"Burning ice". Methane, released by heating, burns; water drips. Inset: clathrate structure (University of Göttingen, GZG. Abt. Kristallographie). Source: United States Geological Survey.
Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.
Methane hydrate is formed when hydrogen-bonded water and methane gas
come into contact at high pressures and low temperatures in oceans.
Methane clathrates are common constituents of the shallow marine geosphere and they occur in deep sedimentary structures and form outcrops
on the ocean floor. Methane hydrates are believed to form by the
precipitation or crystallisation of methane migrating from deep along geological faults.
Precipitation occurs when the methane comes in contact with water
within the sea bed subject to temperature and pressure. In 2008,
research on Antarctic Vostok Station and EPICA Dome C ice cores revealed that methane clathrates were also present in deep Antarcticice cores and record a history of atmospheric methane concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide.
General
Methane
hydrates were discovered in Russia in the 1960s, and studies for
extracting gas from it emerged at the beginning of the 21st century.
Structure and composition
microscope image
The nominal methane clathrate hydrate composition is (CH4)4(H2O)23, or 1 mole
of methane for every 5.75 moles of water, corresponding to 13.4%
methane by mass, although the actual composition is dependent on how
many methane molecules fit into the various cage structures of the water
lattice. The observed density is around 0.9 g/cm3,
which means that methane hydrate will float to the surface of the sea
or of a lake unless it is bound in place by being formed in or anchored
to sediment.
One litre of fully saturated methane clathrate solid would therefore
contain about 120 grams of methane (or around 169 litres of methane gas
at 0 °C and 1 atm), or one cubic metre of methane clathrate releases about 160 cubic metres of gas.
Methane forms a "structure-I" hydrate with two dodecahedral (12 vertices, thus 12 water molecules) and six tetradecahedral
(14 water molecules) water cages per unit cell. (Because of sharing of
water molecules between cages, there are only 46 water molecules per
unit cell.) This compares with a hydration number of 20 for methane in aqueous solution. A methane clathrate MAS NMR spectrum recorded at 275 K and 3.1 MPa shows a peak for each cage type and a separate peak for gas phase methane.
In 2003, a clay-methane hydrate intercalate was synthesized in which a
methane hydrate complex was introduced at the interlayer of a
sodium-rich montmorillonite clay. The upper temperature stability of this phase is similar to that of structure-I hydrate.
Methane
hydrate phase diagram. The horizontal axis shows temperature from -15
to 33 Celsius, the vertical axis shows pressure from 0 to 120,000
kilopascals (0 to 1,184 atmospheres). Hydrate forms above the line. For
example, at 4 Celsius hydrate forms above a pressure of about 50
atm/5000 kPa, found at about 500m sea depth.
Natural deposits
Worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996. Source: USGS
Gas hydrate-bearing sediment, from the subduction zone off Oregon
Specific structure of a gas hydrate piece, from the subduction zone off Oregon
Methane clathrates are restricted to the shallow lithosphere (i.e. < 2,000 m depth). Furthermore, necessary conditions are found only in either continental sedimentary rocks in polar regions where average surface temperatures are less than 0 °C; or in oceanic sediment at water depths greater than 300 m where the bottom water temperature is around 2 °C. In addition, deep fresh water lakes may host gas hydrates as well, e.g. the fresh water Lake Baikal, Siberia. Continental deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less than 800 m depth. Oceanic deposits seem to be widespread in the continental shelf (see Fig.) and can occur within the sediments at depth or close to the sediment-water interface. They may cap even larger deposits of gaseous methane.
Oceanic
There are two distinct types of oceanic deposit. The most common is dominated (> 99%) by methane contained in a structure I clathrate and generally found at depth in the sediment. Here, the methane is isotopically light (δ13C < −60‰), which indicates that it is derived from the microbial reduction of CO2. The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane, since the δ13C values of clathrate and surrounding dissolved methane are similar.
However, it is also thought that fresh water used in the
pressurization of oil and gas wells in permafrost and along the
continental shelves worldwide combines with natural methane to form
clathrate at depth and pressure, since methane hydrates are more stable
in fresh water than in salt water. Local variations may be very common,
since the act of forming hydrate, which extracts pure water from saline
formation waters, can often lead to local, and potentially significant,
increases in formation water salinity. Hydrates normally exclude the
salt in the pore fluid from which it forms, thus they exhibit high
electric resistivity just like ice, and sediments containing hydrates
have a higher resistivity compared to sediments without gas hydrates
(Judge [67]).
These deposits are located within a mid-depth zone around 300–500 m thick in the sediments (the gas hydrate stability zone,
or GHSZ) where they coexist with methane dissolved in the fresh, not
salt, pore-waters. Above this zone methane is only present in its
dissolved form at concentrations that decrease towards the sediment
surface. Below it, methane is gaseous. At Blake Ridge on the Atlantic continental rise, the GHSZ started at 190 m depth and continued to 450 m, where it reached equilibrium with the gaseous phase. Measurements indicated that methane occupied 0-9% by volume in the GHSZ, and ~12% in the gaseous zone.
In the less common second type found near the sediment surface some samples have a higher proportion of longer-chain hydrocarbons (< 99% methane) contained in a structure II clathrate. Carbon from this type of clathrate is isotopically heavier (δ13C
is −29 to −57 ‰) and is thought to have migrated upwards from deep
sediments, where methane was formed by thermal decomposition of organic matter. Examples of this type of deposit have been found in the Gulf of Mexico and the Caspian Sea.
Some deposits have characteristics intermediate between the
microbially and thermally sourced types and are considered to be formed
from a mixture of the two.
The methane in gas hydrates is dominantly generated by microbial
consortia degrading organic matter in low oxygen environments, with the
methane itself produced by methanogenicarchaea. Organic matter in the uppermost few centimetres of sediments is first attacked by aerobic bacteria, generating CO2, which escapes from the sediments into the water column.
Below this region of aerobic activity, anaerobic processes take over,
including, successively with depth, the microbial reduction of
nitrite/nitrate, metal oxides, and then sulfates are reduced to sulfides. Finally, once sulfate is used up, methanogenesis becomes a dominant pathway for organic carbon remineralization.
