Possible causes of the “explosion”
Despite the evidence that moderately complex animals (triploblastic bilaterians) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental, and ecological changes. Any explanation must explain the timing and magnitude of the explosion.
[edit] Changes in the environment
[edit] Increase in oxygen levels
Earth’s earliest atmosphere contained no free oxygen; the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis. As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.[10]
Shortage of oxygen might well have prevented the rise of large, complex animals. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones); but the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal’s size increases equally in all directions. An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen. However, members of the Ediacara biota reached metres in length; clearly oxygen did not limit their growth.[31] Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as collagen, required for the construction of complex structures,[98] or to form molecules for the construction of a hard exoskeleton.[99] However, animals are not affected when similar oceanographic conditions occur in the Phanerozoic; there is no convincing correlation between oxygen levels and evolution, so oxygen may have been no more a prerequisite to complex life than liquid water or primary productivity.[100]
[edit] Snowball Earths
Main article: Snowball Earth
In the late Neoproterozoic (extending into the early Ediacaran period), the Earth suffered massive glaciations in which most of its surface was covered by ice. This may have caused a mass extinction, creating a genetic bottleneck; the resulting diversification may have given rise to the Ediacara biota, which appears soon after the last "Snowball Earth" episode.[101] However, the snowball episodes occurred a long time before the start of the Cambrian, and it is hard to see how so much diversity could have been caused by even a series of bottlenecks;[19] the cold periods may even have delayed the evolution of large size.[42]
[edit] Increase in the calcium concentration of the Cambrian seawater
Newer research suggests that volcanically active midocean ridges caused a massive and sudden surge of the calcium concentration in the oceans, making it possible for marine organisms to build skeletons and hard body parts.[102]
[edit] Developmental explanations
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[19] combines with molecular data[103] 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.[104]
[edit] 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 must explain why the "explosion" happened when it did.[19]
[edit] End-Ediacaran mass extinction
Main article: End-Ediacaran 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. 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.[19]
[edit] Evolution of eyes
Main article: Evolution of the eye
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, 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.[105] Nevertheless many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian.[106] 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 further away than they can be seen under the sea, but the appearance of these other senses apparently did not cause an evolutionary explosion.[19]
[edit] 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: if the predator fails to win a contest, it loses a meal; if the prey is the loser, it loses its life.[107]
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.[42] However, the intensity of predation does appear to have increased dramatically during the Cambrian[108] as new predatory "tactics" (such as shell-crushing) emerged.[109]
[edit] 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.[26]
Mesozooplankton are plankton of a larger size, and early Cambrian specimens filtered microscopic plankton from the seawater. These larger organisms would have produced droppings and corpses that were 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 huge range of new 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).[26]
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.[3][110]
Metazoans have an amazing ability to increase diversity through coevolution.[4] This means that a trait of one organism can cause another to evolve in response; a number of responses are possible, and a different species can potentially emerge for each. As a simple example, the evolution of predation may have caused one organism to develop defence while another developed motion to flee. This would cause the predator lineage to split 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.[4]
[edit] Mis-dating of species
The results of an article published in Nature in 2010,[111] have shown that eukaryotic multicellularity, which had been thought to evolve with the beginning of Cambrian Period, might date back to 2.1 billion years ago, which is approximately 1.55 billion years earlier than the date indicated by currently dominating scientific evidence.[112]
[edit] Discredited hypotheses
Main article: Discredited hypotheses for the Cambrian explosion
As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some hypotheses look improbable. Causes that have been proposed but are now discounted[citation needed] include the evolution of herbivory, vast changes in the speed of tectonic plate movement or of the cyclic changes in the Earth's orbital motion, or the operation of different evolutionary mechanisms from those that are seen in the rest of the Phanerozoic eon.[citation needed]
[edit] No explanation required
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.[113]