Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the best answer to each of the following questions from 31 to 40.
It is estimated that over 99 percent of all species that have ever existed on Earth are now extinct. [I] Extinction occurs when a species can no longer survive in a changing environment. [II] Causes vary but often include climate shifts, food shortages, and competition from better-adapted species. [III] Natural disasters such as volcanic eruptions, earthquakes, and asteroid impacts have also played a role in extinctions. [IV] In other words, when sudden changes make an environment uninhabitable, species may perish.
The fossil record shows that extinction has always been a part of Earth’s history. However, scientists have discovered that, at certain times, large numbers of species have disappeared simultaneously in events called mass extinctions. One of the most famous occurred 65 million years ago, leading to the extinction of dinosaurs and many other life forms. Scientists believe this was caused by an asteroid impact that triggered fires, dust clouds blocking sunlight, and drastic climate changes. Another major mass extinction happened around 225 million years ago, wiping out nearly 95 percent of all species. Mass extinctions can be worsened by the interdependence of species—when one disappears, others that rely on it may also struggle, leading to further extinctions.
An interesting observation is that extinctions over the past 250 million years seem to follow a cycle, occurring roughly every 26 million years. Some scientists suggest this pattern is linked to Earth’s orbit periodically intersecting with a cloud of comets, leading to impacts that cause extinction. However, this theory is speculative, as no solid evidence supports it. Other researchers propose that extinction is sometimes random, meaning species may survive or vanish by chance rather than their ability to adapt. This suggests that parts of evolutionary history may be shaped by unpredictable events.
While extinction is natural, human activities such as deforestation, pollution, and climate change have accelerated species loss. Unlike past extinctions caused by natural forces, the current biodiversity crisis is largely due to human actions. Scientists warn that without stronger conservation efforts, another mass extinction could occur, leading to irreversible ecosystem damage and the loss of countless species essential for ecological balance.
(Adapted from TOEFL reading)
