Two species form a single unit of value to both - symbiosis. The hunter symbionts have skills that their symbiont carriers lack. The hunting ability of the swift hunter symbiont provides enough food both for itself and its slow-moving carrier. The symbiont carrier, in turn, provides both with general movement and protection against the cold.
The symbionts are marching.
A temporary and small retreat of the northern icecap has created vast new tundra areas over the northern continents. For the first time in 5,000 years the rate of melting of the edge of the glaciers is exceeding their rate of southward movement. In effect, the edge of the icecap is melting back. Rocky debris, broken up by the weight of the ice and shoved along the ground by the southward movement, now lies in hummocks and thick beds of mixed clay and boulders. Here and there a long winding esker (a steep ridge of rubble marking the old course of a subglacial river) snakes across the plain. Huge lumps of abandoned ice embedded in the clay melt slowly, gradually becoming lakes.
Yet below the ice-free surface the soil is still permanently frozen. Little grows here, except for the hardy grasses and reeds along the sides of the lakes, and the mosses, lichens and heathers that form tussocks over the rocky soil. Away to the south lie the great forests, which are already spreading northwards into this newly-exposed land, with their outposts of stunted willows, birches and rowans, backed by the dark palisades of spruce and pine. It will be a short-lived advance if the ice moves south once more.
It is the domain of the symbionts. From a distance as they trek across the plain they look like the tundra-dwellers, but they seem to be bigger and rather top-heavy. A closer look shows them each to have what appears to be two heads - a large one surrounded by the woolly ruff of blubber, with small eyes and large nostrils, and beneath the chin a smaller head with big ears and active, darting eyes. The herd consists of about 30 individuals, adult and juvenile. They follow the biggest, whose lower "head" seems to be looking around all the time for the best way to travel.
It stops, staring away into the distance. A dark flock of birds circles in the far sky, something that should be investigated. The leader's arm shoots out in that direction (an amazingly slender arm for a creature of such a size) and it turns towards the distant flock. The rest of the herd turn as well, each one also shooting out an arm.
After a while they come to the site of the disturbance. Most of the birds are hawks, and every now and again one swoops to the ground and carries away something small and furry. There are small arctic foxes there as well, but these turn and scatter as the symbionts approach. The cause of the activity is now visible before them. A mass of small rodents (lemmings) is on the move. Every now and again, in times of relative plenty, they breed prodigiously, until there are so many that the food in their area runs out. Then they move en masse to find new foraging regions. The symbionts have just come upon one such migration, a moving furry layer that stretches in a straight line along the ground towards a possible distant food source.
If the movement of the rodents is remarkable, what then happens to the symbionts is even more strange: about half of the individuals fall apart, literally. Each one resolves itself into two separate creatures. The huge hairy arms of the carriers that were clutched across its chest open up like doors, and to the ground drops a spindly figure - the owner of the "second head" and the pointing arm. The slimly-built creatures are running as soon as they hit the ground, and ten of them plunge into the moving mass of lemmings, snatching and killing as they go. The remaining symbionts, mostly females and young, stand watching, shouting encouragement in words and noises that only members of their own group can understand. The symbiont carrier shapes vacated by the hunter symbionts stand immobile and silent.
After a while the hunter symbionts gather up the rodents that they have killed and bring them back to the group. They are handed around to the figures clutched to the bosoms of the carriers. Then each hunter symbiont returns to its own carrier and, with a touch and a word, it is gathered up into the great arms. For a while they eat. Each lemming is partly eaten by the being at the symbiont carrier's chest, but then the greater part of it is handed up to the great mouth in the head above. The carrier part of the creature receives the food passively, and eats it all.
This strange state of affairs began thousands of years ago. When the temperate woodland-dwellers (the humans that had been engineered to live in the temperate forests) spread out to hunt on the tundra, with the coming of the current ice age they had to adopt all kinds of strategies to keep themselves warm and to survive. Some found that they could live close to the dull tundra-dwellers and share their body heat. The tundra-dwellers did not mind this, if the temperate woodland-dwellers shared their food with them. So the symbiotic situation gradually developed, until now the newly-evolved hunter symbionts could not travel by themselves in the tundra and these particular tundra-dweller descendants (the symbiont carriers) would not be able to survive on their own.
