Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. There was one such event at the end of the Cretaceous period (around 70 million years ago). There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.

The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.

What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents. Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.

American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.

Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.

One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.

An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.

地质年代中，在一个短期的时间间隔有大量物种灭绝的现象就被称为大规模物种灭绝。在白垩纪时期后期（大约七千万年前）就曾经发生过一次大规模物种灭绝。而在二叠纪时期后期（大约两亿五千万年前）还发生过一次规模更大的物种灭绝。由于当时灭绝的物种很少为人类所知，所以二叠纪时期的大规模物种灭绝受到的关注远远不如其他几次大规模物种灭绝。 化石记录显示，历史上至少发生过五次大规模物种灭绝，造成大批海洋生物消亡。如今物种灭绝的比率和之前五次大规模物种灭绝时期一样高。因此许多科学家推断：当前，第六次大规模物种处于灭绝状态。 是什么原因引起如此高的物种灭绝率呢？有几种假说，包括：地球变暖或变冷；季节型波动的改变或洋流变化；大陆位置移动。生物假说包括因昆虫与开花植物之间的合作式进化或海洋底层肉食动物进化引起的生态变化。这些生物机制，有些在极短的时间内就会灭绝，而有些则很有可能经过长时期在不同时代或不同大陆缓慢地进行。有些假说未能解释在陆地和海洋同时发生的物种灭绝。可能每次大规模物种灭绝都有不同的原因。但有证据指出，人类狩猎以及人类破坏栖息地很可能是当前大规模物种灭绝的原因。 美国古生物学家David Raup 和John Sepkoski 曾经从大量化石群中研究物种灭绝的比率。他们指出，自从白垩纪时期中期以来，灭绝的物种不断增多，大约每隔两千六百万年就会定期发生一次。白垩纪时期后期的恐龙和菊石（的灭绝是一系列此类周期性物种灭绝中更为剧烈的一次。对于周期性出现大规模物种灭绝的可能性，引发了这样的假设：一颗具有长周期轨道的伴星体使其他天体从正常轨道偏离，导致其中一些天体变成流星掉落到地球，撞击时造成大范围破坏。 各种假说都试图对白垩纪时期后期物种灭绝做出解释，近年来Luis 和Walter Alvarez最先提出的小行星撞击假说备受人们关注。根据这一假说，地球与一个直径约为10公里的小行星或者总体积与之相当的几个小行星发生碰撞。碰撞的力量把大量碎片喷射到大气中，在这些细小颗粒沉积之前好几年的时间里天空都是灰蒙的。光合作用减弱会造成各种植物的生命大规模下降。这首先会造成草食动物大规模饿死，接着就是肉食动物大规模饿死。按照这种假说，大规模物种灭绝就会突然间发生。 Alvarez 假说基于稀土元素铱的出现做了一个非常有趣的实验。这种元素在地壳中的含量极少，但在大多数小行星中的含量却多得多。因小行星碰撞而被抛进到大气中的碎片可能会含有大量铱元素，并且大气流会把这些物质带到全球各地。白垩纪时期与第三纪时期交替之间的沉积物的研究显示：在这两个时期的交替时期，铱元素的含量急剧增加。尽管还没有发现过撞击的小行星，铱元素异常却为Alvarez假说提供了有力支持。 按理说，这样大小的一个行星，即使受到冲击碎裂之后也会留下一个巨大的陨石坑。撞击所释放的极度高温使得许多种岩石产生热冲击石英。撞击也会将一些大石块抛出去，在主要陨石坑周围形成次级陨石坑。迄今为止，人们沿着墨西哥尤卡坦半岛已经找到了一些此类次级陨石坑。并且在海地和墨西哥找到了热冲击石英。尤卡坦沿海一个叫做希克苏鲁伯的地方，曾被认为是主要的撞击点。