Hope s human cargo had been delivered to Tyre by a crew that wanted rid of their burdensome presence burdensome because of all the obligations that presence entailed. The crew had assessed Tyre as an Earth-clone world capable of sustaining a colony, but their assessment had been optimistic; Tyre was a fraternal twin at best, a dangerous changeling at worst. The first people who actually tried to live on the surface found the going very tough, and they were far from certain that a colony could be maintained, even with the aid of a greater commitment of assistance than the crew wanted to make.
All that had changed when the aliens had been found, and contacted.
The aliens were humanoid, but the similarities were superficial matters of form; at deeper levels of physiology they were radically unhuman. They were naturally emortal and their processes of reproduction were very weird indeed. Each individual was actually a chimera of eight or more distinct cell types, which maintained a balanced competition within the body for the privilege of maintaining different physiological cycles and different organic structures.
The Tyrians evolved as they lived as they had to, given that they lived for such a very long time. Every now and again, they would get together and exchange resources, but not in the simple binary combinations of human sexual intercourse. Tyrians pupated in groups of eight or more, immersing themselves within the massive pyramidal structures that were their own natural SusAn technology, so that their unconscious selves could become fluid, trading chimerical components and forging new, fully grown individuals.
Alice assured us that if this seemed flagrantly promiscuous to us, it was nothing compared to what less complex Tyrian organisms were wont to do. The Tyrian sentients, and their quasi-mammalian kin, kept to themselves because they had minds as well as bodies to maintain, but less intelligent organisms creatures formed like various kinds of Earthly worms and mollusks enjoyed far greater ubiquity. The advantages of this exotic biology had allowed the local soft-bodied animals to enjoy far greater success than their Earthly kin, to the extent that vertebrates were much rarer and more marginal, and insects had never evolved at all.
All of which would have been no more than mildly interesting, story-wise, had the plot not been thickened by two further elements.
Whereas the Earthly ecosphere only has one family of fundamental genetic molecules comprising DNA and its close variant RNA the Tyrian ecosphere had two. One was a DNA-analog which, in purely chemical terms, was a distant cousin to our own and to a number of other analogs animating primitive ecospheres on other worlds. The other was quite different, and so far unique.
Im no biologist so I didnt find it easy to follow the explanation Alice gave, but I think I got the gist of it.
The reproduction of Earthly organisms is a very complicated process, but it has two fundamental components: the reproduction of raw materials and the reproduction of anatomy. What genes do, for the most part, is provide blueprints for all the proteins that make up our bodies. Different kinds of cells use the blueprints in subtly different ways, producing slightly different sets of products,
with those common to numerous cell types sometimes being produced in different quantities. The different cell types then have to be arranged into tissues and organs, and these too have to be distributed according to an anatomical scheme.
You might expect that the blueprint for bodily form would also have to be chemically coded into a set of genes, but its not as straightforward as that. There are bits of DNA whose function is to regulate the productivity of other bits of DNA, so that cells can be differentiated into a series of functional types, but the switching system is a simple one. In the same way, there are bits of DNA that are implicated in the way that different cell types are aggregated into tissues and organs, but their control system is also fairly simple. The process which determines whether an Earthly egg cell produces a cell mass that develops into a man, a bee, a crab, or an ostrich, consists of subtly different modifications of a surprisingly simple set of rules, whose application and enforcement have a lot to do with the environment in which the egg cell produces its embryo.
Figuring out how to simulate and direct an appropriate embryonic environment in an artificial womb was the breakthrough that made Conrad Helier a hero. The genes involved in the process are known as homeotic genes, and because theyre clustered together the whole outfit is sometimes called a homeobox. On Tyre, where the whole system works differently because there is no process of embryonic development the local equivalent of the homeobox isnt just a few extra bits of DNA thrown in with all the rest; its a whole other ballgame. On Tyre, the biochemical system determining the form of organisms is quite separate and distinct from the DNA-analog system providing the raw materials out of which bodies are built.