I will discount the "first" criterion, since I think that will depend on what you end up accepting as candidate plausible answers. I suspect there won't be many. I will offer a review I posted to usenet (rec.arts.sf.written) about 30 years ago, one of a series of reviews of "BioSF" - hard SF based on biology. I don't know of a rasfw archive site to link to, but it's my review so I will copy the whole thing below.
Courtship Rite (1982), Donald Kingsbury.
(UK title: Geta)
If Dreamsnake is the most-awarded book on my list of biological SF, then Courtship Rite must be the least-awarded but most deserving. It did not win either the Hugo or the Nebula for 1982. It did make the Hugo ballot, but lost to Cherryh's Downbelow Station. Courtship Rite has imaginative world-building, engaging characters, good though not brilliant prose, humor, a thriller of a plot, and, key to its appearance on this list, a hard-SF premise based on biochemistry and genetics. Cherryh has shown all of these in one work or another (I'm sure at least one Cherryh title will make it onto my
BioSF list), but Downbelow Station is not one of her stronger novels.
Courtship Rite beats it on every count. Kingsbury's web site says
that he is working on another book set in the same future, so perhaps
Courtship Rite will see republication soon. It certainly deserves it.
Courtship Rite is set in the far future on the world of Geta. Geta was, just barely, suitable for human colonization. The plants and insects are inedible, and conversely the microbial life of Geta cannot tolerate (nor pollinate) imported Terran plants. The colonists had no choice, so they stayed. They survived on laborious hand-cultivation of a small number of Terran plants, equally laborious husbandry of a small number of Terran insects, and extreme measures to ensure the continuing fitness of the human
population. At the start of Courtship Rite, the colonists have survived for long enough that the details of their arrival on Geta have become legend, and highly disputed legend at that. The horse is a remembered only in children's rhymes. The exodus from Earth is remembered only in oral tradition and a few scraps of writing. Terran history was not a survival skill; it was lost. Biochemistry was most definitely a survival skill; it survived, and
biochemists constitute the combined priesthood and ruling class of Geta. The very word "priest" in Getan is perhaps best translated as "biochemist", although it takes a while for the reader to pick up on this. Kingsbury immerses us in the Getan mindset right from the start, but cleverly leaves the significance of many Getan word usages to be picked up by context.
In brief, the plot of Courtship Rite chronicles the start of a technological renaissance on Geta, told from the vantage point of one Getan 'family' (perhaps best translated as 'commune') who are centrally involved. There is political intrigue, an engaging look at the [re]discovery of electromagnetism and information theory, and a central thriller of based on thwarting the simultaneous advance of a bioengineered crop-destroying pest, a mind-destroying plague, and a planned military invasion by a neighboring clan/state.
The Getan preoccupation with 'kalothi' (translation: 'Darwinian fitness') permeates everything from political theory to social planning to marriage (hence the book's title). Getan famines are frequent, and in time of famine only those with high kalothi will survive. To forestall wastage of scarce food supplies, the local priesthood will have ranked their clan members in advance as to their relative kalothi. When the famine comes, those with low kalothi will 'contribute to the clan' so that those with higher kalothi will survive. (translation: they will be killed to feed the rest of the clan when other food is scarce). Yes, cannibalism is common on Geta. One may debate whether it is in fact necessary on Geta, given the paucity of native food sources, but whether or no, it has become ingrained in Getan culture.
Politics follow. Each competing priesthood has some policy for deciding how kalothi is rated, and clans may attach themselves to that priesthood if they feel that policy is to their favor. Some priesthoods rank kalothi through annual competitions in the kol-game, a game that seems to be part board-game, part Sim-City, part genetic modeling. Others, in particular the Kaiel, cull
their clan members in childhood even in good times, pre-selecting for a population that will both be fit and have well-stocked larders. Fortunately for those of low kalothi, times have been good of late, so the hard choice of contributing to the clan has not been required of many. But the advancing crop-destroying pest will change that in a hurry if it is not stopped.
The maran-Kaiel family are members of the Kaiel priesthood, biochemists and politicians all, though Hoemei and Gaet are more inclined to political theory, while Joseai and Teenae are more inclined to advance politics by threatening a rain of pebbles (translation: 'bullets'). The cast of characters is large; in addition to the maran-Kaiel we are introduced to their prospective bride, Oelita, and to a shadowy clan of courtesans, the Liethe, who prove central to Geta's future.
The Liethe, by the way, have dealt with the issue of kalothi by limiting their reproduction to cloning. Courtship Rite manages to present one of the most plausible takes on human cloning I can think of in SF. That alone would be enough to make my BioSF list, but in fact it's just a side note to the main story.
As with Dreamsnake (BioSF review #1), I find the most compelling feature of Courtship Rite to be the vision of future biological research and applied bioengineering carried out largely without recourse to the mechanical tools and laboratory chemistry that are so familiar today. That's a road we are on already, and we've progressed a good way down that it in the 20 years since
Courtship Rite was written. The reagents we use today, from restriction enzymes to co-expressed chaperonins to tailored plasmids to... hmm, let me just summarize by saying that compared to even 20 years ago, an unbelievable number of the tricks and tools used in the lab today are produced by cleverly
engineered microorganisms, domesticated animals if you want to look at it that way. On Geta they don't need a room full of mechanical DNA sequencers to probe the genome of an invading pest; they can read off its DNA sequence by using tailored bacteria. That's a trick we can't quite pull of today, but I can almost see how you might start to work on it. I can only applaud
Kingsbury for having the vision to see that this level and type of technology is well within the range of the possible. Again as with Dreamsnake, it is much more plausible that a marginal society will maintain biological high-tech if it is based on renewable apparatus: bugs rather than NMR machines.
Courtship Rite also tackles issues of human genetics and eugenics. Geta has a planetwide society obsessed with Darwinian selection applied to socially advanced humans. The survival strategies taken by various Getan priesthoods may arguably be unworkable, but that does not detract one bit from the overall plausibility of the story. Some priesthoods get it wrong, and they fail to survive. Others get it right; they survive, and clans flock to their leadership hoping to survive with them at the time of famine. But, right or wrong, all of the factions of Getan society are shaped by their understanding of biological fitness, and Courtship Rite is a marvelous view of how such a world might play out.