If the sedimentation rate is low (about 1 cm/yr), the organic
carbon content is low (about 1% ), and oxygen is abundant, aerobic
bacteria can use up all the organic matter in the sediments faster than
oxygen is depleted, so lower-energy electron acceptors
are not used. But where sedimentation rates and the organic carbon
content are high, which is typically the case on continental shelves and
beneath western boundary current upwelling zones, the pore water in the sediments becomes anoxic
at depths of only a few centimeters or less. In such organic-rich
marine sediments, sulfate then becomes the most important terminal
electron acceptor due to its high concentration in seawater,
although it too is depleted by a depth of centimeters to meters. Below
this, methane is produced. This production of methane is a rather
complicated process, requiring a highly reducing environment (Eh −350 to
−450 mV) and a pH between 6 and 8, as well as a complex syntrophic consortia of different varieties of archaea and bacteria, although it is only archaea that actually emit methane.
In some regions (e.g., Gulf of Mexico, Joetsu Basin) methane in
clathrates may be at least partially derive from thermal degradation of
organic matter (e.g. petroleum generation), with oil even forming an
exotic component within the hydrate itself that can be recovered when
the hydrate is disassociated. The methane in clathrates typically has a biogenic isotopic signature and highly variable δ13C (−40 to −100‰), with an approximate average of about −65‰ . Below the zone of solid clathrates, large volumes of methane may form bubbles of free gas in the sediments.
The presence of clathrates at a given site can often be
determined by observation of a "bottom simulating reflector" (BSR),
which is a seismic reflection at the sediment to clathrate stability
zone interface caused by the unequal densities of normal sediments and
those laced with clathrates.
Gas hydrate pingos
have been discovered in the Arctic oceans Barents sea. Methane is
bubbling from these dome like structures, with some of these gas flares
extending close to the sea surface.
Reservoir size
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an order of magnitude per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 1970s. The highest estimates (e.g. 3×1018 m3)
were based on the assumption that fully dense clathrates could litter
the entire floor of the deep ocean. Improvements in our understanding of
clathrate chemistry and sedimentology have revealed that hydrates form
in only a narrow range of depths (continental shelves), at only some locations in the range of depths where they could occur (10-30% of the Gas hydrate stability zone),
and typically are found at low concentrations (0.9–1.5% by volume) at
sites where they do occur. Recent estimates constrained by direct
sampling suggest the global inventory occupies between 1×1015 and 5×1015 cubic metres (0.24 and 1.2 million cubic miles).
This estimate, corresponding to 500–2500 gigatonnes carbon (Gt C), is
smaller than the 5000 Gt C estimated for all other geo-organic fuel
reserves but substantially larger than the ~230 Gt C estimated for other
natural gas sources. The permafrost reservoir has been estimated at about 400 Gt C in the Arctic,
but no estimates have been made of possible Antarctic reservoirs. These
are large amounts. In comparison, the total carbon in the atmosphere is
around 800 gigatons.
These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2×1016 m3) proposed
by previous researchers as a reason to consider clathrates to be a
geo-organic fuel resource (MacDonald 1990, Kvenvolden 1998). Lower
abundances of clathrates do not rule out their economic potential, but a
lower total volume and apparently low concentration at most sites does suggest that only a limited percentage of clathrates deposits may provide an economically viable resource.
Continental
Methane clathrates in continental rocks are trapped in beds of sandstone or siltstone
at depths of less than 800 m. Sampling indicates they are formed from a
mix of thermally and microbially derived gas from which the heavier
hydrocarbons were later selectively removed. These occur in Alaska, Siberia, and Northern Canada.
In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the Mallik gas hydrate site in the Mackenzie River
delta. This was the second such drilling at Mallik: the first took
place in 2002 and used heat to release methane. In the 2008 experiment,
researchers were able to extract gas by lowering the pressure, without
heating, requiring significantly less energy. The Mallik gas hydrate field was first discovered by Imperial Oil in 1971–1972.
Commercial use
Economic deposits of hydrate are termed natural gas hydrate (NGH) and store 164 m3 of methane, 0.8 m3 water in 1 m3 hydrate.
Most NGH is found beneath the seafloor (95%) where it exists in
thermodynamic equilibrium. The sedimentary methane hydrate reservoir
probably contains 2–10 times the currently known reserves of
conventional natural gas, as of 2013. This represents a potentially important future source of hydrocarbonfuel. However, in the majority of sites deposits are thought to be too dispersed for economic extraction.
Other problems facing commercial exploitation are detection of viable
reserves and development of the technology for extracting methane gas
from the hydrate deposits.
In August 2006, China announced plans to spend 800 million yuan
(US$100 million) over the next 10 years to study natural gas hydrates. A potentially economic reserve in the Gulf of Mexico may contain approximately 100 billion cubic metres (3.5×1012 cu ft) of gas. Bjørn Kvamme and Arne Graue at the Institute for Physics and technology at the University of Bergen have developed a method for injecting CO 2 into hydrates and reversing the process; thereby extracting CH4 by direct exchange. The University of Bergen's method is being field tested by ConocoPhillips and state-owned Japan Oil, Gas and Metals National Corporation
(JOGMEC), and partially funded by the U.S. Department of Energy. The
project has already reached injection phase and was analyzing resulting
data by March 12, 2012.
On March 12, 2013, JOGMEC researchers announced that they had successfully extracted natural gas from frozen methane hydrate.
In order to extract the gas, specialized equipment was used to drill
into and depressurize the hydrate deposits, causing the methane to
separate from the ice. The gas was then collected and piped to surface
where it was ignited to prove its presence. According to an industry spokesperson, "It [was] the world's first offshore experiment producing gas from methane hydrate". Previously, gas had been extracted from onshore deposits, but never from offshore deposits which are much more common. The hydrate field from which the gas was extracted is located 50 kilometres (31 mi) from central Japan in the Nankai Trough, 300 metres (980 ft) under the sea. A spokesperson for JOGMEC remarked "Japan could finally have an energy source to call its own".
