Although outwardly identical to modern humans (except for certain exotic hair and eye colors), internally the humans of the Solar Era are so different as to be essentially a new species. During the age of the Solar Alliance, the full map of the human genome was unlocked, mastered, and altered to provide universal improvements, remove evolutionary glitches, eradicate vulnerabilities and defects, and enhance abilities. Every organ, every cell even, is vastly more efficient and resilient. Although some fringes of society engaged in more esoteric modifications, strict proscriptions were in place by the Solar Alliance against radically altering the “divine form” of man, instead focusing primarily on simply improving it. Likewise, some fringes refused the changes to their genome and lived in “naturalist” enclaves, but, given the almost total devastation of the Judgment War, it is unlikely any of these enclaves still exist.
The average cognitive power of the human mind was drastically improved. The brain's ability to absorb, retain, and process information reached levels that would be considered savant-like today. Even just glancing through a textbook could confer considerable knowledge to the reader. Education in the Solar Era was at a pace that would be considered beyond overwhelming to modern students, learning topics such as algebra in early grade school, and studying quantum physics by the end of primary school. With cybernetic augmentations like an adjutant chip, an everyman of the Solar Era would have an IQ in the hundreds by modern standards.
In the After Fall calendar, these abilities remain, but the potential for growth is limited by the span of human knowledge being a fraction of what it once was, plus lacking educational technology beyond basic books. Even so, by the time of Unto the Breach, academic pursuits were rigorous enough that a modern human would still find their pace and breadth highly challenging, if not impossible. Despite never going to university, someone like Alex would have knowledge and intelligence on par with someone well into their master's program today. A relatively uneducated person like Ali, who mostly learned by glancing through some books with friends, would still be reasonably intelligent and educated in today's world (although her short attention span is another story), perhaps roughly equivalent to a mid-ranked high school education.
Nearly all physical aspects of the human body were improved as well. More efficient lungs allow better breathing under duress or in hazardous environments (survival from limited exposure to the vacuum of space is even possible without adverse effects). A stronger heart withstands extreme stress, the digestive system produces a fraction as much waste (typically only a single visit to the bathroom is needed each day), and improved liver and kidneys means far less disease and vulnerability to toxins (though booze needs to be significantly stronger to have the same effect. A human of the Solar Era could knock back half a dozen shots of 80-proof rum before they even got buzzed). Recovery from injuries is also significantly faster (a non-fatal stab wound that would require surgery and weeks of rest to heal from today would require only basic first aid and a few days rest).
Conversely, many of the same vulnerabilities still apply. Traumatic injury to the brain or another critical organ will still kill (though the improved resiliency may allow a human to survive an injury that would have been fatal to a modern person in many situations). Poisons and diseases with enough strength can still kill. Age still claims most humans in the end, although the average lifespan is double what it is today. Many of the same vices can still be found, and have the same degenerative effects (again, requiring a strength that would be crippling or even fatal to a modern human).
Some vulnerabilities have been at least partially mitigated, however. The most vulnerable arteries and veins are positioned deeper inside the body. Combined with denser muscle tissue that doesn't atrophy nearly as quickly as today, death from one of these arteries being nicked or severed is virtually impossible without concerted effort. The rib cage is now essentially a solid mass of bone, coupled with higher on-average bone density, also means a tremendous amount of force is required to deal any kind of damage to the most critical organs in the chest. The skull and cushioning fluids inside it are also slightly thicker, warding the brain against any kind of concussive injury.
Owing to improved muscle development, the physical differences between men and women in terms of strength and endurance are far smaller than they are today. Fitness and training are still vital, although the rate of muscle atrophy from lack of activity is vastly slower. Fat deposits are also more compact, more efficient, and the body's ability to regulate them is significantly greater too (i.e. it's extremely difficult to be majorly overweight).
Perhaps the most fundamental difference though, is on the cellular level. The DNA of humanity, while a double-helix today, for those of the Solar Era, is instead a triple-helix. So much additional information is packed into the genetics of humanity that such a change was deemed necessary. Every cell of the human body is more efficient and less resistant to decay or defect. Cancer is virtually unheard of in the Solar Era, typically only occurring due to external causes (e.g. excessive radiation exposure). Likewise severe birth defects are so rare as to be functionally impossible, even in sub-optimal conditions during pregnancy (i.e. poor nutrition or other factors).
The most obvious sign of the cellular alterations could probably be found in reproduction. Though the physical act is identical, the internal mechanics are quite different. Today, the menstrual cycle is constant from puberty to menopause; in the Solar Era it only occurs in response to sufficient stimulation, and menopause never occurs at all. Each sex cell contains a single strand of DNA, much like today, however two are not enough to complete a triple-helix. Male cells contain a genetic “header tag”, and the second male cell to merge with a female cell (which can't occur for 24-48 hours after the first merging) must match the tag to complete the helix. Essentially this means it requires at least two acts of sex with the same man, several days apart, to become pregnant. Though “accidents” can still occur, this helps increase the odds children are born to parents in something resembling a committed relationship, and have a more stable environment to develop in. It is certainly not impossible for two different men to have similar enough “header tags” to cause full conception, but the odds of being born from two fathers is so low that in a society of trillions, it only happens maybe a thousand times a year.
The Solar Alliance was considering the idea of implementing mass changes to the human genome to revert back to the “old school” way of reproduction, favoring vastly increased raw birth numbers over quality of early life for the sake of rebuilding humanity's numbers, however the plan was not implemented before the end of the Judgment War.
Growth and Development
The overall stages of life for humans of the Solar Era remain largely the same as they do today, mostly just with some differences to when certain stages begin. From 0-12 years, proper care, nutrition, and stimulation are still vital at this stage, though children are less susceptible to adverse effects that might hinder their ability to function later in life. Adolescence is when the changes become more pronounced. While puberty begins at roughly the same age, some aspects of development are different. The advent of basic sexual impulses tends to happen several years later, though actual fertility doesn't occur until full adulthood, which is generally around 21-22 years of age. After this, they have typically 120-140 years to live their life. 60-90 are considered the “middle-aged” years, and senior status (or retirement age) is usually marked at 125 years. Although having children in one's 20's is certainly possible, it's frequently considered irresponsibly young, and children in the 30's and 40's, or even later, is far more common. Although there is nothing like menopause any more, fertility for both men and women tends to fade away in later decades, and parents older than 100 are exceptionally rare.
In Unto the Breach, many characters have what appear to be magical abilities. While a user's own metabolism provides the energy for spells, this is the extent of the biological component involved. The source of these abilities is actually technological in nature, although extensive training is still required for those who have any “aptitude” in the magical arts. Not everyone has access to these abilities, and the reasons for this, as well as the actual source of these powers, is completely unknown to the people of Terranis, beyond observations that the aptitude for magic seems to generally (but not always) be a hereditary trait, as well as noting that no mention of such abilities was ever made prior to the Fall.