Sebaceans are a bipedal species that largely resemble humans to the untrained eye. John Crichton (the only Human thus far who has made it into the Uncharted Territories as far as it is known) is often mistaken for a Sebacean.
Indeed, there are very few differences. One appears to be eyesight, which is seemingly an improvement on human eyesight (2.04 " Crackers Don't Matter"). However, one detracting factor is that Sebaceans seem to have an intense intolerance for heat. When exposed to extreme heat, the Sebacean falls into a state commonly known as Heat Delirium . Left untreated, a comatose state called the Living Death sets in, reducing the Sebacean to a catatonic-like state ("Exodus From Genesis", "Promises") Once this Living Death takes over, Sebaceans will kill their own out of mercy. ("Exodus From Genesis", "Promises")
Human and Sebacean genetics are compatible. Indeed, the two species are compatible for sexual intercourse ("Relativity") and seemingly compatible enough to produce offspring. (2.10-2.12 "Look at the Princess" trilogy, "Dog with Two Bones", "Terra Firma", "Prayer", 4.19-4.21 "We're So Screwed" trilogy, "Bad Timing")
The ancient Eidelons went to Earth thousands of years ago and picked up several humans to use as the genetic base to create a guardian race to help them keep the peace throughout their territory. This race became the Sebaceans. ("Peacekeeper Wars")
In regards to Scarrans, it would appear there is some benefit that could be reaped by applying Sebacean genetics ("Incubator") though it is not yet revealed what. Perhaps this is due to a Sebacean-Scarran hybrid (in this case Scorpius) being able to detect an aura of sorts around people that indicates whether they are being truthful.
Scarrans appparently have the potential to breed hybrids with Sebaceans, though few survive long after birth as a result of the genetic conflict between the Scarran affinity for heat and the Sebacean aversion to it. Scorpius, the only known Sebacean-Scarran hybrid, must wear a thermal regulator suit with cooling rods inserted directly into his brain in order to maintain a tolerable temperature to function.
Yet more genetic possibilities for the Sebacean race exist with Pilots. Splicing DNA can be a tricky business, but the introduction of Pilot DNA to Sebacean Aeyrn Sun, ("DNA Mad Scientist") allowed for the Sebacean to intuitively understand Leviathan controls via the Pilot console interface ("Through the Looking Glass"), though at the possible expense of various mutations.
Sebaceans can also accept a neural transponder to create a direct neural link to a Leviathan/gunship, Talyn. ("Mind the Baby") This link is more likely due to the Peacekeeper design than natural Leviathan physiology.
While humans cannot stand as much cold as Sebaceans can, alternatively, humans can withstand much more heat than Sebaceans.
Physiology and Pregnancy
Sebacean pregnancy seems to differ from that of Humans, at least in the early stages - this could be part of the natural genetic evolution of Sebaceans, or something bred in by the Peacekeeper military establishment. It is unknown how normal Sebacean pregnancy compares with that of Humans. It is known that Sebacean pregnancies can survive being turned into a metal statue for at least eighty cycles. ("Look at the Princess - I Do, I Think") Labor and delivery of Sebacean babies may take less time and may be less complicated and physically taxing on the mother than Human births. :This is not certain as the only Sebacean labor/delivery noted was that of a Peacekeeper soldier/Human hybrid which took considerably longer than a normal Sebacean birth and was complicated by breach presentation ("Peacekeeper Wars: Part 2")
The reproductive biology of Peacekeeper females born on a command carrier is further modified so that the conceptus enters a state of stasis after the 2nd cellular division. This stasis can be maintained for up to 7 cycles. Within this period, the pregnancy can only progress through medical intervention by a surgeon. (4.06 "Natural Election") Whether foetal development spontaneously progresses or aborts after the 7 cycle stasis period is unknown. Peacekeeper females may be able to keep multiple (possibly dozens) of concepti in stasis (The validity of this point is uncertain as that information was given by Aeryn Sun while she was resisting torture under the influence of pregnancy & drugs; 4.18 "Prayer"). Peacekeeper females in a battle unit have had further genetic modifications to accelerate the gestation process. The exact length of the gestation period is unclear, though it is described as a "geometric pregnancy." Some estimate "a few weeks rather than months" [After Aeryn Sun had her baby transferred back into her some time after the first quadmester, she estimated that the birth would occur in a matter of solar days ("Peacekeeper Wars: Part 1")]
Another difference in their physiology is the lack of redundancies. Where humans have two kidneys to filter toxins out of the bloodstream, Sebaceans have a single "nerve" called the paraphoral nerve that serves the same purpose. Previously, a Sebacean who has had this nerve damaged required a donor. Advances in technology now allow PeaceKeeper medics to synthesize the tissue for the nerve itself, eleminating that as a weakness. It is unknown whether this is available to other Sebaceans, particularly the non-Peacekeeper colonies that lie in the Uncharted Territories.
Deepening the possible relation with humans, it is known that a peace was made between Sebaceans and Scarrans on the planet Arnessk several thousand cycles ago. The peace was handled by priests on Arnessk, and a tile found at an excavation site with a seemingly Egyptian hieroglyph points to possible human participation (4.02 "What Was Lost - Sacrifice").
Sebaceans have somehow thrived and become one of the dominant races in the universe. They have a strong militaristic background and a large military force known as the Peacekeepers - soldiers who can be hired out to defend a planet or a cause, for a price of course. It seems that the Hynerian Empire uses Peacekeepers to enforce their edict, and the Peacekeepers have made dealings with the Luxans. ("Into the Lion's Den - Wolf in Sheep's Clothing", "Bringing Home the Beacon")
The Peacekeeper force currently commands huge fleets of spaceships, including Prowlers, Marauders, and Command Carriers among others, though several Sebaceans have broken off to form their own colonies, such as the Breakaway Colonies deep in the Uncharted Territories. (2.10 - 2.12 "Look At The Princess" trilogy)
Not all Sebaceans are Peacekeepers, as some broke away and developed other societies. Conversely, not all PeaceKeepers are Sebaceans, as members of several other races have been allowed to join or been conscripted over time, but most are Sebaceans.
Science & Speculation (non-canon)
While Sebaceans seem physiologically comparable to humans, their lifespan appears greatly extended by a few hundred cycles (2.15 "The Locket," 3.05 "...Different Destinations"). It is now thought though that this is a benefit brought on by translator microbes slowing down the aging process (see "Horizons" by Rockne S O'Bannon.)
Because of when the Sebacean ancestors were taken from Earth by the Eidelons, Sebaceans could either be a subspecies of the Homo sapiens line (i.e. Homo sapiens sebaceanus), or what could be called a "physiological variant" on the Homo sapiens sapiens sub-species, (i.e what is generally thought of as race).
A range of Sebaceans have been seen on Farscape, it would appear there is a great deal of physiological variation among sebaceans themselves. This may suggest that they are indeed their own subspecies, with their own "physiological variants" that only incidentally resemble our own.
Based on the ability of Sebaceans to hybridize with other radically different species, (i.e. Luxans, Scarrans, etc.) the Eidolons must have utilized some exceedingly advanced technology to enable this type of flexibility within the Sebacean genome. However, based on the structural similarity of certain non-human species, (i.e. Delvians, plant evolved; and Scarran High Caste, reptilian evolved), the possibility exists that these parallel evolutionary processes giving rise to analogous physiological forms also gave rise to a very analogous genome capable of the necessary genetic compatibility.
The close similarity between Humans and Sebaceans, and the even closer neural similarity between Humans and Interions suggest Interions, too come from the original Human anscestors taken from Earth in the ancient past.