After the horrors of the first contact of the Scranja war, Humanity found itself in a predicament that it had not faced since well before the birth of civilization. Scattered to the solar winds, and very nearly the victim of genocide, many of the isolated pockets of mankind faced population bottlenecks, and the possibility of colony failure from inbreeding.
With the basic infrastructure for travel having largely been laid to waste by the destruction of Earth and Jupiter, and a general lack of ships, shipyard, and fuel, many human colonies found themselves with but a few hundred to a thousand souls, and no significant way to cross the interplanetary void. While recycling and isolated farming technology of the 27th century made many of them self sufficient enough to survive, they could not functionally thrive for the generations it would take to rebuild from the war.
Monogamy was an early social more to evaporate in the face of that pressure, but in the smaller colonies, even the greater genetic mixing encouraged by the practice of partner trading would not be enough. Appealed to over the remaining communication channels, the scientists that remained on Mars were able to offer a solution.
Repurposing basic genetic scanning and therapy equipment, they created the ability to read the genetic information of volunteers. Each sequenced genome would be read, and potential major problems red flagged, while healthy chromosomes or easily corrected issues would be green flagged, then stored in a master database on mars. New genomes would be created by randomly selecting two of each chromosome from the green flagged database, then transmitting the synthetic genome to the colonies, where they would be built base pair by base pair by whatever equipment could be adapted to the purpose. They could then be implanted into a volunteer's egg and placed in a volunteer mother, much in the way of the earliest cloning experiments, for nothing yet existed that could replace a human womb.
Biologically, the process was a success. Viable individuals could be created from scratch, and the implant technique could be used to encourage multiple births. The population of the far flung colonies could be maintained indefinitely, until the basic infrastructure for space travel could be reestablished throughout the asteroid belt.
Sociologically, the experiment faced much greater headwinds. The artificially created children were dubbed first Synthkids by the media, then Synthmen as they matured, starting the process of setting them apart, a process only gleefully continued by the remaining religious communities, who denounced them as the product of men playing God where they ought not. Further, despite the official encouragement of the process and subsidation of the resulting families, many were birthed by volunteers on the lower social and economic classes, effectively producing an distrusted underclass. Racism directed against them was common for several generations, until the program was discontinued with the resumption of travel. Slowly, they interbred back into the general population, and were forgotten, until the discovery of FTL travel.
This time, the rest of the species was ready for them. Spreading between the stars now, rather than throughout the solar system, was a task that called for many more living men than the newly formed Neo-Terra had. Billions would be needed to spread out, and for man to hold its own in a galaxy that it was discovering to hold dozens of others.
With the invention of a viable artificial womb in 2912, Synthmen can now be used to rapidly explode the population of new colonies, letting humanity recover itself into an economically viable power, its population once again measuring in the tens of billions rather than the tens of millions. Indeed, the majority of the Starkin are descended from at least one synthetic human.
When born from a human woman, the Synth is generally considered to be the full child of that woman, named and raised by her. However, in the colonies, when the artificial womb is used, they are generally assigned a given name by a lottery containing many of the traditional names of the colony's dominant cultural background. No surname is assigned, just a number. They are then raised in large groups by a handful of trained surrogate parents, both sent with the initial colonization effort and trained locally in the successive generations. Unfortunately, this leads to many of the Synths being well educated, and socializing well with each other and baseline humans, but being very poor at intimate relations and family life. Divorce and child protection interventions are exceedingly and unfortunately common in families that contain one or more Synthmen generated from the artificial womb. This slowly settles down in successive generations as natural families and natural reproduction supplant the use of mass artificial production and mass raising techniques.