Finally, I get a chance to get my thoughts in order on the aggressive xenohumans of my science fiction setting: the Kai'Siri, a human species claiming the planet Sirug as its adopted homeworld...
The ancestors of the Kai'Siri race were bred from live specimens and directly manipulated cell-samples taken from Earth by the alien Strangers some 75,000 years ago, and over time they diverged from their baseline, H. sapiens, in the process and over time evolving into their own species.
Kai'Siri look a lot like baseline humans, but there are slight...ethnic...distinctions from Terrestrial humans, the Tellusine, that enable Kai'Siri to be readily distinguished unless surgically altered:
First, there is the typical eye coloring of the species, having a range of shades unusual for H.sapiens, such as ruby, gold and violet, and less commonly, terrestrial eye coloration. The ears have slightly rounded points at the tips, not so much that you would immediately notice, but more subtly noticeable in that first time visitors notice a vague 'oddness' about them. The last feature is most obvious, but only when they speak or smile (a threat gesture or invitation to a recreational fight. never smile at a Kai'Siri unless you want him unhappy with you or want to have a sparring match.): Kai'Siri have slightly longer, sharper canines on both the upper and lower sets of teeth. These last are just large enough to be seen when the lips are parted or the owner opens his mouth to speak. Kai'Siri are carnivores.
When the early proto-Kai'Siri were first brought to their adopted homeworld, they were still omnivores, but most of the vegetation of Sirug, though superficially similar to terrestrial biochemistry, had certain biochemical quirks in its sugars, starches and other components that were hazardous for humans to eat. Clearly another food source was needed.
The animal life on the other hand, in eating the vegetation, could convert the nutrients of the local vegetation not only into less dangerous substances with their digestive systems, with the exception of maybe one or two quirky amino acids not found in terrestrial life, they could incorporate these nutrients into their own makeup in forms readily digestible to the humans.
While much of the plant life was dangerous, the animal life turned out to be a good source of food. The humans changed over time from being hunter gatherers to hunters, and then ranchers. The digestive tracts and enzymes of the early humans of Sirug evolved to make best use of the available food source, animal life, though they did discover that some of the less immediately poisonous plant life had medicinal value, with a few usable in sparing amounts as spices and grains.
Over time, the Kai'Siri evolved culturally into a powerful, technologically advanced race, and are to this day regarded as producing many of the finest scouts and soldiers in the Local Galaxy.
Their culture encourages strength, assertiveness and recreational violence, while at the same time instilling a strong sense of respect for superiors, tradition, personal discipline, and regard for those of lower station.
The Kai'Siri were the first genetic humans to found and create a major interstellar civilization that exists to the current day, though they, the Tellusine of Earth, and the giant Mokthraga are currently involved in a bit of a kerfuffle with the colossal Broogh from the galaxy's far side, and its an open question as to which side will survive in this war with the Swarm-Fleets.