Marine geologist Mikio Satoh remarked "Now we know that extraction is
possible. The next step is to see how far Japan can get costs down to
make the technology economically viable."
Japan estimates that there are at least 1.1 trillion cubic meters of
methane trapped in the Nankai Trough, enough to meet the country's needs
for more than ten years.
Both Japan and China announced in May 2017 a breakthrough for mining methane clathrates, when they extracted methane from hydrates in the South China Sea. China described the result as a breakthrough; Praveen Linga
from the Department of Chemical and Biomolecular Engineering at the
National University of Singapore agreed "Compared with the results we
have seen from Japanese research, the Chinese scientists have managed to
extract much more gas in their efforts". Industry consensus is that commercial-scale production remains years away.
Environmental concerns
Experts
caution that environmental impacts are still being investigated and
that methane—a greenhouse gas with around 25 times as much global warming potential over a 100-year period (GWP100) as carbon dioxide—could potentially escape into the atmosphere if something goes wrong. Furthermore, while cleaner than coal, burning natural gas also creates carbon emissions.
Hydrates in natural gas processing
Routine operations
Methane
clathrates (hydrates) are also commonly formed during natural gas
production operations, when liquid water is condensed in the presence of
methane at high pressure. It is known that larger hydrocarbon molecules
like ethane and propane can also form hydrates, although longer
molecules (butanes, pentanes) cannot fit into the water cage structure
and tend to destabilise the formation of hydrates.
Once formed, hydrates can block pipeline and processing
equipment. They are generally then removed by reducing the pressure,
heating them, or dissolving them by chemical means (methanol is commonly
used). Care must be taken to ensure that the removal of the hydrates is
carefully controlled, because of the potential for the hydrate to
undergo a phase transition from the solid hydrate to release water and
gaseous methane at a high rate when the pressure is reduced. The rapid
release of methane gas in a closed system can result in a rapid increase
in pressure.
It is generally preferable to prevent hydrates from forming or
blocking equipment. This is commonly achieved by removing water, or by
the addition of ethylene glycol (MEG) or methanol,
which act to depress the temperature at which hydrates will form. In
recent years, development of other forms of hydrate inhibitors have been
developed, like Kinetic Hydrate Inhibitors (which by far slow the rate
of hydrate formation) and anti-agglomerates, which do not prevent
hydrates forming, but do prevent them sticking together to block
equipment.
Effect of hydrate phase transition during deep water drilling
When
drilling in oil- and gas-bearing formations submerged in deep water,
the reservoir gas may flow into the well bore and form gas hydrates
owing to the low temperatures and high pressures found during deep water
drilling. The gas hydrates may then flow upward with drilling mud or
other discharged fluids. When the hydrates rise, the pressure in the annulus
decreases and the hydrates dissociate into gas and water. The rapid gas
expansion ejects fluid from the well, reducing the pressure further,
which leads to more hydrate dissociation and further fluid ejection. The
resulting violent expulsion of fluid from the annulus is one potential
cause or contributor to the "kick". (Kicks, which can cause blowouts, typically do not involve hydrates.
Measures which reduce the risk of hydrate formation include:
High flow-rates, which limit the time for hydrate formation in a volume of fluid, thereby reducing the kick potential.
Careful measuring of line flow to detect incipient hydrate plugging.
Additional care in measuring when gas production rates are low and
the possibility of hydrate formation is higher than at relatively high
gas flow rates.
Monitoring of well casing after it is "shut in"
(isolated) may indicate hydrate formation. Following "shut in", the
pressure rises while gas diffuses through the reservoir to the bore hole; the rate of pressure rise exhibit a reduced rate of increase while hydrates are forming.
Additions of energy (e.g., the energy released by setting cement used in well completion) can raise the temperature and convert hydrates to gas, producing a "kick".
Blowout recovery
Concept
diagram of oil containment domes, forming upsidedown funnels in order
to pipe oil to surface ships. The sunken oil rig is nearby.
At sufficient depths, methane complexes directly with water to form methane hydrates, as was observed during the Deepwater Horizon oil spill in 2010. BP engineers developed and deployed a subsea oil recovery system over oil spilling from a deepwater oil well 5,000 feet (1,500 m) below sea level
to capture escaping oil. This involved placing a 125-tonne (276,000 lb)
dome over the largest of the well leaks and piping it to a storage
vessel on the surface. This option had the potential to collect some 85% of the leaking oil but was previously untested at such depths.
BP deployed the system on May 7–8, but it failed due to buildup of
methane clathrate inside the dome; with its low density of approximately
0.9 g/cm3 the methane hydrates accumulated in the dome, adding buoyancy and obstructing flow.
Climate scientists like James E. Hansen predict that methane clathrates in permafrost regions will be released because of global warming, unleashing powerful feedback forces that may cause runaway climate change.
Research carried out in 2008 in the Siberian Arctic found millions of tonnes of methane being released with concentrations in some regions reaching up to 100 times above normal.
While investigating the East Siberian Arctic Ocean during the
Summer, researchers were surprised by the high concentration of methane,
and theorized that it was being released from pockets of methane
clathrates embedded in ice on the sea floor which had been destabilized
by warmer water.
In 2014 based on their research on the northern United States Atlantic marine continental margins from Cape Hatteras to Georges Bank,
a group of scientists from the US Geological Survey, the Department of
Geosciences, Mississippi State University, Department of Geological
Sciences, Brown University and Earth Resources Technology, claimed there
was widespread leakage of methane.
Scientists from the Center for Arctic Gas Hydrate (CAGE), Environment and Climate at the University of Tromsø, published a study in June 2017, describing over a hundred ocean sediment
craters, some 300 meters wide and up to 30 meters deep, formed due to
explosive eruptions, attributed to destabilizing methane hydrates,
following ice-sheet retreat during the last glacial period, around 15,000 years ago, a few centuries after the Bølling-Allerød warming. These areas around the Barents Sea, still seep methane today, and still existing bulges with methane reservoirs could eventually have the same fate.