Once the food is eaten, the group sets off again. The smaller hunter symbionts can talk to one another, using a simple language, but each communicates with its carrier by nudges and gestures - a pointing of the arm is enough to tell the carrier to go, and which way. They follow the lemming march, as there will be good eating here for a day or two.
By 50,000 years (the 520th Century)
The tundra is dwindling away now, but for many thousands of years groups of tundra-dwellers have been spreading southwards, changing their diet and adapting their lifestyle as they went, through the coniferous forests and into the zone of deciduous woodlands. Because they have been forced to change all the time, they have a better chance of survival than the groups of their symbiont carrier relatives who remain static on the tundra with the hunter symbionts.
By 1 million years (the 10020th Century)Communication between hunter symbiont and carrier has been simplified to a telepathic link - the huge slow-moving symbiont carriers controlled directly by the weaker but agile-minded hunter symbionts. Fights, when they happen, are usually ritual. Death is unexpected.
The leader starts from his sleep because his carrier is uttering grunts of alarm. Dawn is almost here, and the Sun is already shining on the highest snow-clad peaks of the range, although the valleys are still in deep purple shadow. The mountain birds have set up their calling and the short grass beneath him is damp with dew, but his fine pelt keeps him from the chill.
What has disturbed his carrier?
He unwinds his long limbs from around his female and rises to his spindly legs in the cold half-light. Most of the rest of his clan, hunter symbionts and symbiont carriers, are asleep. He can see his fellow hunter symbionts, huddling in pairs or with their children on the slope. The huge white shaggy forms of the carriers are more visible, forming a loose defensive circle around the group. His own carrier, whom he thinks of as Oyo, is awake and alert, disturbed by something that he cannot see.
Could it be one of the distant creatures from the far lowlands? It is not really likely, since they rarely come this far up the mountains, particularly at this time of year. Nor is it likely to be one of the big birds. They do not attack so early in the morning.
On all fours (his usual posture) the leader trots around the group to check that all is well, and realizes that he is not the only one awake. At the far side of the circle two other hunter symbionts are mating, with gentle noises. He looks around and, true enough, their carriers are mating too (a rougher exercise accompanied by hoarse grunts). Certainly nothing is amiss here.
He scrambles over to where his carrier stands, a white silent dutiful column. Without a word he scrambles up the fur and onto his back, resting his narrow chin upon its usual spot on the broad cranium. The massive shaggy arms come up and clutch him firmly. Now he can communicate by thought, without clumsy language.
With thought he commands the great Oyo to turn slowly so that he can scan the lightening landscape. He does not realize it, but this is far from the landscape of his ancestors. The hunter symbionts and the symbiont carriers first came together on the chill wastelands bordering the retreating northern icecap. The tundra-dwellers were well-adapted to the cold, and their great bodies could generate enough heat to keep the slim-limbed temperate woodland-dwellers warm. The hunter symbionts for their part were nimble enough to catch the most evasive of food, and to catch enough to feed both of them. Together they made up more than the sum of each. There is no icecap left now and no tundra, and nowhere in the lowlands are there any environments that suit them; the forests and woodlands are more suited to different humanoid creatures altogether. Only in a few places, on chilly peaks and in cool mountain valleys, are conditions still right. In these isolated places the symbionts linger, marooned as the colder conditions withdrew up the mountains, and towards the pole where they disappeared.
Nevertheless there is still a good living in the mountains: plenty of smaller mammals and birds for the hunter symbionts to hunt for themselves and to share with their carriers, and plenty of grasses, mosses and lichens for the carriers to scrape up and share with their hunter symbionts. Hunter symbionts and symbiont carriers mate at the same time - the mating of a pair of hunters inducing mating in their respective carriers, and vice versa. This usually results in the birth of a hunter symbiont baby at the same time as a carrier baby. Both babies are carried by the parent carriers for about six years, at the end of which the young hunter symbionts choose their own carrier of the same age and same sex.
The family groups move with the seasons, from the grassy slopes of the valleys in the winter and spring to the flower-strewn bluffs and crags of the peaks in the summer and autumn. The habitable areas, although productive, are few and scattered, and the tribes of symbionts have their own ranges.
