The question of how life first came to be on early earth has been of fascination to scientists for decades. By stark contrast, the question of how life may end on Earth in the distant future has received little attention, probably because the question itself forces us to face the rather depressing reality that one day, our star will die, our planet’s center will cool, and Earth will no longer be capable of supporting life. But the end of the road for life on Earth will not dawn on us overnight. Just as it took billions of years for our earliest prokaryotic ancestors to evolve into complex, multicellular life (these early microbial communities, spread far and wide across the earth, had the big responsibility of oxidizing the atmosphere so that multicellular organisms could have something to breathe) it will likely take millions if not billions of years for life on Earth to devolve as the planetary systems that allow complex ecosystems to exist slowly shut down.
How will the end of life happen? On present day Earth, the weathering of silicate rocks draws carbon dioxide out of the atmosphere and returns it to the continental and oceanic crusts. This process occurs quite slowly (millions – billions of year timescales), but ultimately if there were no source of carbon to replenish the CO2 drawn out of the atmosphere by weathering, it would eventually be depleted. This would make our planet inhospitable to plant life, which requires CO2 to do photosynthesis. Unfortunately this is exactly what will happen when our star begins to die. As the Sun ages, its luminosity will increase, meaning Earth’s surface is going to get hotter. This will speed up silicate weathering and the speed with which CO2 leaves our atmosphere. Normally, this CO2 would be replenished with carbon recycled through the crust and mantle due to tectonic activity. However, the heating up of the planet’s surface will also increase surface water loss due to evaporation, which will increase the friction between the plates until eventually plate tectonics shuts down entirely.
When atmospheric CO2 concentrations drop below a critical threshold (< 10 ppm), plants will begin to die off. With the plants will go both the food and oxygen source for animals. Large ectotherms (large mammals and birds) will likely go first, due to their high oxygen requirements, followed by smaller ectotherms (i.e., small birds, rodents). Reptiles, amphibians and fish will likely outlast the rest of us due to their lower oxygen requirements, with marine species lasting longer in general because the oceans will likely heat up less rapidly than the land. Eventually, within perhaps 100 million years of the end of plant life, the world will be returned to the microbes.
And what might the final microbes on Earth be? In all likelihood, they will be extremophiles capable of tolerating multiple environmental stressors (polyextremophiles)- low oxygen concentrations and high temperatures, for example. They will recede from the planet’s surface where high heat and rapid evaporation will be too stressful. In all likelihood, they will return to deep ocean vents and similar subsurface environments that are both buffered from surface temperatures and capable of supplying their own source of energy.
I’ve just described the devolving (or perhaps mass extinction occurring on along a trajectory of evolutionary complexity over geologic timescales) of life of Earth in broad brushstrokes. As with most scientific theories, there are details, nuances, unanswered questions and questionable assumptions. Of course, this is also assuming that some anthropogenic force doesn’t end life on Earth far sooner (nuclear war, chemical warfare, climate change, GMOs, encountering an unfriendly and technologically superior alien race, creation and subsequent uprising of AI, zombie apocalypse). And it’s also possible that I’m being far too pessimistic, and some future technological or geoengineering solution will be able to keep our planet liveable far beyond its natural lifetime.
I’ll leave you with a plug for SpaceRip, a really awesome astronomy youtube show that has several episodes focused on questions of planetary habitability. These guys know way more than I do.