5.7 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 8/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
==== Increase in oxygen levels ==== Earth's earliest atmosphere contained no free oxygen (O2); the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis. Cyanobacteria were the first organisms to evolve the ability to photosynthesize, introducing a steady supply of oxygen into the environment. Initially, oxygen levels did not increase substantially in the atmosphere. The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water. Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in its diatomic form. Oxygen levels in the atmosphere increased substantially afterward. As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years. Oxygen levels seem to have a positive correlation with diversity in eukaryotes well before the Cambrian period. The last common ancestor of all extant eukaryotes is thought to have lived around 1.8 billion years ago. Around 800 million years ago, there was a notable increase in the complexity and number of eukaryotes species in the fossil record. Before the spike in diversity, eukaryotes are thought to have lived in highly sulfuric environments. Sulfide interferes with mitochondrial function in aerobic organisms, limiting the amount of oxygen that could be used to drive metabolism. Oceanic sulfide levels decreased around 800 million years ago, which supports the importance of oxygen in eukaryotic diversity. The increased ventilation of the oceans by sponges, which had already evolved and diversified during the late Neoproterozoic, has been proposed to have increased the availability of oxygen and powered the Cambrian's rapid diversification of multicellular life. Molybdenum isotopes show that increases in biodiversity were strongly correlated with expansion of oxygenated bottom waters in the Early Cambrian, lending support for oxygen as a driver of the Cambrian evolutionary radiation. The 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), while 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 tens of millions of years before the Cambrian explosion. Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as collagen, which is required for the construction of complex structures, or the biosynthesis of molecules for the construction of a hard exoskeleton. However, animals were not affected when similar oceanographic conditions occurred in the Phanerozoic; therefore, some see no forcing role of the oxygen level on evolution.
==== Ozone formation ==== The amount of ozone (O3) required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence around the Cambrian explosion. The presence of the ozone layer may have enabled the development of complex life and life on land, as opposed to life being restricted to the water.
==== 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. However, the snowball episodes occurred a long time before the start of the Cambrian, and it is difficult to see how so much diversity could have been caused by even a series of bottlenecks; the cold periods may even have delayed the evolution of large size organisms. Massive rock erosion caused by glaciers during the "Snowball Earth" may have deposited nutrient-rich sediments into the oceans, setting the stage for the Cambrian explosion.
==== Increase in the mineral content of the Cambrian seawater ==== Newer research suggests that volcanically active mid-ocean 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. Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell's Great Unconformity. An increase of calcium may also have been caused by erosion of the Transgondwanan Supermountain that existed at the time of the explosion. The roots of the mountain are preserved in present-day East Africa as an orogen. In tandem, the levels of phosphorus in the Ediacaran oceans were substantially below those in later seas; this may have delayed the origin of phosphatic-shelled organisms.
==== Increase in ocean alkalinity ==== New research done in 2014 using boron isotope analysis found that ocean alkalinity was rising until the Early Cambrian. This would have had the effect of making it easier for organisms to precipitate calcium carbonate, facilitating easier formation of hard parts, including shells, exoskeletons, and spines.
=== Developmental explanations ===