Natural gas hydrates for gas storage and transportation
Since methane clathrates are stable at a higher temperature than liquefied natural gas (LNG)
(−20 vs −162 °C), there is some interest in converting natural gas into
clathrates (Solidified Natural Gas or SNG) rather than liquifying it
when transporting it by seagoing vessels.
A significant advantage would be that the production of natural gas
hydrate (NGH) from natural gas at the terminal would require a smaller
refrigeration plant and less energy than LNG would. Offsetting this, for
100 tonnes of methane transported, 750 tonnes of methane hydrate would
have to be transported; since this would require a ship of 7.5 times
greater displacement, or require more ships, it is unlikely to prove
economically feasible..
Recently, methane hydrate has received considerable interest for large
scale stationary storage application due to the very mild storage
conditions with the inclusion of tetrahydrofuran (THF) as a co-guest. With the inclusion of tetrahydrofuran,
though there is a slight reduction in the gas storage capacity, the
hydrates have been demonstrated to be stable for several months in a
recent study at −2 °C and atmospheric pressure. A recent study has demonstrated that SNG can be formed directly with seawater instead of pure water in combination with THF.
The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when practically all major animalphyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversifications in other groups of organisms as well.
Before the Cambrian explosion,
most organisms were relatively simple, composed of individual cells, or
small multicellular organisms, occasionally organized into colonies.
As the rate of diversification subsequently accelerated, the variety of
life became much more complex, and began to resemble that of today. Almost all present-day animal phyla appeared during this period.
The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, and in his 1859 book On the Origin of Species, Charles Darwin
discussed the then inexplicable lack of earlier fossils as one of the
main difficulties for his theory of descent with slow modification
through natural selection. The long-running puzzlement about the appearance of the Cambrian fauna,
seemingly abruptly, without precursor, centers on three key points:
whether there really was a mass diversification of complex organisms
over a relatively short period of time during the early Cambrian; what
might have caused such rapid change; and what it would imply about the
origin of animal life. Interpretation is difficult, owing to a limited
supply of evidence, based mainly on an incomplete fossil record and
chemical signatures remaining in Cambrian rocks.
The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698.
Although their evolutionary importance was not known, on the basis of
their old age, William Buckland (1784–1856) realized that a dramatic
step-change in the fossil record had occurred around the base of what we
now call the Cambrian. Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the Cambrian and Silurian periods.
By 1859, leading geologists including Roderick Murchison, were
convinced that what was then called the lowest Silurian stratum showed
the origin of life on Earth, though others, including Charles Lyell, differed. In On the Origin of Species, Charles Darwin
considered this sudden appearance of a solitary group of trilobites,
with no apparent antecedents, and absence of other fossils, to be
"undoubtedly of the gravest nature" among the difficulties in his theory
of natural selection. He reasoned that earlier seas had swarmed with
living creatures, but that their fossils had not been found because of
the imperfections of the fossil record. In the sixth edition of his book, he stressed his problem further as:
To the question why we do not find
rich fossiliferous deposits belonging to these assumed earliest periods
prior to the Cambrian system, I can give no satisfactory answer.
American paleontologist Charles Walcott, who studied the Burgess Shale fauna,
proposed that an interval of time, the "Lipalian", was not represented
in the fossil record or did not preserve fossils, and that the ancestors
of the Cambrian animals evolved during this time.
Earlier fossil evidence has since been found. The earliest claim is that the history of life on earth goes back 3,850 million years: Rocks of that age at Warrawoona, Australia, were claimed to contain fossil stromatolites, stubby pillars formed by colonies of microorganisms. Fossils (Grypania) of more complex eukaryotic cells, from which all animals, plants, and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana. Rocks dating from 580 to 543million years ago contain fossils of the Ediacara biota, organisms so large that they are likely multicelled, but very unlike any modern organism. In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose.
Opabinia made the largest single contribution to modern interest in the Cambrian explosion.
The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington
and colleagues, who, in the 1970s, reanalysed many fossils from the
Burgess Shale and concluded that several were as complex as, but
different from, any living animals. The most common organism, Marrella, was clearly an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia
were so different from anything else known that Whittington's team
assumed they must represent different phyla, seemingly unrelated to
anything known today. Stephen Jay Gould's popular 1989 account of this work, Wonderful Life,
brought the matter into the public eye and raised questions about what
the explosion represented. While differing significantly in details,
both Whittington and Gould proposed that all modern animal phyla had
appeared almost simultaneously in a rather short span of geological
period. This view led to the modernization of Darwin's tree of life and
the theory of punctuated equilibrium, which Eldredge
and Gould developed in the early 1970s and which views evolution as
long intervals of near-stasis "punctuated" by short periods of rapid
change.
Other analyses, some more recent and some dating back to the
1970s, argue that complex animals similar to modern types evolved well
before the start of the Cambrian.
Dating the Cambrian
Radiometric
dates for much of the Cambrian, obtained by analysis of radioactive
elements contained within rocks, have only recently become available,
and for only a few regions.
Relative dating (A was before B) is often assumed
sufficient for studying processes of evolution, but this, too, has been
difficult, because of the problems involved in matching up rocks of the
same age across different continents.
Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available.
Body fossils
Fossils
of organisms' bodies are usually the most informative type of evidence.
Fossilization is a rare event, and most fossils are destroyed by erosion or metamorphism
before they can be observed. Hence, the fossil record is very
incomplete, increasingly so as earlier times are considered. Despite
this, they are often adequate to illustrate the broader patterns of
life's history.
Also, biases exist in the fossil record: different environments are
more favourable to the preservation of different types of organism or
parts of organisms. Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs.
Since most animal species are soft-bodied, they decay before they can
become fossilised. As a result, although 30-plus phyla of living animals
are known, two-thirds have never been found as fossils.
The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow paleontologists
to examine the internal anatomy of animals, which in other sediments
are only represented by shells, spines, claws, etc. – if they are
preserved at all. The most significant Cambrian lagerstätten are the
early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland); the middle Cambrian Burgess Shale (British Columbia, Canada); and the late Cambrian Orsten (Sweden) fossil beds.
