Physiologically and genetically, Syreen are human: the only visable differences between the races are that Syreen skin tint is somewhat more bluish and their retinas reflect light like an Earth cat's. Additionally, the typical Syreen is more comfortable in a slightly warmer and more humid environment than the typical Human.
Syreen are also more psychicly active than humans, to such an extent that the captain of a Penetrator starship can make use of a psychic amplifier to coerce enemy crew. It is not known if Penetrator captains are chosen for exceptional psychic potential, or how common psychic potential is in the general Syreen population, although they do not evince casual telepathic powers that races such as the Pkunk or even the Zoq-Fot-Pik do.
On Earth, stereotypes of the Syreen are rampant: matriarchal, "Space Amazons," "Space Gypsies," "warm, breathing pinups," and other sorts abound. Of course, the truth is more complex than such stereotypes would imply.
Syreen society, like Human society, developed from hunter-gatherer tribes. But unlike Humanity, the Syreen were unplagued by war or other great social upheaval, instead joining together until their whole world was a single, peaceful, agrarian super-community. Also unlike Humanity, Syreen society and Syreen military roles were dominated by the females of the species. How connected these two facts are is a topic of some debate in both Human and Syreen intellectual circles.
Shortly before the advent of the First War, the Syreen suffered a terrible set back. Their homeworld, Syra (located at the star the Humans call "Beta Copernicus I"), was by all accounts a tropical Eden. However, a catastrophe destroyed nearly all life on the planet, reducing the population to those stationed in orbit as part of the Syreen Space Patrol (the vast majority of which were female) and a few planet-bound survivors. The Syreen gathered the remains of their species into space habitats (mostly incapable of HyperSpace travel) and began to search the galaxy for a new home.
As the First War began, and with no homeworld to fall back to, the Syreen maneuvered into Human space to avoid VUX raiders. It is implied but not explicitly acknowledge that Humanity's industrial base supported the Syreen fleet. The Humans and Syreen also participated in at least one crew exchange, confirming the viability of Human/Syreen crossbreeding and bolstering the Syreen's flagging population.
After the defeat of the Chenjesu, the Mmrnmhrm, and finally Humanity, the Syreen were abandoned by the remainder of the Alliance and left to face the Ur-Quan alone. They surrendered, and in exchange the Ur-Quan searched their planetary database to find them a new homeworld. This homeworld, renamed Gaia, orbits the star Betelgeuse.
The Syreen were content to remain neutral at this point until Zelnick proved to them that the destruction of Syra was in fact engineered by the Mycon using a Deep Child. After wreaking their revenge upon the Mycon, the Syreen joined with the New Alliance.