If all the people on Earth 1) (DISAPPEAR) tomorrow, nature 2) (BEGIN) to reclaim the planet. For a start, if people no longer 3) (POLLUTE) the atmosphere, the air 4) (SOON BECOME) clean again. If there 5) (BE) no people to maintain buildings, they 6) (SOON BEGIN) to decay, but more solid parts 7) (TAKE) thousands of years to disappear. In general, if the 6.5 billion humans no longer 8) (COMPETE) with other species on Earth, most species 9) (BENEFIT).

For example, if humans no longer 10) (CATCH) fish, the numbers of fish worldwide 11) (EVENTUALLY INCREASE). However, if humans 12) (VANISH) from the Earth, endangered species of animals 13) (NOT NECESSARILY RECOVER), as some are already too few in number. Some endangered species 14) (HAVE) greater difficulty surviving if no humans 15) (TAKE) the trouble to protect them from other species.

Even if we no longer 16) (POISON) the planet, several decades 17) (GO BY) before all dangerous chemicals 18) (DISAPPEAR). And even if the burning of fossil fuels 19) (CEASE) tomorrow, the oceans 20) (NOT ABSORB) all the CO₂ in the atmosphere for thousands of years.

In the end, though, if alien visitors 21) (LAND) on the Earth in 100,000 years time, they 22) (FIND) no signs that an advanced civilization had ever lived here.