The Cambrian explosion, also called the Cambrian radiation or Cambrian diversification, is a time period that began about 538.8 million years ago during the Cambrian period of the early Paleozoic era. During this time, a rapid increase in complex life forms occurred, and most major animal groups first appeared in the fossil record. This event lasted approximately 13 to 25 million years and led to the splitting of most modern animal phyla. Other groups of organisms also diversified significantly during this time.
Before the Cambrian explosion, most life forms were simple, consisting of single cells or small multicellular organisms, sometimes arranged in colonies. As diversification increased, life became more complex and began to resemble the variety seen today. Nearly all animal groups that exist today first appeared during this period, including the earliest chordates.
History and significance
William Buckland noticed in the 1840s that fossils in the "Primordial Strata" appeared quickly. In his 1859 book On the Origin of Species, Charles Darwin discussed the lack of older fossils as a major challenge for his idea that life changed slowly over time through natural selection. Scientists have long puzzled over the sudden appearance of complex animals in the Cambrian period, focusing on three questions: whether many new life forms developed quickly during this time, what caused this change, and what it means about how animals began. Understanding this is hard because evidence is limited, mainly due to an incomplete fossil record and chemical clues in Cambrian rocks.
The first Cambrian fossils discovered were trilobites, described by Edward Lhuyd in 1698. Though their importance was not known, William Buckland realized that a major change in the fossil record happened near the start of the Cambrian period. Nineteenth-century geologists like Adam Sedgwick and Roderick Murchison used Cambrian fossils to date rock layers and define the Cambrian and Silurian periods. By 1859, some geologists, including Roderick Murchison, believed the lowest Silurian stratum showed Earth’s earliest life, but others, like Charles Lyell, disagreed. Darwin called the sudden appearance of trilobites without earlier fossils a serious problem for his theory. He thought earlier seas had many creatures, but their fossils were missing due to gaps in the fossil record. In the sixth edition of his book, he wrote: "I can give no satisfactory answer" to why older fossil layers are not found.
Charles Walcott, an American paleontologist, studied the Burgess Shale and proposed a time called the "Lipalian" might explain the lack of earlier fossils, suggesting Cambrian animals evolved during this period. Later discoveries showed life existed 3,850 million years ago, with fossil stromatolites in Australia. Fossils of complex eukaryotic cells were found in rocks 1,400 million years old in China and Montana. Ediacaran biota, large but unlike modern organisms, lived 580 to 543 million years ago. In 1948, Preston Cloud suggested a "eruptive" evolutionary period in the Early Cambrian, but by the 1970s, no clear link was found between these early life forms and Cambrian animals.
Modern interest in the Cambrian explosion grew after Harry B. Whittington and colleagues reanalyzed Burgess Shale fossils in the 1970s. They found creatures like Marrella (an arthropod not classified into any known group) and Opabinia (a five-eyed organism) that were unlike any living animals. Stephen Jay Gould’s 1989 book Wonderful Life brought this topic to public attention. Both Whittington and Gould suggested most modern animal groups appeared quickly in the Cambrian, influencing ideas like punctuated equilibrium, which describes long periods of little change followed by rapid evolution.
Some studies argue complex animals evolved before the Cambrian. Radiometric dating of Cambrian rocks, which uses radioactive elements, has only recently become available for limited areas. Relative dating (A came before B) is often used but is challenging because matching rocks of the same age across continents is difficult. Dates for the Cambrian’s start were revised over time: 542 million years ago in 2004, 541 million years ago in 2012, and 538.8 million years ago in 2022.
Theories suggest the Cambrian explosion occurred during the final stages of Gondwana’s formation, which followed the splitting of Rodinia and the opening of the Iapetus Ocean. The largest Cambrian fossil area was around Gondwana, spanning from low northern to high southern latitudes. By the Cambrian’s middle and later periods, continents like Laurentia, Baltica, and Siberia drifted apart.
Fossils of entire organisms are the most useful evidence, but fossilization is rare. Most fossils are destroyed by erosion or metamorphism before being studied. The fossil record is incomplete, especially for older times. Biases exist, as certain environments preserve specific types of organisms or parts, like shells. Most animals are soft-bodied and decay before fossilizing, so only about one-third of living animal phyla have been found as fossils.
