Found Objects - a sci-fi

Abrasive nano particles of plastic in human systems erase everyone through a process of attritional damage to human bodies, over a period of around 300 years. 

Plant and insect life abides; albeit reduced; and over eons the temperature stabilises for long enough for other types of life to endure. The big wheel keeps on turning.

Initial symptoms started appearing in the late twentieth century with regular reports of nano scale plastic particles being discovered in the atmosphere to the height of Mt Everest and beyond; and in the deep sea. Lifeforms were eating, drinking and breathing plastic. 

By 2030, widespread studies from epidemiologists worldwide, had identified nanoplastics as an increasingly key driver in the early deterioration of human and other higher lifeforms' organs and other holobionomic functions. If nothing else it was getting a little crowded at microbe scale.  

Essentially there was too much abrasion by too many particles over, what to most humans is a long, long time.  Between 10 and 50 years in most cases; before symptoms began appearing. Cancer rates; especially along the respiratory tracts and into the lungs also increased significantly.

The problem was amplified by the sheer accumulation of nano particles over time. Degradation of plastic began as soon as they started to be produced en masse after the Second World War. Alongside an exponential rise in CO2 from the continuing burning of fossil fuels for energy; the use of fossil fuel in the huge expansion of plastic production meant the next human generations faced an already chemically and particle insulted atmosphere. And for each successive generation the insult was progressively greater. 

By 2400, a mere few thousand biological humans remained. There were a billion more digital ones, dormant; waiting for someone or some future entity to embed them with consciousness and a new body. 

But there was to be no reincarnation for these digihumes in a live-forever body and the chance to skip about the universe forever at the speed of light. The fantasy of your digital YOU etched in particles of light going further and further; being incarnated in a 'robo body' to have a gander at planets while you were passing through star systems was not to be realised once the remaining humans lost touch with advanced technologies; and power hungry data systems collapsed.

There were also other huge ruptures here and there - famine, conventional wars, disease and so on; but humans avoided nuke-ing each other, endured an 8C rise in average global temps; the impacts of the aforementioned plastics; and the scarcity of resources from a total exhaustion of the Earth's reserves of materiality. 

The remnant humans retained a lot of intermediate tech skills, but continued to suffer shorter life spans than their ancestors; memories of whom passed into oblivion over the passing thousands of years.