Good point re Australian aboriginals. Even in PNG jungles, bows were used albeit the short variety? A long bow would be next to useless in thick vegetation!
I think the usual triggers which lead to the development of the bow are;
A. Animals that are just too big and scary that mess you up when you prick em with a spear.
B. Animals that are tasty but tricky to reach I.E monkeys in trees
C. People that you want to be dead but can kick your ass.
-The animals we have here tend to be delt with by hand spear and big Roos or buffalo can just be left alone.
-from what I have seen there seems to be a tendency to eat grubs over try to hit a cockatoo with a rock so perhaps food on ground level is plentiful?
-being fairly nomadic I suppose the territory wars wherent quite as much an issue..
Honestly my knowledge of the various aboriginal cultures is woefully bad...
There are some cultures who dwell in the jungle who are forced to use hugely long bows for a few reasons;
A, the timbers (or grasses in some cases) don’t dry to a low enough humidity due to the rainforest environment so a shorter bow takes a lot of set (limbs stay permanently bent) and loses poundage.
B. Longer bows tend to be more accurate by virtue of their comfort to shoot.
Note: some cultures will use poison when hunting monkey from treetops so the bows just need to be strong enough to puncture the skin.
A noteworthy design is what we consider the English longbow which was man tall and bent through its entire length, typically of a D cross section, this design was also independently developed in Africa and interestingly by the Pygmys.