I hope I'm not too much of a TC (trepanned chihuahua) here :-)
One suggestion for the value of art is that it serves for sexual selection.
This is certainly the case for a few other species; bower birds strive to
build beautiful nests and mockingbirds strive to develop large vocabularies
of mocked songs. It's also at least part of the game for humans today; we
dress up, dance, get flowers, sing love songs, etc. as part of courtship.
Let's suppose that other hominid species used other things, like appearance,
for sexual selection.
Now Neanderthals, and presumably Erectus, had a social organization similar
to other social great apes - relatively small groups of tribes. Presumably
these groups related to each other as other apes did - minimally or with
hostility, save for occaisional exchange of mating partners. I do have to
mention bonobos as an exception, in which different groups do often get
along.
After all, if one male Neaderthal encountered another from a different tribe,
what would they have to gain by interacting? They're unlikely to have much
to trade - both will have similar, limited, technologies and hence similar
items. If they can communicate (not a given), they could exchange knowledge,
but most knowledge will be of their own ranges and it's obviously
double-edged to tell a potential rival how to forage or survive in your own
territory. Good general knowledge is pretty hard to come by and even harder
to check - if one gave another a bum tip, he wouldn't know until much later,
and checking tips often can be risky even later.
Now let's suppose that proto-sapiens developed an sexual selection for art
during the isolation in which the species sapiens developed. Rampant sexual
selection during speciation is quite possible, particularly if the species is
pretty well adapted to the conditions it's in. Irish deer, whose horns got
so large they drove themselves into extinction, are an example. It would
explain our funny voice boxes - bad for choking, but real good at carrying a
tune.
Now what happens when two Sapiens from different tribes meet? Unlike the
Neanderthals, they *do* have something to trade - their art. They can swap
stories, share tunes, recount jokes, trade clothes (maybe), share designs,
etc. If they do, each goes home to his or her tribe a little better equipped
to joust in the sexual selection. Plus, they can check the traded items at
the point of transaction - is the song catchy, the story moving, the design
pretty? They've got a built-in protection from shoddy merchandise.
Now while humans don't always "all get along", this provides an opportunity
for cooperation between members of different groups quite absent in any
modern ape, and possibly in any other hominid as well. While other hominids
might as well be mildly xenophobic (nothing to gain from dealings), sapiens
benefit from having friends in other tribes to keep an ear out for the latest
trends, as it were. So instead of family groups or small tribes of up to a
few dozen, humans are organized into meta-tribes with hundreds or perhaps
even thousands of members.
The real killer, of course, comes from the combination of poetry/song/epigram
with verbal meaning - the oral tradition. Over time, you will tend to see
catchy and meaningful combine. These oral traditions travel freely over the
meta-tribes. While oral traditions do contain a lot of drek, people want to
teach their children true and useful things and so oral traditions contain a
lot of truth as well.
So what happens when the isolation is breached and Sapiens faces off with
Erectus, Heidelbergensis, Neanderthal, and possibly other hominids we don't
know about? Well, the other hominids were apparently stronger; those brow
ridges were probably handy in a fight, and Erectus at least might well have
been a better runner. But their small isolated social groups can't mantain
nearly the knowledge base of the Sapiens meta-tribes, and based on human
history, those meta-tribes will tend to get together if it comes to a fight.
Neanderthal was probably more than a match for Sapiens 1-on-1; but 1-on-10
and Neanderthal would have been bird food. They'd have been outcompeted
and/or outfought.
This doesn't require that the other hominids lack languages. Neanderthal in
particular had quite a big brain. Human children spontaneously invent
languages to communicate with each other if not provided with any (and
sometimes even then). That kind of linguistic facility would have been quite
handy for Neanderthal or any other hominid, and they might well have had it.
It's just that without art they would tend not to have a social system that
would best make use of their ability to communicate.
*End of main point*
*Major digression and TC alert*
As I write this, my Warcraft II game manual sits on the table in front of me.
(Yeah, yeah, I have a real useful life) As I look at the picture of the
evil orcs, I realize they are depicted with brow ridges, protruding jaws, and
a low forehead. They eat meat, they are violent and xenophobic, they are
inhumanly strong, and they have no appreciation for beauty. In other words,
they are passable caricatures of non-Sapiens hominids, as I am hypothesizing
them. (the protruding canines are definitely wrong, and the green skin is
pretty unlikely). Isn't it interesting how readily we assign an imaginary
evil so many features of our dead rivals?
No doubt Sapiens was equally unsavory to them. I'm sure our heads would have
appeared deformed, and our bodies sickly. Our preoccupation with art would
no doubt have seemed insane to them - humming to ourselves, arranging
seashells on strings (and I'll bet Neanderthal at least would have been smart
enough to notice). So we saw them as orcs and they saw us as deformed
residents of an insane asylum. No wonder we didn't interbreed much, if at
all.