The Cambrian period has many lagerstätten, which preserve soft tissues and allow scientists to study internal anatomy. Key examples include the Chengjiang (China), Sirius Passet (Greenland), Burgess Shale (Canada), and Orsten (Sweden) fossil beds. Though lagerstätten provide more detailed information than typical fossils, they are still incomplete. They form in rare environments where soft-bodied organisms are preserved quickly, like mudslides. These conditions likely do not reflect normal living conditions. Known lagerstätten are rare and hard to date, limiting their usefulness.
Explanation of key scientific terms
A phylum is the highest level in the Linnaean system for classifying organisms. Phyla are groupings of animals based on general body plans. Even though organisms may look very different on the outside, they are placed into the same phylum if they share similar internal structures and developmental patterns. For example, spiders and barnacles are both in the phylum Arthropoda, but earthworms and tapeworms, despite looking similar, are in different phyla. As scientists improve chemical and genetic testing methods, some previously suggested phyla are often changed or removed entirely.
A phylum is not a basic division of nature, like the difference between electrons and protons. It is a high-level group in a classification system designed to describe living organisms. This system is not perfect, even for modern animals: different books list different numbers of phyla, mainly because scientists disagree about how to classify many worm-like species. Since the system is based on living organisms, it does not work well for extinct ones.
The concept of stem groups was introduced to describe evolutionary relatives of living groups. These groups are based on scientific theories. A crown group includes a group of closely related living animals, their last common ancestor, and all descendants of that ancestor. A stem group includes branches of the evolutionary family tree that split off before the last common ancestor of the crown group. This is a relative idea: for example, tardigrades are a crown group on their own, but some scientists have also considered them a stem group related to arthropods.
The term "triploblastic" means having three layers of tissue formed early in an animal’s development from a single-celled egg to a larva or juvenile. The innermost layer becomes the digestive tract (gut), the outermost layer becomes skin, and the middle layer becomes muscles and internal organs (except the digestive system). Most living animals are triploblastic. Exceptions include Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.).
Bilaterians are animals that have right and left sides at some point in their life. This means they also have top and bottom surfaces and distinct front and back ends. All known bilaterians are triploblastic, and all known triploblastic animals are bilaterians. Living echinoderms (such as sea stars, sea urchins, and sea cucumbers) appear radially symmetrical (like wheels) as adults, but their larvae have bilateral symmetry. Some early echinoderms may have had bilateral symmetry. Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic. However, the planula larva of the common ancestor of Bilateria and Cnidaria is thought to have been bilaterally symmetrical.
The term "coelomate" means having a body cavity (coelom) that contains internal organs. Most phyla involved in debates about the Cambrian explosion are coelomates, including arthropods, annelid worms, molluscs, echinoderms, and chordates. Priapulids are an important exception because they are not coelomates. All known coelomates are triploblastic bilaterians, but some triploblastic bilaterians do not have a coelom. For example, flatworms have organs surrounded by unspecialized tissues instead of a coelom.
Precambrian life
Changes in the number and variety of certain fossils have been seen as signs of attacks by animals or other organisms. Stromatolites, short, pillar-like structures made by groups of tiny organisms, were a major part of the fossil record from about 2,700 million years ago. However, their numbers and variety dropped sharply after about 1,250 million years ago. Scientists think this drop happened because animals that ate or dug through the mats disturbed the stromatolites.
Before the Cambrian period, the ocean had many small fossils called acritarchs. This term includes almost any small fossil with an organic outer layer, such as egg cases from tiny animals or resting structures from green algae. Acritarchs first appeared around 2,000 million years ago, but their numbers, variety, size, and complexity increased around 1,000 million years ago. Their spiny shapes in the last 1 billion years may show they needed better protection from predators. Other small organisms from the Neoproterozoic era also show signs of defenses against predators. Looking at how long different groups of organisms existed suggests that predator pressure increased around this time.
In general, the fossil record shows that these lifeforms appeared very slowly during the Precambrian, with many cyanobacterial species making up much of the sediment.
At the start of the Ediacaran period, many acritarch species, which had not changed much for hundreds of millions of years, became extinct. They were replaced by a range of larger, new species that did not last as long. This change, the first major one in the fossil record, was soon followed by a group of unfamiliar, large fossils called the Ediacara biota. These organisms thrived for 40 million years until the start of the Cambrian period. Most of the Ediacara biota were at least a few centimeters long, much larger than earlier fossils. These organisms formed three distinct groups, growing larger and more complex over time.
