The earliest fossil evidence for eukaryotes complex organisms whose cells contain a distinct nucleus dates to only about 1.2 billion years ago. The fossil record suggests that animal evolution progressed slowly, with relatively little change seen between fossils from 1.2 billion years ago and those from a half-billion years later. But then something quite dramatic happened as can be judged by the many different animal groups that suddenly appear in the fossil record.

Biologists classify animals according to their basic body plans. For example, the basic body plan shared by mammals and reptiles is fundamentally different from that of insects. Animals are grouped by body plan into what biologists call phyla. Mammals and reptiles both belong to the single phylum Chordata, which includes animals with internal skeletons. Insects, crabs, and spiders belong to the phylum Arthropoda, which contains animals with body features such as jointed legs, an external skeleton, and segmented bodies. Classifying animals into phyla is an ongoing project for biologists, but modern animals appear to comprise about 30 different phyla, each representing a different body plan.

Remarkably, nearly all of these different body plans, plus a few others that have gone extinct, make their first known appearance in the geological record during a period spanning only about 40 million yearsless than about 1 percent of Earth's history. This remarkable flowering of animal diversity appears to have begun about 545 million years ago, which corresponds to the start of the Cambrian period. Hence it is called the Cambrian explosion.

The fact that the Cambrian explosion marks the only major diversification of body plans in the geological record presents us with two important and related questions: Why, so long after the origin of eukaryotes, did the pace of evolution suddenly accelerate dramatically at the beginning of the Cambrian, and why hasn't there been another period of similarly explosive diversification since then

We can identify at least four factors that might have contributed to the Cambrian explosion. First, the oxygen level in our atmosphere may have remained well below its present level until about the time of the Cambrian explosion. Thus, the rapid diversification in animal life may have occurred at least in part because oxygen reached a critical level for the survival of larger and more energy-intensive life forms.

A second factor that may have been important was the evolution of genetic complexity. As eukaryotes evolved, they developed more and more genetic variation in their DNA. Some scientists believe that the Cambrian explosion marks the point at which organisms developed certain kinds of genes (homeobox genes) that control body form and that could be combined in different ways, allowing the evolution of a great diversity of forms over time.

A third factor may have been climate change. Geological evidence points to a series of episodes in which Earth froze over before the Cambrian began. The extreme climate conditions of these episodes eliminated many species, leaving a wide array of ecological niches available into which new species could rapidly evolve when climate conditions eased at the beginning of the Cambrian.

A fourth factor may have been the absence of efficient predators. Early predatory animals were probably not very sophisticated, so some evolving animals that later might have been eliminated by predation were given a chance to survive, making the beginning of the Cambrian period a window of opportunity for many different adaptations to establish themselves in the environment.

This last idea may partly explain why no similar explosion of diversity has taken place since the Cambrian: once predators were efficient and widespread, it may have been virtually impossible for animals with entirely new body forms to find an environmental niche in which they could escape predation. Or it may be that while more body plans may have been possible at some early point in evolution, it was not possible to evolve into those other body plans from the body plans that evolved in the Cambrian. Or perhaps the various body forms that arose during the Cambrian explosion represent the full range of forms possible given the basic genetic resources that characterize all Earth's organisms. In any case, no fundamentally new body forms have emerged since the Cambrian explosion.

真核生物的复杂生物体在其细胞内含有独特的细胞核，其最早的化石依据可追溯到大约12亿年前。化石记录显示，动物进化缓慢，12亿年前的化石与5亿年前的化石相差不大。但是在此之后，可以从突然出现在化石记录中的许多不同的动物群判断出一些非常戏剧性的事情。 生物学家根据动物的基本躯干构造对其进行分类。例如，哺乳动物和爬行动物的基本躯干构造与昆虫的基本躯干构造有着本质上的区别。根据动物的基本躯干构造，可以按照生物学家所称的“门”进行分类。哺乳动物和爬行动物都属于脊索动物门，其中包括具有内骨骼的动物。昆虫，螃蟹和蜘蛛属于节肢动物门，它包含身体特征有如关节腿、外骨骼和分节身体的动物。将动物按“门”进行分类是生物学家正在进行研究的项目，但现代动物似乎可分为大约30个不同的门，每个门代表不同的躯干构造。 值得注意的是，几乎这些所有的不同躯干构造以及其他一些已经灭绝的躯干构造，在一段只有约4千万年的时期中才首次出现在地质记录中，这段时期仅占到整个历史长河的百分之一。这一显著的物种多样性似乎始于大约5亿4500万年前，也就是寒武纪的开始阶段。因此这一现象被称为寒武纪大爆发。 事实上，寒武纪大爆发标志着地质记录中唯一一次的躯干构造多样化，这给我们提出了两个重要且相关的问题：真核生物已经出现很长时间了，为什么其进化速度在寒武纪之初迅速提高，为什么从那以后还没有第二个类似大爆发的多样化的时期。 我们可以确定至少有四个因素可能导致了寒武纪大爆发的发生。首先，直到大约寒武纪大爆发开始时，大气中的氧含量可能仍然低于目前的水平。因此，动物生命多样化的迅速出现至少部分是因为，氧含量已经达到了一个临界水平，可以保证更大、更耗能的生命形式的存在。 第二个可能重要的因素是遗传复杂性的进化。随着真核生物的进化，他们的DNA发生了越来越多的遗传变异。一些科学家认为，寒武纪大爆发标志着生物体发展出了某种控制身体形态的基因（同源盒基因），并且这些基因可以以不同的方式进行排列组合，从而随着时间的推移形成多样性。 第三个因素可能是气候变化。地质证据表明在寒武纪开始之前地球发生了一系列冻结现象。这种极端气候条件淘汰了许多物种，留下了大量的生态位，这使得在寒武纪之初，气候条件有所缓解时，新物种可以迅速进化。 第四个因素可能是缺乏强大的食肉动物。早期的捕食性动物可能经验不足，因此那些进化后的动物到了生存的机会，虽然它们之后会被捕食性动物所消灭，这给了寒武纪初期的动物一个机会，让这些动物可以适应不同环境。 这最后一个概念可能部分地解释了为什么寒武纪之后没有发生类似的多样性大爆发：一旦食肉动物捕食效率提高，数量增多，根本不可能让新形态的动物找到一个可以逃避捕食的环境生态位。或者，虽然在进化的初期阶段可能有更多的躯干构造，但是不可能从寒武纪时代进化的躯干构造进化成其他的躯干构造。或许，在寒武纪大爆发期间出现的各种身体形态囊括了地球上所有生物的基本遗传资源。总之，自寒武纪大爆发以来，根本没有出现任何新的身体形态。