, but I've seen the lack of stakemarks touted elsewhere as evidence of otherworldly origins.
Silliness. The dead giveaway of human origins on 90% of these things, like the one picture in the thread here, is that they're basically six-pointed designs. That's exceedingly easy to do with nothing more than a long piece of rope and something to flatten the stalks. This is basically the old middle-school geometry method of inscribing a hexagon within a circle using a compass and straightedge, writ large. Your rope functions as both the compass and the straightedge in these cases, and it's absurdly easy to do.
I'm halfway tempted to call some friends and give it a whirl, just to see how fast we can pull it off. You can really get some surprisingly complex designs with just a bit of planning....