Many of these organisms were unlike anything before or since, looking like discs, mud-filled bags, or quilted mattresses. One scientist suggested that the strangest of these should be classified as a separate group, called Vendozoa.
At least some of these organisms may have been early forms of the animal groups central to the "Cambrian explosion" debate. They have been compared to early mollusks (Kimberella), echinoderms (Arkarua), and arthropods (Spriggina, Parvancorina, Yilingia). However, scientists debate their classification because the features that help classify modern organisms, such as similarities to living species, are often missing in Ediacaran fossils. Still, there is strong evidence that Kimberella was a triploblastic bilaterian animal. These organisms are important in the debate about how sudden the Cambrian explosion was. If some were early members of today’s animal groups, the "explosion" seems less sudden than if these organisms were unrelated and quickly replaced by the animal kingdom.
Tracks left by organisms moving on and under the microbial mats covering the Ediacaran seafloor are preserved from about 565 million years ago. These tracks were likely made by organisms similar in shape, size, and movement to earthworms. The burrowers themselves were never found as fossils, but their need for a head and tail suggests they had bilateral symmetry, which would make them bilaterian animals. These organisms likely fed above the sediment surface but burrowed to avoid predators.
Cambrian life
Trace fossils, such as burrows, show what kinds of life existed long ago. These fossils suggest that life became more diverse at the start of the Cambrian period, with animals quickly moving into freshwater environments as well as oceans.
Fossils called "small shelly fauna" have been found worldwide and date from just before the Cambrian to about 10 million years after its start (Nemakit-Daldynian and Tommotian ages). These fossils include spines, armor plates, tubes, sponge-like animals, and tiny shells similar to those of brachiopods and snail-like mollusks. Most are only 1 to 2 mm long.
Although small, these fossils are much more common than complete fossils of the organisms that made them. They fill a gap in the fossil record between the start of the Cambrian and the first well-preserved fossil sites (lagerstätten), which otherwise lack fossils. This helps scientists extend the known time ranges of many ancient life groups.
The first cnidarian larvae, named Eolarva, appeared in the Cambrian. However, scientists disagree about whether Eolarva is truly a cnidarian larva. If it is, Eolarva would be the earliest evidence of indirect development in animals from the Cambrian period.
Medusozoans, a group of animals, developed complex life cycles with a medusa stage during the Cambrian. This is shown by the discovery of Burgessomedusa phasmiformis.
The oldest trilobite fossils are about 530 million years old. However, trilobites were already diverse and found globally, suggesting they existed for a long time before being preserved as fossils. The first trilobite fossils found are those with hard, mineral-based shells, not the earliest trilobites themselves.
Crustaceans, one of the four main groups of arthropods today, are rare in Cambrian fossils. Some early Cambrian fossils, like those from the Burgess Shale, were once thought to include crustaceans, but none belong to the "true" crustacean group. Cambrian crustacean fossils are mostly found in tiny microfossils, such as those in the Swedish Orsten horizons. These fossils are only of very small organisms, like juveniles or tiny adults.
A more detailed record of Cambrian crustaceans comes from organic microfossils in the Mount Cap formation in Canada. These fossils, from about 510 to 515 million years ago, include tiny pieces of arthropod cuticle. When the rock is treated with hydrofluoric acid, these fragments are revealed. The variety of these fossils is similar to modern crustacean communities. Fossilized feeding parts in the Mount Cap formation show highly specialized feeding structures, unlike most other Cambrian arthropods, which had simpler feeding methods. These structures belonged to a large organism (about 30 cm) and likely helped it diversify by allowing different ways to eat and avoid being eaten.
The earliest accepted echinoderm fossils appeared in the Late Atdabanian stage of the Cambrian. Unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical. These fossils provide clear evidence of the end of the Cambrian explosion, or at least show that modern animal groups were already present.
