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Scientists believe that over 99 percent (D answer option) of species that once existed on Earth are now extinct. This can be known by archeologist research, which allows us to know what species lived in the past by studying the soil, its different layers, and the found fossils, dating with high precision how many years ago those organisms inhabited our planet. With these research tools, we have known that there were at least five massive extinctions on the Earth, where aquatic and terrestrial species perished, and they are the Ordovician-Silurian (around 440 mya), Devonian, the Permian-Triassic, the Triassic-Jurassic, and the Cretaceous-tertiary (65 mya) extinctions.