Of course this is all speculative, and I suspect that the point of the Nautilus article was to entertain the thought, so in that respect it's a great article.
>Long-distance running (for "endurance" hunting, where you just tire out the thing you're chasing)
I've recently read claims (though I forget where -- primary sources, if memory serves) that directly contradict this point (which I'd also heard). From memory:
- foot structure is ill-adapted to continuous running (stress fractures, I believe)
- knees are also problematic
- some physiology / metabolitic arguments
I must also admit that this hypothesis seems fishy if only because I can't think of many tasty animals that man can actually keep up with...
"Long-distance" doesn't have to imply "in a straight line", keep in mind. If we can herd prey in arbitrary directions, we can have other members of our tribe stationed where we're herding them, turning the marathon into either a pincer trap or a relay race.
I'll try to dig up the refs tonight, because it makes for interesting reading.
Of course, bipedal might have evolved before spear carrying, rather than being driven by it.
"On all fours" isn't quite what our common primate ancestors do on the ground, though. Picture a gorilla: the "hands" are knuckled. I imagine it'd work alright to hold a small tool (e.g. a stone dagger) in them, though not anything as long as a spear.
The question is what forced humans into this particular, odd body-shape that no other ape has. Long-distance running (for "endurance" hunting, where you just tire out the thing you're chasing) has been the hypothesis I've heard the longest, but running and throwing works too.