Fear has been a helpful trait that has kept our race alive since back in the day. It continues to be necessary for survival as a species. However, it can be just as harmful. When faced with death or severe injury, people do things that are more primal than practical. Individuals can become paralyzed with fear, scream and wail with no clear objective, or simply freeze in place. These actions are not conducive to their survival, for it leaves them vulnerable and prevents then from taking decisive action against the thing that is meant to harm them. It takes a good amount of training to be able to conquer those emotions, which is why police, firefighters, or soldiers are able to run into dangerous situations.
In this species of human, I need for them to be able to control their actions and emotions while going through the physical process of fear. I want this to develop naturally in their biology through evolution, instead of artificial scientific shenanigans. This is not meant to reduce or remove the amount of fear that a person experiences, but to allow them to "keep their wits about them" so they can think practically and act in the moment. If death is the only outcome, they should be able to accept their fate. They should also avoid turning into Spock from Star Trek, a member from a race called Vulcans who are completely unemotional beings that are always rational.
Psychopathy is a broad spectrum of behavioral disorders, one of which is boldness or fearlessness.
Low fear including stress-tolerance, toleration of unfamiliarity and danger, and high self-confidence and social assertiveness.
Not all psychopaths have this trait, but those who do experience less fear and have a different physiological reaction to frightening situations. They can understand that people who literally stare down the barrel of a gun fear for their life, but they cannot recreate this feeling in the same situation.
Results: As predicted, psychopathy was associated with reductions in the subjective experience of fear relative to other emotions. Children and adolescents with psychopathic traits reported fewer symptoms associated with sympathetic nervous system arousal during fear‐evoking experiences.
Conclusions: Rather than being related to uniformly impoverished emotional experience, psychopathic traits appear to be associated with greater deficits in subjective experiences of fear. This pattern of responding supports and extends previous observations that psychopathy engenders deficits in fear learning, physiological responses to threats, and the recognition of fear in others.
And another study examined measurable physiological reactions to fear in psychopaths:
Inmates with high scores on both PCL-R factors and inmates with high scores on only the social deviance simension (Factor 2) display smaller increases than nonpsychopathic inmates in both heart rate and skin conductance when asked to imagine fear-provoking versus neutral situations. Thus, some of the reduced responsiveness to fear stimuli may be a function of ASPD, not psychopathy.
Patrick and his colleagues have also reported that psychopaths display an unusual attenuation of startle responses to intense stimuli presented during the viewing of negative effective slides. Whereas nonpsychopaths display larger eyeblinks if startled while viewing slides of negative valence, psychopaths display smaller blinks under such conditions. [...]
Such group differences are interpreted as reflecting reduced activity in a fedence/withdrawal system in psychopathic individuals. (Source: page 117)
The MMPI Pd scale was used to separate Ss [subjects] into High and Low Pd [psychopathic disorder] groups of 10 Ss each. Skin resistance was recorded while the numbers 1 to 12 were consecutively presented on a memory drum, with Ss having been previously informed that an electric shock would be received when 8 appeared. The results indicated that as shock became imminent, the increase in conductance was greater, more rapid, and began earlier for the Low Pd Ss than for the High Pd ones. The results were related to the clinical observation that the psychopathic person is relatively unaffected by the threat of punishment.
The predisposition to develop psychopathy is inheritable:.
In this respect, psychopathic personality traits [...], specifically callous-unemotional (CU) traits, have been proposed as the most important specifiers within antisocial populations16 and have been included as such in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V). This is not only because of their clinical relevance [...] but also for their associated genetic risk and neurobiological deficits, including aberrant reactivity in limbic and prefrontal brain regions (Marsh et al.18 and Finger et al.,19 for a review see Viding and McCrory20). (Source)
Results: DeFries–Fulker extremes analysis indicated that exhibiting high levels of CU [callous-unemotional] is under strong genetic influence. Furthermore, separating children with AB [antisocial behaviour] into those with high and low levels of CU showed striking results: AB in children with high levels of CU is under extremely strong genetic influence and no influence of shared environment, whereas AB in children with low levels of CU shows moderate genetic and shared environmental influence.