At the start of the Cambrian (about 539 million years ago), new types of traces appeared, such as vertical burrows like Diplocraterion and Skolithos, and traces made by arthropods like Cruziana and Rusophycus. These burrows suggest that worm-like animals developed new behaviors and possibly new physical traits. Some Cambrian trace fossils show that their makers had hard exoskeletons, though not always made of minerals. Both small and large bilaterian animals (animals with left and right sides) were involved in creating these burrows.
Burrows are strong evidence of complex organisms because they are easier to preserve than body fossils. The lack of trace fossils has sometimes been used to suggest that large, mobile bottom-dwelling animals were not present. Burrows support the idea that the Cambrian explosion was a real increase in life diversity, not just a result of better fossil preservation.
The earliest skeletal fossils from the Ediacaran and lowest Cambrian (Nemakit-Daldynian) include tubes and unclear sponge spicules. The oldest known sponge spicules are siliceous (made of silica) and are about 580 million years old, found in China and Mongolia. However, some scientists question whether these fossils are truly spicules. In the late Ediacaran and early Cambrian, many mysterious organisms lived in tubes. These included organic-walled tubes (like Saarina) and chitinous tubes (like Sokoloviina and Sabellidites) that thrived until the start of the Tommotian stage. Mineralized tubes, such as those from Cloudina and Anabarites, appeared near the end of the Ediacaran period and into the early Cambrian. These tubes were often found in carbonate rocks like stromatolites and thrombolites, suggesting they lived in environments that were not ideal for most animals.
Although these fossils are hard to classify, they are important for two reasons. First, they are the earliest known calcifying organisms (organisms that built shells from calcium carbonate). Second, the tubes helped these organisms rise above the seafloor and compete for food, and they also provided some protection from predators and harsh conditions. Some Cloudina fossils have small holes, which might be signs that predators drilled through their shells. This could be part of an "evolutionary arms race" between predators and prey, a theory that explains the Cambrian explosion.
In the earliest Cambrian period, stromatolites (layered rock structures formed by algae) declined. This allowed animals to colonize warm-water pools with carbonate sediments. Early Cambrian fossils include those of anabaritids and Protohertzina (spines from chaetognaths). By the end of the Nemakit-Daldynian stage, hard structures like shells, sclerites, thorns, and plates appeared. These were the earliest known members of groups like halkierids, gastropods, and hyoliths. The start of the Tommotian stage is marked by a sudden increase in the number and variety of fossils, including molluscs, hyoliths, sponges, and skeletal remains of unknown animals. Early archaeocyathids, brachiopods, and tommotiids also appeared. Soft-bodied animals like comb jellies and horseshoe worms had armored forms. This sudden increase may partly be due to missing rock layers in the Tommotian section, as many of these organisms actually diversified gradually during the Nemakit-Daldynian and into the Tommotian.
Some animals may have had hard structures like sclerites and plates in the Ediacaran period, such as Kimberella, which had carbonate-based sclerites. However, thin carbonate skeletons are
Stages
The early Cambrian period of diversification lasted about 20 to 25 million years. The high rates of evolution during this time ended by the start of Cambrian Series 2, which was 521 million years ago. This time also marks the first appearance of trilobites in the fossil record. Different scientists describe the intervals of diversification during the early Cambrian in various ways:
Ed Landing identifies three stages: Stage 1, which begins at the Ediacaran-Cambrian boundary, includes the diversification of animals that form hard parts and the development of deep and complex burrows. Stage 2 involves the spread of mollusks and early brachiopods (hyoliths and tommotiids), which likely originated in intertidal waters. Stage 3 includes the diversification of trilobites in deeper waters, with little change in intertidal areas.
Graham Budd combines different systems to create a unified view of the Cambrian explosion, dividing it into four intervals: a "Tube world" from 550 to 536 million years ago, which spans the Ediacaran-Cambrian boundary and includes fossils like Cloudina, Namacalathus, and pseudoconodont-type elements; a "Sclerite world," which features the rise of halkieriids, tommotiids, and hyoliths, lasting until the end of the Fortunian (about 525 million years ago); a "brachiopod world," possibly linked to the unconfirmed Cambrian Stage 2; and a "Trilobite World," which begins in Stage 3.
