The islanders have evolved parasitic feeding habits that rely on the tundra-dweller's metabolic need to produce surplus fat. In this way, the obese hosts have found an ecological niche that allows them to exist now that the tundra plains have disappeared and the mountain tribes failed.
A hiver party, 100 strong, takes its usual route along the undulating foothills, skirting the dreadful slimelands on the right, and the barren rocky uplands on the left. Beyond, the slope widens out into a valley in which water flows for much of the year, and where plants can grow and there are usually tubers or other thick roots to be had.
Before their narrow path widens the leader of the party grunts an order to halt. The seeker they carry with them is agitated, but is not telling them that there is food close by: it is telling them that others approach.
With another grunt the leader calls the warriors together in a protective wall; but they need not have worried. Those who approach pose no threat.
It is full day now, and the party can see five or six shambling creatures moving down the rocky slope towards the slimelands. The bodies are bulky (very bulky for the size of their legs) with thick hummocks and rolls of fat seeming to engulf them. Dull faces look out from the folds of pale flesh. In the dim light, however, the parasites are just visible: tiny and spiderlike, four or five of them are embedded in the deep fat of each figure, their faces buried and unseen, feeding continually from the creature's surplus.No threat to the hive, and so of no interest to the party; but the leader does recollect that more and more of them are seen nowadays wandering over their domain. They seem to be spreading from the forest areas that are their home. Dimly the leader wonders what they find to eat here, and how they protect themselves from the harsh Sun. He does not wonder for long, however. With a backhanded gesture, he brushes the first of the day's sand out of his moustache and signals for the party to move onwards. Soon he has the party on the move once more and the strangers have been completely forgotten.
Had the party stayed to watch, they would have observed the lumbering creatures scramble down into the flats of the slimelands and wade out amongst the disgusting blue-green sogginess. Dumbly they scoop up handfuls of the slime, exposing the yellow stench beneath, and begin to feed on it. The parasites embedded in their fat ignore all this. The food, be it nuts, leaves or slime, will be converted into huge deposits of fat and tissue that will sustain them.
The parasites and their hosts are not the first communal creatures to arise since the days of the engineers, but they are some of the only surviving types. The hunter symbionts and the symbiont carriers, in which the temperate woodland-dwellers teamed up with the tundra-dwellers to live on the cold plains, are extinct now. They took to the mountains after the cold tundra plains faded away, and there they existed for some time; but they were never really developed as mountain creatures, and all kinds of maladaptations began to show themselves. Eventually the populations dwindled and the whole two species died out.
That is not the case amongst the parasites and their hosts. The hosts, too, are descended from the tundra-dwellers, but unlike the symbiont carriers they changed as the conditions changed. Gone are the woolly coats and the resistance to extreme cold, but they still retain the thick deposits of fat. Indeed their metabolism generates more fat than they could possibly need, and that is what sustains the parasites. The energy and raw materials for all this production comes from the constant consumption of plants - any kind of plants and similar organisms, including the blue-green algal cultures that the aquatics developed as their own food source and spread over the lowland areas of the globe, turning them into the foul slimelands so despised by most of the land-living creatures.
It is not only the hivers that ignore the parasites and their hosts as they wade into the featureless slippery mat. Also ignoring them are the aquatics, not far away, looping and slithering about in the moist yellow depths below the slime crust. They are grazing their way through the algal culture that their ancestors established ages ago on the lowlands above the surface of the ocean. There is plenty of food for them now, not like in the days of want. They know very well that some creatures from the land come and steal from the edges, but the losses are small. The only trouble is dehydration. If the algal covering is breached there may be a considerable water loss before it has a chance to grow again; but with all the world's lowlands covered in the self-sustaining food-generator there is little to worry about.
By 3 million years (the 30020th Century)The tiny parasites still infest hosts and live on the excess of fat.
By 5 million years (the 50020th Century)
The moving stars have come. The decadent parasites, embedded in the fat layers of their grotesquely misshapen hosts, care nothing beyond their hosts' continuing survival; and their hosts are mere feeding machines, dumbly eating, eating, eating.
Decades later, they are wiped by the travelers of the stars.