Hmm well, whatever the yautja were BEFORE they became what they are today was, if I were to guess, much more brutal and physically capable.
Based on simple theories of evolution and physical structuring, it can be assumed that the yautjas were once quadrupedal organisms, which became bipedal in order to spot predators more efficiently.
(That is the only purpose of any organism to develop bipedal capabilities aside from reaching high vegetation)
The yautja were most likely always carnivores, explaining their sharpened teeth and front-facing eyes. This is also why they only sense body heat.
In the past their mandibles must have been larger, allowing their ancestral selves to both bite and seize prey in one swift motion. As the yautja evolved, these mandibles shrank, up to the point where they are only used to protect their jaws, enhance communication, and operate certain machinery.
The ancestral yautja also most likely always had the tough, ridged hide which they possess today, but the flesh their ancestral selves must have been much more durable and resilient than the flesh of the current yautja, whose epidermis has weakened with the introduction of technology and lack of physical abuse.
Their rough claw-like fingernails hint that the yautja also may have used to possess claws for seizing prey and enhanced locomotion, but these have shrank for improved dexterity and bipedal movement.
The tendrils which cover their heads have always been for protection, but in the past this was most likely from the claws and teeth of other yautja and the more fearsome carnivores which they had to aviod.
As time progressed, however, the yautja developed technological advancements which enabled them to eliminate their predators.
I believe that xenomorphs not only existed on the same planet, but that they were actually the yautja's ancestral predator. Because xenomorphs give off no body heat, the yautja must have had to develop masks to view other spectrums in order to combat their timeless foe.
This also helps explain why the yautja consider it a holy ceremony to fight and kill xenomorphs. After all, what better way to prove one's bravery and hunting skills than to kill that which once plagued your ancestors?
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.