The aliens are definitely going to take the general theme of the Tyranid/Zerg: often large, numerous, insectoid-bestial creatures with a singular desire to consume life that the encounter. In our case, they will be almost solely focused on consuming Humans for reasons that i'm sure will become apparent later into the story.  
This means, of course, they will not be blasting us with lasers, nor be utilising any kind of vehicles, though there will be some monstrous aliens who tower above the trees, for example. 
Expect to get eviscerated. It's going to be great. 
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[img]http://orig13.deviantart.net/d220/f/2013/091/0/0/zergling_ii_by_mr__jack-d1unnnx.jpg[/img]
[img]http://orig01.deviantart.net/63da/f/2014/056/d/5/tyranid_mawloc_artwork_by_zergwing-d780398.jpg[/img]
[img]http://media.moddb.com/images/groups/1/9/8153/TYRANID_CONCEPTS_11.jpg[/img][/center]

In terms of technology of involved characters, we're looking at rudimentary bows, spears, clubs, slings, and other forms of stone-aged equipment. As this will be taking place in the palaeolithic era, we are restricted to stone based tools, weapons, and torches. The nature of this time period does not allow for metal elements in equipment. Furs, leathers, reeds, skins, and other natural resources are the materials through which most things are crafted during this time period. 

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[@Halvtand] mentioned a 'religious' character being involved, which is an interesting subject to bring up that is worthy of discussion. I, like every other human being on Earth, have absolutely no idea how palaeolithic humans worshipped or otherwise revered a divine or religious practice. My assumption would be that 'gods' are more likely to be spirits of the wild in some sense: spirit of the wolf or bear, perhaps. This does not restrict certain characters from having other beliefs, of course, but I think in the spirit of deviating from the standard practice of civilised modern religion, we should stay well clear of common religious tropes in favour of more eccentric models. 
I imagine Humans of the time focus on survival first and foremost. Design any religious elements around this paradigm if possible. 

Any questions are gladly welcome as I shape this up.