valich said:
Initially, bipedalism was an efficient thermoregulation, and an advantageous transitional form.
The only problem with that is that bipedalism is not efficiently thermoregulatory until after the animal is efficiently bipedal - before that, it's hard work that overheats the suffering beastie. There are very few advantages to the transitional stages between quad and bipedalism - and consequently, despite the widespread need for such common abilities as thermoregulation, very few animals that have made that transition.
valich said:
The efficiency and morphological fitness to becoming permanently bipedal would have evolved afterwards.
Yes. So we need an environment in which inefficient and unfit bipedalism, for long times and long distances, by an ape covered with hair, is an advantage. The hot - or cold - savannah is not one of those environments.
valich said:
Apes and chimps lived in the jungles and trees while hominids went into the open savannah. They were no longer able to scamper back up into trees. Thus another reason for the transition.
Loss of trees for safety would select powerfully for speed and agility on the ground - four legs, like every other savannah-adapted tree animal. You cannot be seriously suggesting that a transitional hairy ape, almost unable to walk, let alone run, "went out into the savannah" full of leopards on its short back legs.
valich said:
Effective sweating requires as little hair cover as possible, as it needs air contact - moving air - over the skin to remove the heated sweat.
That isn't true. Horses, for example, sweat quite effectively - using sweat glands that don't waste oil and salt. Humans have an extra layer of insulating fat, and poorly designed sweat glands that waste oil and salt - these are strange adaptations, for an animal originally and fundamentally interested in cooling itself.
So the need for cooling probably came later - after the hominids had invaded the savannah, and had the ability to cover long distances on the ground, they could find advantage in doing this under the noonday sun.
(There are a lot more hairy humans than quadrupedal ones - hair loss is more recent by far than bipedalism, by that evidence. )