While lagerstätten preserve far more than the conventional fossil
record, they are far from complete. Because lagerstätten are restricted
to a narrow range of environments (where soft-bodied organisms can be
preserved very quickly, e.g. by mudslides), most animals are probably
not represented; further, the exceptional conditions that create
lagerstätten probably do not represent normal living conditions.
In addition, the known Cambrian lagerstätten are rare and difficult to
date, while Precambrian lagerstätten have yet to be studied in detail.
The sparseness of the fossil record means that organisms usually
exist long before they are found in the fossil record – this is known as
the Signor–Lipps effect.
In 2019, a "stunning" find of lagerstätten, known as the Qingjiang biota, was reported from the Danshui river in Hubei province, China.
More than 20,000 fossil specimens were collected, including many soft
bodied animals such as jellyfish, sea anemones and worms, as well as
sponges, arthropods and algae. In some specimens the internal body
structures were sufficiently preserved that soft tissues, including
muscles, gills, mouths, guts and eyes, can be seen. The remains were
dated to around 518 Mya and around half of the species identified at the
time of reporting were previously unknown.
Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.
Trace fossils are particularly significant because they represent a
data source that is not limited to animals with easily fossilized hard
parts, and reflects organisms' behaviour. Also, many traces date from
significantly earlier than the body fossils of animals that are thought
to have been capable of making them.
While exact assignment of trace fossils to their makers is generally
impossible, traces may, for example, provide the earliest physical
evidence of the appearance of moderately complex animals (comparable to earthworms).
Geochemical observations
Several chemical markers
indicate a drastic change in the environment around the start of the
Cambrian. The markers are consistent with a mass extinction, or with a massive warming resulting from the release of methane ice.
Such changes may reflect a cause of the Cambrian explosion, although
they may also have resulted from an increased level of biological
activity – a possible result of the explosion.
Despite these uncertainties, the geochemical evidence helps by making
scientists focus on theories that are consistent with at least one of
the likely environmental changes.
Phylogenetic techniques
Cladistics
is a technique for working out the "family tree" of a set of organisms.
It works by the logic that, if groups B and C have more similarities to
each other than either has to group A, then B and C are more closely
related to each other than either is to A. Characteristics that are
compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or protein. The result of a successful analysis is a hierarchy of clades –
groups whose members are believed to share a common ancestor. The
cladistic technique is sometimes problematic, as some features, such as
wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.
From the relationships, it may be possible to constrain the date
that lineages first appeared. For instance, if fossils of B or C date to
X million years ago and the calculated "family tree" says A was an
ancestor of B and C, then A must have evolved more than X million years
ago.
It is also possible to estimate how long ago two living clades
diverged – i.e. about how long ago their last common ancestor must have
lived – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks",
however, are fallible, and provide only a very approximate timing: they
are not sufficiently precise and reliable for estimating when the
groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques vary by a factor of two.
However, the clocks can give an indication of branching rate, and when
combined with the constraints of the fossil record, recent clocks
suggest a sustained period of diversification through the Ediacaran and
Cambrian.
A phylum is the highest level in the Linnaean system for classifying organisms. Phyla can be thought of as groupings of animals based on general body plan.
Despite the seemingly different external appearances of organisms, they
are classified into phyla based on their internal and developmental
organizations. For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda, but earthworms and tapeworms,
although similar in shape, belong to different phyla. As chemical and
genetic testing becomes more accurate, previously hypothesised phyla are
often entirely reworked.
A phylum is not a fundamental division of nature, such as the difference between electrons and protons. It is simply a very high-level grouping in a classification system
created to describe all currently living organisms. This system is
imperfect, even for modern animals: different books quote different
numbers of phyla, mainly because they disagree about the classification
of a huge number of worm-like species. As it is based on living
organisms, it accommodates extinct organisms poorly, if at all.
Stem group
The concept of stem groups
was introduced to cover evolutionary "aunts" and "cousins" of living
groups, and have been hypothesized based on this scientific theory. A crown group
is a group of closely related living animals plus their last common
ancestor plus all its descendants. A stem group is a set of offshoots
from the lineage at a point earlier than the last common ancestor of the
crown group; it is a relative concept, for example tardigrades
are living animals that form a crown group in their own right, but Budd
(1996) regarded them as also being a stem group relative to the
arthropods.
A coelomate animal is basically a set of concentric tubes, with a gap between the gut and the outer tubes.
Triploblastic
The term Triploblastic means consisting of three layers, which are formed in the embryo, quite early in the animal's development from a single-celled egg to a larva or juvenile form. The innermost layer forms the digestive tract
(gut); the outermost forms skin; and the middle one forms muscles and
all the internal organs except the digestive system. Most types of
living animal are triploblastic – the best-known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.).
Bilaterian
The bilaterians
are animals that have right and left sides at some point in their life
histories. This implies that they have top and bottom surfaces and,
importantly, distinct front and back ends. All known bilaterian animals
are triploblastic, and all known triploblastic animals are bilaterian.
Living echinoderms (sea stars, sea urchins, sea cucumbers,
etc.) 'look' radially symmetrical (like wheels) rather than bilaterian,
but their larvae exhibit bilateral symmetry and some of the earliest
echinoderms may have been bilaterally symmetrical. Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic.
Coelomate
The term Coelomate
means having a body cavity (coelom) containing the internal organs.
Most of the phyla featured in the debate about the Cambrian explosion are coelomates: arthropods, annelid worms, molluscs, echinoderms, and chordates – the noncoelomate priapulids
are an important exception. All known coelomate animals are
triploblastic bilaterians, but some triploblastic bilaterian animals do
not have a coelom – for example flatworms, whose organs are surrounded by unspecialized tissues.
Changes in the abundance and diversity of some types of fossil have
been interpreted as evidence for "attacks" by animals or other
organisms. Stromatolites, stubby pillars built by colonies of microorganisms, are a major constituent of the fossil record from about 2,700 million years ago, but their abundance and diversity declined steeply after about 1,250 million years ago. This decline has been attributed to disruption by grazing and burrowing animals.
Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil – from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around 2,000 million years ago, acritarchs underwent a boom around 1,000 million years ago,
increasing in abundance, diversity, size, complexity of shape, and
especially size and number of spines. Their increasingly spiny forms in
the last 1 billion years may indicate an increased need for defence
against predation. Other groups of small organisms from the Neoproterozoic era also show signs of antipredator defenses. A consideration of taxon longevity appears to support an increase in predation pressure around this time.
In general, the fossil record shows a very slow appearance of these
lifeforms in the Precambrian, with many cyanobacterial species making up
much of the underlying sediment.
Fossils of the Doushantuo formation
The layers of the Doushantuo formation from around 580 million year old
harbour microscopic fossils that may represent early bilaterians. Some
have been described as animal embryos and eggs, although some may
represent the remains of giant bacteria.
Another fossil, Vernanimalcula, has been interpreted as a coelomate bilaterian,
but may simply be an infilled bubble.
These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators.
Burrows
An Ediacaran trace fossil, made when an organism burrowed below a microbial mat.
The traces of organisms moving on and directly underneath the
microbial mats that covered the Ediacaran sea floor are preserved from
the Ediacaran period, about 565 million years ago. They were probably made by organisms resembling earthworms
in shape, size, and how they moved. The burrow-makers have never been
found preserved, but, because they would need a head and a tail, the
burrowers probably had bilateral symmetry – which would in all
probability make them bilaterian animals. They fed above the sediment surface, but were forced to burrow to avoid predators.
Around the start of the Cambrian (about 542 million years ago), many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus.
The vertical burrows indicate that worm-like animals acquired new
behaviours, and possibly new physical capabilities. Some Cambrian trace
fossils indicate that their makers possessed hard exoskeletons, although they were not necessarily mineralised.
Burrows provide firm evidence of complex organisms; they are also
much more readily preserved than body fossils, to the extent that the
absence of trace fossils has been used to imply the genuine absence of
large, motile, bottom-dwelling organisms.
They provide a further line of evidence to show that the Cambrian
explosion represents a real diversification, and is not a preservational
artefact.
This new habit 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 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.
Ediacaran organisms
Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance
At the start of the Ediacaran period, much of the acritarch
fauna, which had remained relatively unchanged for hundreds of millions
of years, became extinct, to be replaced with a range of new, larger
species, which would prove far more ephemeral. This radiation, the first in the fossil record, is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota, which flourished for 40 million years until the start of the Cambrian.
Most of this "Ediacara biota" were at least a few centimeters long,
significantly larger than any earlier fossils. The organisms form three
distinct assemblages, increasing in size and complexity as time
progressed.
Many of these organisms were quite unlike anything that appeared
before or since, resembling discs, mud-filled bags, or quilted
mattresses – one palæontologist proposed that the strangest organisms
should be classified as a separate kingdom, Vendozoa.
Fossil of Kimberella, a triploblastic bilaterian, and possibly a mollusc
At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs (Kimberella), echinoderms (Arkarua); and arthropods (Spriggina, Parvancorina, Yilingia).
Still, debate exists about the classification of these specimens,
mainly because the diagnostic features that allow taxonomists to
classify more recent organisms, such as similarities to living
organisms, are generally absent in the ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal. These organisms are central to the debate about how abrupt the Cambrian explosion was.
If some were early members of the animal phyla seen today, the
"explosion" looks a lot less sudden than if all these organisms
represent an unrelated "experiment", and were replaced by the animal
kingdom fairly soon thereafter (40M years is "soon" by evolutionary and
geological standards).
Beck Spring Dolomite
Paul Knauth, a geologist at Arizona State University, maintains that photosynthesizing organisms such as algae may have grown over a 750- to 800-million-year-old formation in Death Valley
known as the Beck Spring Dolomite. In the early 1990s, samples from
this 1,000-foot thick layer of dolomite revealed that the region housed
flourishing mats of photosynthesizing, unicellular life forms which
antedated the Cambrian explosion.
Microfossils have been unearthed from holes riddling the
otherwise barren surface of the dolomite. These geochemical and
microfossil findings support the idea that during the Precambrian
period, complex life evolved both in the oceans and on land. Knauth
contends that animals may well have had their origins in freshwater
lakes and streams, and not in the oceans.
Some 30 years later, a number of studies have documented an
abundance of geochemical and microfossil evidence showing that life
covered the continents as far back as 2.2 billion years ago. Many
paleobiologists now accept the idea that simple life forms existed on
land during the Precambrian, but are opposed to the more radical idea
that multicellular life thrived on land more than 600 million years ago.
Ediacaran–Early Cambrian skeletonisation
The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules. The oldest sponge spicules are monaxon siliceous, aged around 580 million years ago,
known from the Doushantou Formation in China and from deposits of the
same age in Mongolia, although the interpretation of these fossils as
spicules has been challenged.
In the late Ediacaran-lowest Cambrian, numerous tube dwellings of
enigmatic organisms appeared. It was organic-walled tubes (e.g. Saarina) and chitinous tubes of the sabelliditids (e.g. Sokoloviina, Sabellidites, Paleolina) that prospered up to the beginning of the Tommotian. The mineralized tubes of Cloudina, Namacalathus, Sinotubulites, and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from 549 to 542million years ago, as well as the triradially symmetrical mineralized tubes of anabaritids (e.g. Anabarites, Cambrotubulus) from uppermost Ediacaran and lower Cambrian. Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and thrombolites, i.e. they could live in an environment adverse to the majority of animals.
Although they are as hard to classify as most other Ediacaran
organisms, they are important in two other ways. First, they are the
earliest known calcifying organisms (organisms that built shells from calcium carbonate).
Secondly, these tubes are a device to rise over a substrate and
competitors for effective feeding and, to a lesser degree, they serve as
armor for protection against predators and adverse conditions of
environment. Some Cloudina fossils show small holes in shells.
The holes possibly are evidence of boring by predators sufficiently
advanced to penetrate shells. A possible "evolutionary arms race" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion.
