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The Cambrian Explosion

The Cambrian Explosion, a rather abrupt surge in the evolution of higher organisms, began in the early Cambrian period around 541 million years ago and lasted for only about 13 until 25 million years. This article summarizes the evolutionary steps during this time. In particular, a very recent scientific study is introduced, which suggests that altered solar radiation led to an increase in oxygen levels on Earth and in the ocean. The Cambrian Explosion is so named not only because a diverse array of multicellular organisms evolved within a remarkably short period of up to 25 million years, but also because it brought about significant leaps in the phylogenetic tree of animals. Sponges and cnidarians evolved, and from this lineage, the bilaterally symmetrical animals (Bilateria) developed. Primeval protostomes, such as some Arthropoda, as well as primeval deuterostomes, including the echinoderms (Echinodermata), also evolved. Well-known organisms classified as Arthropoda include Anomalocaris and the trilobites. Anomalocaris was a predator that grew up to 120 cm long and is difficult to precisely place in the cladogram of Arthropoda, while trilobites are mostly classified as belonging to the early ancestral lineage of the Chelicerata (spider-related major clade). Early deuterostomes within the Echinodermata are for example Crinoidea, which include the sea lilies. Research does not assume that Cambrian evolution is due to an artifactual fossil record, i.e., that fossils survived in closely spaced sediments over a significantly longer period of time only by chance, but rather research considers the remarkably short Cambrian Explosion to be a real event based on multiple pieces of evidence. As an evolutionary biologist and paleontologist, one must naturally ask what might have triggered such a massive radiation of evolution (including in the algae plant kingdom). The following factors, for example, are considered responsible: the oxygen content in the Earth's atmosphere and oceans increased significantly. Furthermore, calcium-containing compounds in the oceans increased, enabling the evolution of hard skeletal elements in sponges, corals, arthropods, and e.g. echinoderms. Initially, these skeletons likely served to excrete excess calcium from tissues. However, calcareous skeletons then allowed animals to modify their environment, for example, by burrowing through the ocean floor in search of food. This resulted in a more complex structure of the seabed, providing a greater number of ecological licenses, which in turn enabled a wider range of speciation processes. The evolutionary "invention" of bilaterally symmetrical animals was a key feature that allowed animals to use directed locomotion. This enabled enormous evolutionary leaps. In their recent study, Y. Zhang et al. (2025) presented a hypothesis to explain why the oxygen content on Earth, which formed an important basis for the Cambrian Explosion, could increase so suddenly. According to the researchers' model, Earth's orbit around the sun changed. The resulting change in solar radiation is thought to have accelerated weathering processes on land, leading to an increase in nutrients entering the oceans. These nutrients, in turn, promoted plant life, which then exhibited greater photosynthetic activity. This significantly increased the oxygen content, which occurred during a period when important prerequisites for the radiation of organisms were already in place. © all video aspects including the written narrative, Stefan F. Wirth Berlin December 2025