Statistically, it is highly unlikely that the Earth will ever suffer a cometary or asteroid impact sufficiently large to exterminate all life, though future serious collisions are to be expected at roughly hundred million year intervals. It is also unlikely that intense radiation from a supernova will ever sterilise the planet, though this is possible in principle. No star close enough to cause total extinction by these means (within 50 light years) is likely to become a supernova during the expected lifetime of the sun.
However, life on Earth will end. As the sun continues to heat up, the buffering effect of the carbon dioxide cycle will ultimately be swamped. The present global warming effect of carbon dioxide production by human activity will have no significance in the long term. Increased temperature will fix more and more carbon dioxide in the form of weathered rock, and although the Gaia effect will mitigate this to some extent - granted the adaptation of future species to changing conditions - the average surface temperature will reach 50oC in some 1,500 million years' time. Only some protists and prokaryotes will survive this temperature. Increased evaporation of water will enhance the greenhouse effect until, in 2,000 million years or so from now, the oceans will boil dry. There is no life as we know it without liquid water.
The last survivors of life on Earth will probably be archaea, organisms not dissimilar to the earliest pioneers. We already know about the archaea that live in deep ocean trenches and require high temperatures to survive. No other known organism will survive when the end of the oceans approaches, and even the archaea might die out when they cease to receive the products of photosynthesis.
But of course we have no way of knowing what species will evolve in the future or what capabilities they will have. When we consider how life on Earth has transformed itself and the planet during the past thousand million years, how can we hope to predict the next thousand million? One thing we can predict fairly safely, though: our own species will not be around for anything like that long.
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