In the lowest Cambrian, the stromatolites were decimated. This
allowed animals to begin colonization of warm-water pools with carbonate
sedimentation. At first, it was anabaritids and Protohertzina (the fossilized grasping spines of chaetognaths) fossils. Such mineral skeletons as shells, sclerites, thorns, and plates appeared in uppermost Nemakit-Daldynian; they were the earliest species of halkierids, gastropods, hyoliths and other rare organisms. The beginning of the Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs, hyoliths, and sponges, along with a rich complex of skeletal elements of unknown animals, the first archaeocyathids, brachiopods, tommotiids, and others. Also soft-bodied extant phyla such as comb jellies, scalidophorans, entoproctans, horseshoe worms and lobopodians had armored forms.
This sudden increase is partially an artefact of missing strata at the
Tommotian type section, and most of this fauna in fact began to
diversify in a series of pulses through the Nemakit-Daldynian and into
the Tommotian.
Some animals may already have had sclerites, thorns, and plates in the Ediacaran (e.g. Kimberella had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized in siliciclastic deposits.
Older (~750 Ma) fossils indicate that mineralization long preceded the
Cambrian, probably defending small photosynthetic algae from
single-celled eukaryotic predators.
Trace
fossils (burrows, etc.) are a reliable indicator of what life was
around, and indicate a diversification of life around the start of the
Cambrian, with the freshwater realm colonized by animals almost as
quickly as the oceans.
Small shelly fauna
Fossils known as "small shelly fauna"
have been found in many parts on the world, and date from just before
the Cambrian to about 10 million years after the start of the Cambrian
(the Nemakit-Daldynian and Tommotian ages; see timeline). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals), and small shells very like those of brachiopods and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.
While small, these fossils are far more common than complete
fossils of the organisms that produced them; crucially, they cover the
window from the start of the Cambrian to the first lagerstätten: a
period of time otherwise lacking in fossils. Hence, they supplement the
conventional fossil record and allow the fossil ranges of many groups to
be extended.
Early Cambrian trilobites and echinoderms
A fossilized trilobite, an ancient type of arthropod: This specimen, from the Burgess Shale, preserves "soft parts" – the antennae and legs.
The earliest trilobite
fossils are about 530 million years old, but the class was already
quite diverse and worldwide, suggesting they had been around for quite
some time.
The fossil record of trilobites began with the appearance of trilobites
with mineral exoskeletons – not from the time of their origin.
The earliest generally accepted echinoderm fossils appeared a little bit later, in the Late Atdabanian; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.
These provide firm data points for the "end" of the explosion, or
at least indications that the crown groups of modern phyla were
represented.
Burgess Shale type faunas
The Burgess Shale and similar lagerstätten preserve the soft parts of
organisms, which provide a wealth of data to aid in the classification
of enigmatic fossils. It often preserved complete specimens of organisms
only otherwise known from dispersed parts, such as loose scales or
isolated mouthparts. Further, the majority of organisms and taxa in
these horizons are entirely soft-bodied, hence absent from the rest of
the fossil record. Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed.
However, the assemblages may represent a "museum": a deep-water
ecosystem that is evolutionarily "behind" the rapidly diversifying fauna
of shallower waters.
Because the lagerstätten provide a mode and quality of
preservation that is virtually absent outside of the Cambrian, many
organisms appear completely different from anything known from the
conventional fossil record. This led early workers in the field to
attempt to shoehorn the organisms into extant phyla; the shortcomings of
this approach led later workers to erect a multitude of new phyla to
accommodate all the oddballs. It has since been realised that most
oddballs diverged from lineages before they established the phyla known today – slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did.
The preservational mode is rare in the preceding Ediacaran
period, but those assemblages known show no trace of animal life –
perhaps implying a genuine absence of macroscopic metazoans.
Early Cambrian crustaceans
Crustaceans, one of the four great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans
were once thought to be common in Burgess Shale-type biotas, but none
of these individuals can be shown to fall into the crown group of "true
crustaceans". The Cambrian record of crown-group crustaceans comes from microfossils. The Swedish Orsten
horizons contain later Cambrian crustaceans, but only organisms smaller
than 2 mm are preserved. This restricts the data set to juveniles and
miniaturised adults.
A more informative data source is the organic microfossils of the Mount Cap formation, Mackenzie Mountains, Canada. This late Early Cambrian assemblage (510 to 515million years ago) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with hydrofluoric acid.
The diversity of this assemblage is similar to that of modern
crustacean faunas. Analysis of fragments of feeding machinery found in
the formation shows that it was adapted to feed in a very precise and
refined fashion. This contrasts with most other early Cambrian
arthropods, which fed messily by shovelling anything they could get
their feeding appendages on into their mouths. This sophisticated and
specialised feeding machinery belonged to a large (about 30 cm)
organism, and would have provided great potential for diversification;
specialised feeding apparatus allows a number of different approaches to
feeding and development, and creates a number of different approaches
to avoid being eaten.
Early Ordovician radiation
After an extinction at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic.
During this radiation, the total number of orders doubled, and families tripled, increasing marine diversity to levels typical of the Palaeozoic, and disparity to levels approximately equivalent to today's.
Stages
The event lasted for about the next 20–25 million years, and its elevated rates of evolution had ended by the base of Cambrian Series 2, 521 million years ago, coincident with the first trilobites in the fossil record.
Different authors break the explosion down into stages in different ways.
Ed Landing recognizes three stages: Stage 1, spanning the
Ediacaran-Cambrian boundary, corresponds to a diversification of
biomineralizing animals and of deep and complex burrows; Stage 2,
corresponding to the radiation of molluscs and stem-group Brachiopods (hyoliths and tommotiids),
which apparently arose in intertidal waters; and Stage 3, seeing the
Atdabanian diversification of trilobites in deeper waters, but little
change in the intertidal realm.
