[9급 국가직 영어 2020년 20번]

All creatures, past and present, either have gone or will go extinct. Yet, as each species vanished over the past 3.8-billion-year history of life on Earth, new ones inevitably appeared to replace them or to exploit newly emerging resources. From only a few very simple organisms, a great number of complex, multicellular forms evolved over this immense period. The origin of new species, which the nineteenth-century English naturalist Charles Darwin once referred to as “the mystery of mysteries,” is the natural process of speciation responsible for generating this remarkable ___________________ with whom humans share the planet. Although taxonomists presently recognize some 1.5 million living species, the actual number is possibly closer to 10 million. Recognizing the biological status of this multitude requires a clear understanding of what constitutes a species, which is no easy task given that evolutionary biologists have yet to agree on a universally acceptable definition.