In addition to the shelly fossil record, trace fossils are divided into five subdivisions: "Flat world" (late Ediacaran), where traces are limited to the sediment surface; Proterozoic III (after Jensen), with increasing complexity; "pedum world," which starts at the base of the Cambrian with the base of the T. pedum zone; "Rusophycus world," spanning 536 to 521 million years ago, matching the "Sclerite World" and "Brachiopod World" under the SSF system; and "Cruziana world," which clearly corresponds to the "Trilobite World."
Validity
There is strong evidence that species of Cnidaria and Porifera lived during the Ediacaran period. Some Porifera may have existed even earlier, during the Cryogenian period. Bryozoans, once believed to appear in the fossil record only after the Cambrian period, have now been found in Cambrian Age 3 rocks from Australia and South China.
The fossil record that Darwin studied seemed to show that major groups of metazoans (animals) appeared quickly during the early to mid-Cambrian period. This idea was still widely accepted in the 1980s.
However, evidence of Precambrian animals is growing. If the Ediacaran Kimberella was a mollusc-like protostome (one of two main groups of coelomates), then the protostome and deuterostome lineages (the other main group of coelomates) must have split before 550 million years ago. Even if Kimberella was not a protostome, it is widely accepted as a bilaterian. Fossils of cnidarians (jellyfish-like organisms) found in the Doushantuo lagerstätte show that the cnidarian and bilaterian lineages must have split more than 580 million years ago.
Trace fossils and signs of predation on Cloudina shells provide evidence of Ediacaran animals. Some fossils from the Doushantuo formation have been interpreted as embryos, and one (Vernanimalcula) as a bilaterian coelomate, though these interpretations are not universally accepted. Predatory pressure on stromatolites and acritarchs has been recorded as far back as 1,250 million years ago.
Some scientists say evolutionary changes happened much faster than previously thought. However, the presence of Precambrian animals suggests the "Cambrian explosion" was not as sudden as once believed. Animals may have appeared gradually, and their diversification may not have been as rapid as earlier thought. Statistical analysis shows the Cambrian explosion was not faster than other major evolutionary events in animal history. However, certain innovations, like resistant armor, evolved only once in the animal lineage, making it harder to support a long Precambrian animal history. The traditional idea that all animal phyla originated in the Cambrian is incorrect. While phyla may have diversified during this time, representatives of many phyla's crown groups appeared much later in the Phanerozoic. Also, the fossil record may not represent all phyla, as most mineralized phyla evolved in benthic (seafloor) environments. The fossil record supports a Cambrian explosion limited to benthic animals, with pelagic (open-ocean) phyla evolving later.
Ecological complexity among marine animals increased in the Cambrian and later in the Ordovician. Recent research challenges the old idea that diversity was extremely high throughout the Cambrian and then declined. In fact, diversity remained relatively low during the Cambrian, reaching modern levels only after the early Ordovician radiation.
The diversity of many Cambrian assemblages is similar to today's. At a high level (class/phylum), some scientists believe diversity rose gradually through the Cambrian, stabilizing in the Ordovician. However, this view overlooks the complex and sudden branching of major animal lineages near the Cambrian boundary. Questions raised by Harry Blackmore Whittington about the abrupt nature of the Cambrian explosion remain unanswered.
Budd and Mann suggested the Cambrian explosion was caused by a type of survivorship bias called the "Push of the Past." Groups that survive tend to diversify rapidly early in their history, creating the illusion of a sudden increase in diversification rates. Even if diversification rates stayed normal overall, this effect could still appear in surviving lineages.
Possible causes
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Okay, I need to rewrite the given text at a 6th-grade reading level. Let me start by understanding the original content. The text discusses the Cambrian Explosion, explaining possible reasons for the rapid evolution of complex life forms during that period. The reasons include environmental factors like oxygen levels, developmental changes involving genes, and ecological interactions like predator-prey relationships.
First, I should simplify the vocabulary. Words like "triploblastic bilaterians" can be replaced with "complex animals" since the original says "moderately complex animals (triploblastic bilaterians)" and the user wants to keep factual content. I need to avoid technical terms unless necessary, but the original uses some, so I have to keep them but explain them in simpler terms if possible.
Next, sentence structure. The original has long, complex sentences. I need to break them into shorter ones. For example, the sentence about Earth's earliest atmosphere can be split into two or three sentences for clarity.
