Their musculature seems very similar to ours with the whole pectorals and six pack. Now you could say that such a muscle design would make sense on a humanoid life form since the pecs are designed to pull the arms forward.
But there's other things about the Predators that's also very human. They have five fingered hands and walk on their heels like us.
They're just too human for an alien lifeform, and I did like the idea of them being the ultra super rare one in a billion chance example of convergent evolution, I am also liking this idea as an explanation for their humanoid form.
I still think the original Predator form wouldn't be too different from how they look now seeing as lots of their technology looks to be outfitted for something humanoid.
