The dominant species of the Class 12 deathworld planet known in the most widely-used native language as Earth.

We're going to assume here that the reader is mostly familiar with humans outside the context of the Jenkinsverse and skip a lot of the basics like physiology, habitat, diet, society, biology, etc. If you need to know such things, rather than abducting them; you can peruse the Big Wiki here.

As a result of evolving on a the most dangerous temperate world in the galaxy; higher than Galactic Standard (9.81 m/s² vs 7.55 m/s²) gravity, poisonous plants and animals, competing with carnivores, infectious microorganisms, natural disasters e.g. earthquakes, volcanoes, hurricanes, high background radiation, Humans have developed unique physiology to galactic species at large.

Humans have a calcium based endoskeleton which is stronger and heavier than the silica based structures of most non-deathworld species.  

Human muscles are unique as well; they only pull in one direction, and with great force.  

Human tissue is very dense, so even though they are much shorter than the galactic average, they tend to weigh much more. As a result they are comparatively highly resistant to kinetic pulse fire

Deathworlders in general have much faster and more sensitive neurological systems as a result of constant threats of predators and potentially fatal injuries.  

These factors combined make humans very dangerous compared to other races; even when non-hostile. Humans risk seriously injuring ET's with any physical contact. In a fight, humans can easily overwhelm alien opponents with their sheer mass, strength, and speed.

However, humans aren't almighty gods, and have some weaknesses. For one, Earth's gravity creates a thicker atmosphere, and humans can experience shortness of breath and altitude sickness in the rarer air of some planets, for example, Origin. In extreme cases, this can even result in hypoxia and embolisms, though this is rare - the galactic standard is perfectly tolerable to most humans. Another drawback to lower gravity is due to the 'use it or lose it' energy saving measures in deathworlder biology; humans grow weaker in prolonged microgravity.  

The dense human musculature and skeleton requires a vast intake not only of calories but of vitamins and mineral supplements or else humans can become badly weakened or even killed by conditions such as Scurvy or Beriberi. Humans away from Earth require at least four times as much food and water as even the most voracious non-human species just to remain healthy, and the only food source which reliably meets a human's nutritional demands are tasteless and unappetizing emergency ration spheres that most beings will only resort to when there's nothing better on offer. As omnivores, humans are able to eat and get usable nutrition from an exceptionally wide variety of sources, many of which other races would find inedible, toxic, or even lethal.

The human nervous system is a finely-tuned organ that operates at astounding efficiency. Humans are the equals in technological and logical intellect to Corti despite having physically smaller brains. It is outside of those fields, however, that humanity truly shines. Humans are more creative, more imaginative, better lateral-thinkers, more emotional, have a deeper sense of spirituality and a finely-honed instinct for survival. Human artwork explores themes never even considered by other species, invents fantastic realms and ideas wholly beyond the imaginative capacity of non-human life, and does so with an emotional depth and power that can reduce even jaded and galaxy-hardened beings to tears (or the equivalent).

Human senses are overall sharper, although certain other species have more specialized senses. Human reflexes are preternaturally fast, and even when acting reflexively they are capable of astonishing precision. Finally, humans retain the ability to learn "muscle memory" and ingrained reflexes throughout their lives rather than only during their childhood years.

Again, this astonishing ability is not without cost. The human brain is a precision machine, engineered by the most brutal trial-and-error regime in the galaxy and adapted to serve as much as a weapon in humanity's arsenal of survival as is the rest of the body. Like all precision-engineered equipment however, the human nervous system has very fine tolerances, without any of the inherent margin of error built into most other beings. Humans are therefore more vulnerable to certain effects, principle among them being a weapons technology known as Nervejam Grenades. Ethanol, while just as toxic in terms of tissue damage to every other species in the galaxy, only has an inebriating effect on humans. Also, by galactic standards, Humans are intensely neurotic, superstitious, susceptible to brainwashing, prone to delusions, and vulnerable to mental illness.

This quirk has, in a strange way, contributed to humanity's survival on Earth: Treating everything as a narrative and seeing intent and agency behind everything - even where other species would think it shockingly irrational, paranoid and delusional to see such things - primes a Human to see threats coming from a long way off. The many false alarms this raises have not adversely affected any human's reproductive fitness, while the hair-trigger identification of threats (or, more often, the superstitious avoidance of hazardous situations) has saved uncountably many lives over the generations.

Humans experience a unique emotional state known as "spirituality". Sometimes also known as "grace", "faith", or "contact with God", this euphoric emotion - a feeling of connection to the universe, a sense of cosmic purpose and a place in the grand scheme of things - forms the emotional foundation for all human religions, and is the ultimate secret to humanity's success on Earth. Without it, the relentless grinding hardship and death would long since have driven humanity into the same depressive species suicide that awaits all sapient lifeforms in the long run.

Psychologically, humanity is well adapted for a world where there is always something more dangerous than they are. Humans subconsciously consider themselves to be the underdogs, and thrive on it - humans are never happier than when facing adversity, overcoming obstacles and having to apply themselves in new and challenging ways. In fact, the entire art form of "computer games" - which never progressed beyond being a minor curiosity away from Earth - is founded on rewarding this instinctive hunger for opposition or competition. By the standards of the rest of the galaxy, Humans are freakishly good at strategy and tactical contests, be it games or combat.