Down the brightening slope, with the grey mist of the valley behind it, stands a stranger. That is what had disturbed Oyo: the massive shape of another male symbiont carrier, with the squat hummock of a male hunter symbiont lying over his shoulders and head.
With a burst of thought the leader asks Oyo if he recognizes the newcomer, but the dimwitted reply is inconclusive (direct questions like this between hunter and carrier rarely yield anything useful). The stranger strides purposefully up the hill towards them.
It is a challenge. Evidently this is a rogue male, thrown out of a clan, possibly even thrown out of the leader's own clan some time in the past. Wherever he came from his intentions are now clear. With thin yells and reedy shouts (strange noises to be coming from the huge bulk of a symbiont carrier) the newcomer utters his threats and challenges. The leader replies in like voice.
The result is ritual. The hunter symbionts pull themselves back from the great heads of their carriers, and hang on tightly with their own hands to the long fur of the shoulders and back. This frees the carriers' arms for the combat. Then, spurred on by the hunter symbionts' thoughts, the great carriers that bear them wade into each other, striking, slapping and pushing with the flats of their vast hands.
Most of the blows land harmlessly on the great areas of muscle and fur on the chest and forearms. An occasional blow that lands on the face brings blood from the nose or the lip, but does no serious damage. This sparring will continue until one of the combatants, usually the attacker, tires and turns away, or else falls over separating hunter from carrier.
On this occasion the combat is quite predictable. Although the attacker's carrier is big (bigger than Oyo, in fact) the hunter symbiont does not have the mental skill to guide his blows and punches to best effect. If, by chance, he did become the leader of the clan, its future would not look good. Mental skill is needed by a leader in order to judge the timing of the fruiting of food plants, and to plan the routes of migration.
This time, however, the leader's mental agility is not proving to be enough to counteract the strength of the attack. Oyo is cringing in pain from the bruises and cuts from the blows which the opponent's carrier is hammering down with particular ferocity. The sharpness of the pain and fear is picked up by the leader, through the same nerves and ganglions along which he gives Oyo his orders.
It is no good! He is going to have to step down. Oyo will die if this continues, so after all these years he must relinquish the leadership of the clan. He had not anticipated anything like this. Yesterday he was at the height of his powers and virility; now he must give way to a younger hunter symbiont. He will live out his days as an old and revered clan member, nothing else.
He steps back and turns around, presenting his naked back to his opponent: the age-old sign of surrender. The clan now belongs to the attacker.
What happens then is totally unexpected, and quite against any tradition. The opponent's carrier seizes him by the exposed neck and shoulder with his huge hands. Strange thoughts and emotions burst through him from the contact with his enemy - thoughts of rage and hate and uncontrolled violence from the carrier, unchecked by the feeble commands of the controlling hunter.
The leader is torn free from Oyo's fur and flung onto the ground. The flood of alien thoughts ceases, as do the sensations of pain and panic from Oyo. It is just as well. The attacking carrier brings down his great hands on Oyo's back and shoulders, flinging the dear creature to the ground, and wrenches back his head, breaking his neck.
The silence that follows is not just the silence of the horrified clan, who have been roused from their sleep and are watching the fight earnestly. Nor is it the silence of the hillside, produced when the birds are quieted by the violence of alarming events. It is the aching silence of loneliness.
Oyo is gone. Half of the leader's being is dead, and the other half must follow soon. He can no longer be a part of the clan, but must seek out a life of his own and exist as best he can.
This is always a failure. A hunter symbiont without a symbiont carrier, like a symbiont carrier without a hunter symbiont, is always dead within a few days.
Yet cutting through the searing grief is an even more troubling thought. The clan (his clan) is now in the charge of a symbiont that consists of a powerful violent carrier that cannot be controlled by his hunter. The hunter symbiont, as well as being weak, does not have the mental versatility to lead a clan. That much was evident during the fight. It is not just his own death and that of Oyo he mourns, but the death of his entire clan and family.
By 2 million years (the 20020th Century)
Both symbionts go extinct. The reason for this was because 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.