<h2>The Cambrian Explosion of Animal Life</h2>
<p>The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. It is believed that most of the animal phyla in existence today had their origins during this time, often referred to as the <strong>Cambrian explosion</strong> (see the figure below).</p>
<div class="wp-caption alignnone" style="width: 544px"><img alt="The illustration shows a sea bed abundant with odd organisms, including tube-shaped worms anchored to the sea floor and animals that resemble cockroaches crawling along it. Swimming creatures somewhat resemble modern insects." height="540" src="https://data.ulearngo.com/assets/cd54a57b-5394-42fa-b384-fee1f0ca902f" width="544"/><p class="wp-caption-text">An artist’s rendition depicts some organisms from the Cambrian period.</p></div>
<p>Echinoderms, mollusks, worms, arthropods, and chordates arose during this period. One of the most dominant <a data-lesson="summarizing-community-ecology" data-subject="biology" data-tutorial="population-community-ecology" href="https://app.ulearngo.com/biology/population-community-ecology/summarizing-community-ecology">species</a> during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision (see figure (a), (b), (c) and (d) below).</p>
<div class="wp-caption alignnone" style="width: 544px"><img alt="Parts a–d show four trilobite fossils. All are teardrop shaped, with a smooth wide end. About one-third of the way down, the body is segmented into horizontal ridges." height="530" src="https://data.ulearngo.com/assets/2c3ba752-33ed-4a46-8ed2-f443884a76a9" width="544"/><p class="wp-caption-text">These fossils (a–d) belong to trilobites, extinct arthropods that appeared in the early Cambrian period, 525 million years ago, and disappeared from the fossil record during a mass extinction at the end of the Permian period, about 250 million years ago.</p></div>
<p>The cause of the Cambrian explosion is still debated. There are many theories that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period (see the figure below). Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist. There is also support for theories that argue that ecological relationships between species, such as changes in the food web, <a data-lesson="competition" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/competition">competition</a> for food and space, and predator-prey relationships, were primed to promote a sudden massive <a data-lesson="coevolution" data-subject="biology" data-tutorial="evolution-overview" href="https://app.ulearngo.com/biology/evolution-overview/coevolution">coevolution</a> of species.</p>
<p>Yet other theories claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of <em>Hox </em>control <a href="https://app.ulearngo.com/tutorials/introducing-genetics/eukaryotic-gene-regulation">genes</a> may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Theories that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened <em>when </em>it did. There is evidence that both supports and refutes each of the theories described above, and the answer may very well be a combination of these and other theories.</p>
<div class="wp-caption alignnone" style="width: 800px"><img alt="The chart shows the percent oxygen by volume in the Earth’s atmosphere. Until 625 million years ago, there was virtually no oxygen. Oxygen levels began to rapidly climb around this time, and peaked around 275 million years ago, at about 35 percent. Between 275 and 225 million years ago, oxygen levels dropped precipitously to about 15 percent, and then climbed again and dropped to the modern-day concentration of 22 percent." height="531" src="https://data.ulearngo.com/assets/397876f6-4df2-4b74-9ff1-471a761d9ad2" width="800"/><p class="wp-caption-text">The oxygen concentration in Earth’s atmosphere rose sharply around 300 million years ago.</p></div>
<p>However, unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Was there really an “explosion” of <a href="https://app.ulearngo.com/tutorials/introducing-study-of-life/what-is-life">life</a> at this particular time? Some scientists question the validity of the this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well.</p>
<p>Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively rapid <a data-lesson="animal-evolution" data-subject="biology" data-tutorial="evolution-overview" href="https://app.ulearngo.com/biology/evolution-overview/animal-evolution">animal evolution</a> and diversification that is unmatched elsewhere during history.</p>
<p>View an animation of what <a data-lesson="summarizing-aquatic-biomes" data-subject="biology" data-tutorial="ecology-biosphere" href="https://app.ulearngo.com/biology/ecology-biosphere/summarizing-aquatic-biomes">ocean</a> life may have been like during the Cambrian explosion.</p>
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The Cambrian Explosion of Animal Life

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Learn about the evolutionary history of the animal kingdom, their diverse features, tissue structures, reproduction and development, and the classification and characterization of animals based on body symmetry and embryological features, as well as the construction of an animal phylogenetic tree and modern advances in phylogenetic understanding.


The Cambrian Explosion of Animal Life

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