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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 9/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
A range of theories are based on the concept that minor modifications to animals' development as they grow from embryo to adult may have been able to cause very large changes in the final adult form. The Hox genes, for example, control which organs individual regions of an embryo will develop into. For instance, if a certain Hox gene is expressed, a region will develop into a limb; if a different Hox gene is expressed in that region (a minor change), it could develop into an eye instead (a phenotypically major change). Such a system allows a large range of disparity to appear from a limited set of genes, but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity. Evidence of Precambrian metazoans combines with molecular data to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian. This apparent paradox is addressed in a theory that focuses on the physics of development. It is proposed that the emergence of simple multicellular forms provided a changed context and spatial scale in which novel physical processes and effects were mobilized by the products of genes that had previously evolved to serve unicellular functions. Morphological complexity (layers, segments, lumens, appendages) arose, in this view, by self-organization. Horizontal gene transfer has also been identified as a possible factor in the rapid acquisition of the biochemical capability of biomineralization among organisms during this period, based on evidence that the gene for a critical protein in the process was originally transferred from a bacterium into sponges.
=== Ecological explanations === These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the food chain; some suggest arms races between predators and prey, and others focus on the more general mechanisms of coevolution. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they do not explain why the "explosion" happened when it did.
==== End-Ediacaran mass extinction ====
Evidence for such an extinction includes the disappearance from the fossil record of the Ediacara biota and shelly fossils such as Cloudina, and the accompanying perturbation in the δ13C record. It is suspected that several global anoxic events were responsible for the extinction. Mass extinctions are often followed by adaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction. However, once the dust had settled, overall disparity and diversity returned to the pre-extinction level in each of the Phanerozoic extinctions.
==== Anoxia ==== The late Ediacaran oceans appears to have suffered from an anoxia that covered much of the seafloor, which would have given mobile animals with the ability to seek out more oxygen-rich environments an advantage over sessile forms of life.
==== Increase in sensory and cognitive abilities ====
Andrew Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time, hunting and evading were both close-range affairs—smell (chemoreception), vibration and touch were the only senses used. When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines and similar defenses may also have evolved in response to vision. He further observed that, where animals lose vision in unlighted environments such as caves, diversity of animal forms tends to decrease. Nevertheless, many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian. It is also difficult to understand why the evolution of eyesight would have caused an explosion, since other senses, such as smell and pressure detection, can detect things at a greater distance in the sea than sight can, but the appearance of these other senses apparently did not cause an evolutionary explosion.
One hypothesis posits that the development of increased cognitive abilities during the Cambrian drove diversity increase. This is evidenced by the fact that the novel ecological lifestyles created during the Cambrian required rapid, regular movement, a feature associated with brain-bearing organisms. The increasing complexity of brains, positively correlated with a greater range of motion and sensory abilities, enabled a wider range of novel ecological modes of life to come into being.
==== Arms races between predators and prey ==== The ability to avoid or recover from predation often makes the difference between life and death, and is therefore one of the strongest components of natural selection. The pressure to adapt is stronger on the prey than on the predator because if the predator fails to win a contest, it loses a meal, but if the prey is the loser, it loses its life. But there is evidence that predation was rife long before the start of the Cambrian, for example in the increasingly spiny forms of acritarchs, the holes drilled in Cloudina shells, and traces of burrowing to avoid predators. Hence, it is unlikely that the appearance of predation was the trigger for the Cambrian "explosion", although it may well have exhibited a strong influence on the body forms that the "explosion" produced. However, the intensity of predation does appear to have increased dramatically during the Cambrian as new predatory "tactics" (such as shell-crushing) emerged. This rise of predation during the Cambrian was confirmed by the temporal pattern of the median predator ratio at the scale of genus, in fossil communities covering the Cambrian and Ordovician periods, but this pattern is not correlated to diversification rate. This lack of correlation between predator ratio and diversification over the Cambrian and Ordovician suggests that predators did not trigger the large evolutionary radiation of animals during this interval. Thus the role of predators as triggerers of diversification may have been limited to the very beginning of the "Cambrian explosion".