Graham Budd
synthesises various schemes to produce a compatible view of the SSF
record of the Cambrian explosion, divided slightly differently into four
intervals: a "Tube world", lasting from 550 to 536million years ago,
spanning the Ediacaran-Cambrian boundary, dominated by Cloudina,
Namacalathus and pseudoconodont-type elements; a "Sclerite world",
seeing the rise of halkieriids, tommotiids, and hyoliths, lasting to the
end of the Fortunian (c. 525 Ma); a brachiopod world, perhaps
corresponding to the as yet unratified Cambrian Stage 2; and Trilobite
World, kicking off in Stage 3.
Complementary to the shelly fossil record, trace fossils can be
divided into five subdivisions: "Flat world" (late Ediacaran), with
traces restricted to the sediment surface; Protreozoic III (after
Jensen), with increasing complexity; pedum world, initiated at the base of the Cambrian with the base of the T.pedum zone (see Cambrian#Dating the Cambrian); Rusophycus world, spanning 536 to 521million years ago and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm; and Cruziana world, with an obvious correspondence to Trilobite World.
Validity
There is strong evidence for species of Cnidaria and Porifera existing in the Ediacaran and possible members of Porifera even before that during the Cryogenian. Bryozoans do not appear in the fossil record until after the Cambrian, in the Lower Ordovician.
The fossil record as Darwin knew it seemed to suggest that the
major metazoan groups appeared in a few million years of the early to
mid-Cambrian, and even in the 1980s, this still appeared to be the case.
However, evidence of Precambrian Metazoa is gradually accumulating. If the Ediacaran Kimberella was a mollusc-like protostome (one of the two main groups of coelomates), the protostome and deuterostome lineages must have split significantly before 550 million years ago (deuterostomes are the other main group of coelomates). Even if it is not a protostome, it is widely accepted as a bilaterian. Since fossils of rather modern-looking cnidarians (jellyfish-like organisms) have been found in the Doushantuolagerstätte, the cnidarian and bilaterian lineages must have diverged well over 580 million years ago.
Trace fossils and predatory borings in Cloudina shells provide further evidence of Ediacaran animals. Some fossils from the Doushantuo formation have been interpreted as embryos and one (Vernanimalcula) as a bilaterian coelomate, although these interpretations are not universally accepted. Earlier still, predatory pressure has acted on stromatolites and acritarchs since around 1,250 million years ago.
Some say that the evolutionary change was accelerated by an order of magnitude,
but the presence of Precambrian animals somewhat dampens the "bang" of
the explosion; not only was the appearance of animals gradual, but their
evolutionary radiation
("diversification") may also not have been as rapid as once thought.
Indeed, statistical analysis shows that the Cambrian explosion was no
faster than any of the other radiations in animals' history.
However, it does seem that some innovations linked to the explosion –
such as resistant armour – only evolved once in the animal lineage; this
makes a lengthy Precambrian animal lineage harder to defend.
Further, the conventional view that all the phyla arose in the Cambrian
is flawed; while the phyla may have diversified in this time period,
representatives of the crown groups of many phyla do not appear until
much later in the Phanerozoic.
Further, the mineralised phyla that form the basis of the fossil record
may not be representative of other phyla, since most mineralised phyla
originated in a benthic
setting. The fossil record is consistent with a Cambrian explosion that
was limited to the benthos, with pelagic phyla evolving much later.
Ecological complexity among marine animals increased in the Cambrian, as well later in the Ordovician.
However, recent research has overthrown the once-popular idea that
disparity was exceptionally high throughout the Cambrian, before
subsequently decreasing.
In fact, disparity remains relatively low throughout the Cambrian, with
modern levels of disparity only attained after the early Ordovician
radiation.
The diversity of many Cambrian assemblages is similar to today's,
and at a high (class/phylum) level, diversity is thought by some to
have risen relatively smoothly through the Cambrian, stabilizing
somewhat in the Ordovician. This interpretation, however, glosses over the astonishing and fundamental pattern of basal polytomy and phylogenetic telescoping at or near the Cambrian boundary, as seen in most major animal lineages. Thus Harry Blackmore Whittington's questions regarding the abrupt nature of the Cambrian explosion remain, and have yet to be satisfactorily answered.
The Cambrian explosion as survivorship bias
Budd and Mann suggested that the Cambrian explosion was the result of a type of survivorship bias called the "Push of the past".
As groups at their origin tend to go extinct, it follows that any
long-lived group would have experienced an unusually rapid rate of
diversification early on, creating the illusion of a general speed-up in
diversification rates. However, rates of diversification could remain
at background levels and still generate this sort of effect in the
surviving lineages.
Possible causes
Despite the evidence that moderately complex animals (triploblasticbilaterians)
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 both the timing and magnitude of the explosion.
Changes in the environment
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 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 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, required for the construction of complex structures, or to form molecules for the construction of a hard exoskeleton.
However, animals were not affected when similar oceanographic
conditions occurred 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.
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.
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.
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.
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 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 able to seek out more oxygen-rich environments an advantage over
sessile forms of life.
Evolution of eyes
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.
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.
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.
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".
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 Devoniancolonization 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.
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.
Uniqueness of the explosion
The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a coevolutionary
rise in diversity as animals explored niches on the Ediacaran sea
floor, followed by a second expansion in the early Cambrian as they
became established in the water column.
The rate of diversification seen in the Cambrian phase of the explosion
is unparalleled among marine animals: it affected all metazoan clades of which Cambrian fossils have been found. Later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans. Although the recovery from the Permian-Triassic extinction
started with about as few animal species as the Cambrian explosion, the
recovery produced far fewer significantly new types of animals.
Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailable ecological niches.
When these were all occupied, limited space existed for such
wide-ranging diversifications to occur again, because strong competition
existed in all niches and incumbents
usually had the advantage. If a wide range of empty niches had
continued, clades would be able to continue diversifying and become
disparate enough for us to recognise them as different phyla;
when niches are filled, lineages will continue to resemble one another
long after they diverge, as limited opportunity exists for them to
change their life-styles and forms.
There were two similar explosions in the evolution of land plants: after a cryptic history beginning about 450 million years ago, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about 400 million years ago. Furthermore, angiosperms (flowering plants) originated and rapidly diversified during the Cretaceous period.
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