Conclusions: The remarkably high heritability for CU, and for AB children with CU, suggests that molecular genetic research on antisocial behaviour should focus on the CU core of psychopathy. Our findings also raise questions for public policy on interventions for antisocial behaviour.
Being unfazed when faced with danger is actually deletary.
Evolution is not about the survival of one individual, but about the perpetuation of traits that are successful over generations. The thing about fight or flight is that while on ond hand it may get a handful people killed due to paralysis or bad decisions, it does save millions of lives on the other.
If you lack a fight or flight instinct and you don't have a primal fear of certain situations, you are in for a Darwin Award at some point of your life. A cursory search over tje awards will provide many examples of why this is so.
(8 January 2001, Florida) Greed knows no bounds, even when the situation is life threatening. In an Orlando restaurant a fire spread when a rusty metal chimney ignited the old dry wood surrounding it. Flames shot from the roof and the dining area began to fill with smoke. Patrons rushing to the exits to flee the burning restaurant... at least, most of them did. But one greedy patron rushed to the buffet to fill his plate with food so he could eat outside, and another was seen stealing tips from the tables in the smoke-filled room. A third patron asked fire officials if the restaurant would reopen later in the day so she could have her pudding.
(...) The scientist believed that sharks could sense fear, and that his mastery of his heartbeat through yoga techniques made sharks regard him as a fellow predator, not fearful prey. Other shark experts advocate dressing in black wetsuit, hood, and gloves, to cover skin that resembles pale-colored prey in murky waters, but not Erich. He had "waded with sharks" for years. And this Wednesday, a video crew was prepared to tape throwing fish into the water to attract bull sharks, then wading into the sea with bare legs to observe their body language.
The sharks are often accompanied by remora, quasi-parasite fish that clean the sharks and sometimes attach to them with a suction cup for long rides. Just after one remora swam between Erich's legs a shark followed, and--unaware that Erich's yoga techniques had turned him into a fellow predator--snapped off a huge chunk of his left calf. He was pulled from the water in shock and flown by air ambulance to West Palm Beach, Florida, where doctors tried to save the remains of his leg and his life.
He spent six weeks in the hospital trying to figure out what went wrong. He concluded that nothing went wrong; the shark simply mistook his leg for the remora in the murky water.
This is just the fight or flight reaction when faced with fear inducing situations. this is very hard to overcome as it is an innate reaction, no Thinking happens, but as you've already stated through training it is already possible to overcome.
There are many examples of people doing things that others find terrifying. A prime example is parents running into a burning building to save a child. overcoming these fears or at least still being able to think methodically takes training, it is possible for natural occurrences to train people but these occurrences would need to happen commonly enough for people to not be struck by the shock of the event, while still fearing it. and unfortunately the more often an event happens are the person survives, the less people fear it... This is where the issue lies with your situation.
Fear is sliding scale, not a binary function.
People fear different things, plenty of people fear spiders, but then again many love them, and keep them as pets. Those that fear heights, those that jump of of planes, etc etc...
Then there's the exposure, when Cars were first invented, there were people who feared it... These days there are still those that fear excessive speed but travelling by car is literally like a trip to the shops for most. People feared flying at first, yet many people fly. and again... some of those people even jump out the plane strapped to some string and a big tablecloth!!!
So a singular fear inducing event is unlikely to train everyone to cope in most situations and recurring natural events reduce the fear
TLDR: I don't think this level or reaction is possible naturally... only with training
Do nothing for them and let this go on for millenia. Make sure as many people as possible come in contact with multiple dangerous situations without outright killing them.
The reactioms you describe used to be useful, either for the individual or the species (screaming alerts of danger for example, good for the group but not necessarily for the individual). By exposing people to modern threats, the "bad" reactions remove them from the gene Pool and eventually new reactions develop through evolution.
Adrenaline does this already. It allows people to power through physical pain and mental fears in threatening situations. You can extrapolate a future where adrenaline has a stronger effect due to constant external pressures like fear over millennium, to the point that humans use it all the time and has resulted in hyper focused thinkers. They could have active and passive adrenal glands. They could have more then normal amounts of adrenal glands. They could have evolved a more complex and useful version of adrenaline. Or more receptors for it.
Just make it a natural mutation over time in the species.