Set the scene: galactic-scale society. Somehow, we managed to get faster-than-light (FTL) travel working, and as a species got it together enough to actually get out into galactic society. For some reason or another, we haven’t yet hit the singularity, and so we’re still running around in our hundreds-of-thousands-of-years-old meat shells.
It’s at this point that we start imagining all the horrible aliens running around out there. Blob monsters. Things with tentacles, oh god the tentacles. Sharp teeth, claws, wings. Nightmares given shape by the infinite variety of life on a galactic scale.
But now, look at us from their perspective. We’re pretty well-evolved ourselves. Earth is a planet full of predators, threats that may be inconceivable to other species out there.
Humans, then, enter late into the scene of the galactic society. They’re very low-level, technologically. They reproduce rather slowly – something close to once a year, at most. Not threatening like the hyper-advanced machine races, or the overwhelming mass of the species that could overpopulate a planet in the space of a couple months.
But, on an individual scale, they’re terrifying. Humans shortly earn themselves a reputation as great fighters, brutal or calculating as the situation called for it.
There are a few things that make them notable: Though their primary sense is based upon the fairly conventional 390-700 nanometer spectrum, they can also detect low-frequency vibrations in their surrounding atmosphere; this sense is capable of triangulation, making them dangerous even in low-light situations. They can also detect (though not well identify) chemicals in the air.
In the upper limb of their body (also notable for containing the neurological center, making it a prime target for ranged weapons) they have numerous sharp teeth. From that mouth they produce a fluid notable for being incredibly bio-active, one of the most toxic substances in the galaxy. It remains one of their unique weapons – the multitude of microorganisms has yet to be reproduced in a laboratory, and dies rapidly after secretion.
By far the most terrifying aspect about humans, though, is their endurance. Even amongst the other species (non-sentient and legally limited to being on their home planet) of their home planet, their endurance is incredible. Breaking the internal solid structures of their limbs is, in general, not a killing blow. If they aren’t killed almost immediately by the shock, they’re capable of recovering from most wounds. Even without medical assistance available – they’ve been recorded performing field surgery on themselves. Their bodies have hyperactive scar tissue, making their recovery time incredibly short as compared to any other known species.
Uninjured, humans are capable of impressive feats. They can maintain long periods of activity without rest – on the order of several days.
The combination of these factors results in one thing: pursuit predation. Though not always the strongest or fastest in any battle, humans don’t give up. They will happily stalk their target for days on end without rest, and can carry on even after being injured.
Keep an eye out for humans. They’re terrifying.