EVOLUTION'S ERROR

Molady and Molodran glanced back to where they had left their spaceship anchored in the planet's atmosphere, just out of reach from the inhabitants of the fetid swamp below their anti-grav boots. A huge body beneath the slimy, green crust moved towards the vessel glinting in the yellow sun and threatened to flick it out of the sky with its long, scaly tail. Hopefully the creature would survive being stunned by the anchor beams: points were deducted from researchers' permits for killing a member of any exotic species. While the ecology on this planet was still experimenting on how large its occupants could grow before collapsing under their own weight, there was unlikely to be sentient life here, however basic.
 As the sinuous tail passed beneath their boots, Molady and Molodran became aware of something even more monstrous rearing up behind them. A creature with massive jaws and long, spiked teeth was threatening to swallow both of them in one gulp.
Molady immediately stunned the beast's snout with her shoulder gun. 'You claimed that these swamp creatures were browsers.'
Molodran refused to apologise. 'So they browse off whatever they can reach.'
The monster plunged back into the fetid water, yowling in annoyance. The anti-grav boots of the researchers were obviously disturbing the inhabitants beneath the green slime so they descended to firm ground on the other side of the swamp. Here there were trees dripping with ribbons of moss and others topped by huge fronds towering into the sky. The fleshy stems of succulents erupted from their trunks, while other flowering plants hugged the ground as though aware that the only creatures it was wise to attract were the ones which would fertilise their seeds. 
A small animal with protruding teeth and short tail browsing in the undergrowth disappeared as soon as it saw them. This was very much a world of hunters and hunted. Omnivores, like the distant ancestors of Molady and Molodran were probably rare. Their species were reluctant to admit that, at some point in their past, they might have pounced on other living creatures and devoured them, and their early scavenging natures were never mentioned in polite conversation, only discussed in ecological research departments. Molady and Molodran always lived in hope of discovering a species evolving intelligence on a vegetarian diet as though that would give more credence to the school of thought which believed that the flesh of another living creature had never passed through the digestive tracts of their ancestors.
Molodran was about to suggest that it was a waste of time searching this swampy world when another meat eater charged through the trees towards him, jaws wide in agreement. He stunned its knees and the attacker crumpled into an undignified heap.
There was a flurry of activity in the bushes on the edge of the clearing. Something was trying to decide how friendly the visitors were. Stunning the huge carnivore had apparently counted in their favour.
'Company at last?' suggested Molady.
A lump of rock struck her helmet and ricocheted back into the bushes it had been hurled from.
Molodran laughed. 'They seem to like you.'
'I have the sinking feeling that this may be the intelligent species it was recommended we look for.'
Overcome by curiosity, the small, rock throwing animals emerged from the bushes.
'What are they?'
The hairy tribe shuffled closer, making hooting sounds.
'As much as this goes against my better judgement, I think we'd better humour them,' said Molady. 'Offer the male with the surly expression one of the coloured crystals while I scan him.'
Molodran delved into his satchel. 'They've all got surly expressions.' He cautiously held out a glittering crystal to what appeared to be the leader of the group. 
As the gift was accepted, the two dominant males immediately pounced on the unfortunate individual and proceeded to kick and punch him until he gave it up. Then the other members of the group joined in the fight, kicking, biting and scratching each other.
'It's people like you who break up happy families,' observed Molady.
Handfuls of fur and ear splitting squeals filled the air as every male, female, and child indiscriminately punched each other to the point of senselessness. The absence of any old or infirm seemed hardly surprising.
The crystal rolled out of the melee.
Molodran retrieved it. 'Shame to waste it. Keep it from another time perhaps?'
'Yes.' Molady turned off her scanner. 'A very long while as far as these specimens are concerned.'
The researchers turned to go back to their ship. Only then did the squabbling creatures become aware that they were taking away the precious bauble. As much as they had been disunited before, the small tribe closed ranks and pursued the tricksters, hurling rocks and branches at them.
'Well, they seem to be on the cusp of least one evolutionary step,' noted Molodran. 'Offensive weapons.'
'Yet another species to evolve despite itself,' Molady agreed morosely. A particularly large, sharp missile almost slashed her spacesuit. 'I wonder if they eat crustaceans as well as using their shells to attack people?' She dictated a brief memo to her wrist monitor. 'Subjects to watch. Potential threat if they develop space travel.'
Once over the swamp, the occupants snapping at their feet, the visitors ascended to their ship. The ludicrous tribe dare not follow and stood on their hind legs, shrieking in rage. As the researchers' vessel lifted off another colossal head reared out of the fetid water and lunged at it.
Once safely out of the planet's atmosphere, Molodran and Molady collated the information they had gleaned from the primitives.
'So you really think they could evolve?' asked Molodran.
'On balance, yes. Notice how they co-operated when they lost what they were fighting over?'
'Don't see how it's possible on a world like that. Everything else is against them. Their metabolisms depend mostly on oxygen, which is never a good sign, and they only have two arms, two legs and two eyes. How could any basically intelligent species develop with that disadvantage?'
Molady set the spaceship's course out of the solar system with her third and forth hands. 'Let's get back to civilisation before we're diverted to find any more "intelligent" life out here in the back of nowhere.'
Molodran looked back at the bright, blue world laced with wispy white clouds. 'Shame if they do eventually trash that planet. It looks so beautiful from out here.'