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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 10/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
==== Increase in size and diversity of planktonic animals ==== Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic. Before the start of the Cambrian, their corpses and droppings were too small to fall quickly towards the seabed, since their drag was about the same as their weight. This meant they were destroyed by scavengers or by chemical processes before they reached the sea floor. Mesozooplankton are plankton of a larger size. Early Cambrian specimens filtered microscopic plankton from the seawater. These larger organisms would have produced droppings and ultimately corpses large enough to fall fairly quickly. This provided a new supply of energy and nutrients to the mid-levels and bottoms of the seas, which opened up a new range of possible ways of life. If any of these remains sank uneaten to the sea floor they could be buried; this would have taken some carbon out of circulation, resulting in an increase in the concentration of breathable oxygen in the seas (carbon readily combines with oxygen). The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during the Ediacaran period. Metazoans have an amazing ability to increase diversity through coevolution. This means that an organism's traits can lead to traits evolving in other organisms; a number of responses are possible, and a different species can potentially emerge from each one. As a simple example, the evolution of predation may have caused one organism to develop a defence, while another developed motion to flee. This would cause the predator lineage to diverge into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual coevolution is somewhat more subtle, but, in this fashion, great diversity can arise: three quarters of living species are animals, and most of the rest have formed by coevolution with animals.
=== Ecosystem engineering === Evolving organisms inevitably change the environment they evolve in. The Devonian colonization of land had planet-wide consequences for sediment cycling and ocean nutrients, and was likely linked to the Devonian mass extinction. A similar process may have occurred on smaller scales in the oceans, with, for example, the sponges filtering particles from the water and depositing them in the mud in a more digestible form; or burrowing organisms making previously unavailable resources available for other organisms.
==== Burrowing ==== Increases in burrowing changed the seafloor's geochemistry, and led to decreased oxygen in the ocean and increased CO2 levels in the seas and the atmosphere, resulting in global warming for tens of millions of years, and could be responsible for mass extinctions. But as burrowing became established, it allowed an explosion of its own, for as burrowers disturbed the sea floor, they aerated it, mixing oxygen into the toxic muds. This made the bottom sediments more hospitable, and allowed a wider range of organisms to inhabit them—creating new niches and the scope for higher diversity.
=== Complexity threshold === The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed: for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed. This genetic threshold may have a correlation to the amount of oxygen available to organisms. Using oxygen for metabolism produces much more energy than anaerobic processes. Organisms that use more oxygen have the opportunity to produce more complex proteins, providing a template for further evolution. These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments. With the help of oxygen, genes that code for these proteins could contribute to the expression of complex traits more efficiently. Access to a wider range of structures and functions would allow organisms to evolve in different directions, increasing the number of niches that could be inhabited. Furthermore, organisms had the opportunity to become more specialized in their own niches.
== Relationship with the Great Ordovician Biodiversification Event ==
After an extinction at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic. This event, known as the Great Ordovician Biodiversification Event (GOBE), has been considered a "follow-up" to the Cambrian explosion. Recent studies have suggested that the Cambrian explosion were not two discrete events but one long evolutionary radiation. Analytical study of the Geobiodiversity Database (GBDB) and Paleobiology Database (PBDB) failed to find a statistical basis for separating the two radiations. Some researchers have proposed the existence of a biodiversity gap during the Furongian separating the Cambrian explosion and GOBE known as the Furongian Gap. Studies of the Guole Konservat-Lagerstätte and similar fossil sites in South China have instead found the Furongian to instead be a time of rapid biological turnovers though, making the existence of the Furongian Gap highly controversial.