Temperate climates encourage the evolution of large creatures, bulk retains body heat and large leaf-eaters can find enough nourishment to support their mass. By a process of convergent evolution the slothman is now similar to the large ground sloths of South America and North America from previous times. But two factors were needed to allow the tundra-dwellers to evolve into slothmen: plentiful food and no enemies. Sustenance is still there but now they face a newly-evolving predator, the spiketooth.
A huge mound of fur-covered flesh, an indistinct lump in the luxuriant undergrowth, pushes its way through the bracken and brambles. It makes contented burbling noises to itself as it goes. Insects, smaller mammals and birds burst from cover to get out of the path of the great creature as it crunches slowly through the greenery. It is quite harmless, but its immense weight causes a great deal of damage as it passes.
It stops by a tree and looks slowly upwards. There are appetizing fresh green leaves up there. Using its forelimbs against the trunk, it slowly pulls itself upright. Now it begins to look more like a human being, to be precise the tundra-dwelling human being, that was its distant ancestor.
From the bushes beyond, a number of other creatures stop feeding and move out of the way. They are also descendants of the tundra-dwellers and have grown large, but not nearly as big as this great creature. Nor have they changed much in the last million years or so: they still produce the huge quantities of superfluous fat, and are still infested by the tiny parasites that live on the excess.
The tundra-dwellers that adapted to woodland life did so very successfully. Their heavy bodies were well supplied by the voluminous plant life of the habitat. Evolution had produced the right shapes by trial and error; man copied them, and then evolution took the copies and modified them further. If these big creatures have a parallel (a convergence) with any creature from the fossil past it would be with the massive ground sloths of the ancient Americas. Like these, firstly, they developed successfully, even with their great bulk and sluggish habits, because there was the food supply to sustain them and they had little to no natural enemies; secondly, they spend most of their time on all fours, so that their bulk can be well supported, but they can also rise to their hind legs to feed from tall trees; and, thirdly, they have become about three times as tall, and so about ten times as heavy, as their ancient ancestors.
Like the giant ground-dwelling sloths, too, they are succumbing to a newly-evolving predator.
The hunters have been evolving into many specialized types, each one hunting a specific type of prey: some hunt birds, some hunt smaller mammals, some hunt fish. One, however, has evolved to hunt the descendants of the big tundra-dwellers. The spiketooth is larger and heavier than the other hunters, not needing stealth or speed for its hunting since its prey is large and slow-moving. What it does need, however, is a specialized killing weapon, and this it possesses in the shape of its front teeth.
Amongst the traditional carnivorous mammals, of which there are only a few small species left, the killing teeth were normally the pointed canines. In extreme types, like the sabre-toothed cats, they developed into long slashing blades that were able to penetrate the thick hides of very large animals. In the spiketooth the weapons have developed instead in the incisor teeth at the front, rather like the only remaining teeth of the parasites that also feed on the flesh of the descendants of the tundra-dwellers. The spiketooth's mouth is very large, allowing its jaw to drop clear of the upper teeth so that they can be wielded efficiently. The hands are large and powerful, with strong fingernails that allow the spiketooth to hang onto the fur of the slothman while it stabs at the neck, or onto the fatty rolls of the parasitehost while it slashes its way through the blubber.
This may seem like cannibalism, since both predator and prey are descended from ancient human beings; but their common ancestor existed so far back in time that the creatures involved now comprise entirely different species. The preying of one upon the other is merely the natural result of the development of a stable ecological system.
The male slothman munches placidly at the leaves and twigs, unaware of the approaching danger. Away below him in the undergrowth the parasitehosts have already left, their dim wits sensing the approach of a pair of spiketooths. If the distant crashing caused by their lumbering flight through the thickets causes any concern to the slothman, he does not react to it. He does not react at all until the familiar form of a spiketooth steps out from the shade of the forest and he suddenly recognizes the shape and the smell. Slowly he turns away from the tree, turning his back on his enemy, and begins to descend to all fours.
The first spiketooth, less experienced than the other, leaps for the broad back, hooks onto the long fur, throws up his head and drops his jaw ready for the strike. This is a mistake, as it enables the slothman to use his only weapon -his weight. He slowly topples backwards, while the attacking spiketooth tries frantically to untangle his claws from the fur. Remorselessly the attacker is pressed back down into the bracken and the soil of the forest floor, and the slothman lands spreadeagled on his back with his enemy crushed to death beneath him. However, this makes him vulnerable to the spiketooth's mate. She now leaps upon the unprotected chest and plunges her long killing incisors into the slothman's neck.
The kill is a success, which is all she knows. There is no grief for her dead mate. The spiketooth has evolved so far from the original human state that she feels no emotion at all.
By 5 million years (the 50020th Century)
They are wiped out by the travelers of the stars.