I must ensure that all factual information is retained. The user emphasized not adding opinions or new information. So, I have to be careful not to paraphrase in a way that changes the meaning. For instance, when talking about oxygen levels and their effect on eukaryotes, I need to keep the timeline and the correlation with diversity.
Also, the user wants the tone to be formal and educational, not too casual. So, avoid contractions like "it's" and use full terms. However, the reading level is 6th grade, so the language should be simple but still formal.
Looking at specific parts: the section on ozone and UV radiation. The original mentions wavelengths, which might be too technical. Maybe replace "200 to 300 nanometers" with "a certain range of light" unless the exact numbers are crucial. But the original includes them, so I should keep them but explain in simpler terms if possible.
Another part is about Hox genes. The original says they control which organs develop. I can simplify that to "genes that help decide which body parts develop in certain areas."
Check for any jargon. Terms like "biomineralization" might need simplification. Maybe "forming hard parts like shells" instead of "biomineralization."
Also, ensure that all the categories (environmental, developmental, ecological) are clearly mentioned and explained in simpler terms.
I need to make sure that each paragraph is concise and flows logically. For example, the part about the Ediacara biota reaching meters in length before the Cambrian explosion can be rephrased to "some large, simple animals existed before the Cambrian period."
Avoiding complex terms like "orogen" unless necessary. The original mentions "orogen" as the roots of the mountain in East Africa. Maybe replace with "mountain range" or "geological formation."
Finally, check that all the key points are covered: oxygen levels, ozone layer, glaciations, calcium and phosphorus levels, Hox genes, horizontal gene transfer, and ecological interactions. Each of these should be mentioned in simpler language without omitting any facts.
I think that's a good start. Now, I'll go through each paragraph and rewrite them step by step, ensuring clarity and simplicity while maintaining the original facts.
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Despite evidence that complex animals existed before the Cambrian period, the rate of evolutionary change during the early Cambrian was unusually fast. Possible reasons for this include environmental, developmental, and ecological changes. Any explanation must account for both the timing and the large number of new life forms that appeared.
Earth's earliest atmosphere had no free oxygen (O₂). The oxygen we breathe today, found in air and water, was created over billions of years through photosynthesis by cyanobacteria. At first, oxygen levels in the atmosphere did not rise much because it quickly reacted with iron and other minerals in rocks and ocean water. Once these reactions reached their limit, oxygen began to build up as a gas. Over time, oxygen levels in the atmosphere gradually increased.
Oxygen levels are linked to the diversity of eukaryotes (a group of complex cells) long before the Cambrian period. The last common ancestor of all living eukaryotes is believed to have lived about 1.8 billion years ago. Around 800 million years ago, the variety and complexity of eukaryote species in the fossil record increased significantly. Before this, eukaryotes lived in environments with high sulfur levels. Sulfur can interfere with how cells use oxygen, limiting energy production. As sulfur levels dropped around 800 million years ago, oxygen became more available, supporting greater diversity. Sponges, which had already evolved, may have helped increase oxygen levels in the ocean, contributing to the Cambrian explosion. Evidence from molybdenum isotopes shows that rising oxygen levels in the ocean were closely tied to increased biodiversity during the Early Cambrian.
Low oxygen levels may have limited the growth of large, complex animals. The amount of oxygen an animal can absorb depends on the surface area of its oxygen-absorbing organs (like lungs or gills), while the amount of oxygen needed depends on its body volume. As an animal grows larger, its volume increases faster than its surface area, making it harder to get enough oxygen. However, some large, simple animals from the Ediacara biota existed millions of years before the Cambrian period. Other processes, such as building tissues like collagen or hard exoskeletons, may also require oxygen. Yet, similar low-oxygen conditions in later periods did not prevent the evolution of complex life, leading some scientists to question oxygen's role.
Ozone (O₃), which protects Earth from harmful ultraviolet (UV) radiation (light with wavelengths between 200 and 300 nanometers), is thought to have formed around the time of the Cambrian explosion. This ozone layer may have allowed life to develop on land, in addition to water.
During the late Neoproterozoic era (ending in the Ediacaran period), Earth experienced extreme cold, with most of its surface covered in ice. This may have caused a mass extinction, reducing genetic diversity and leading to a later diversification of life, such as the Ediacara biota. However, these ice ages occurred long before the Cambrian period, and it is unclear how they could have caused such a rapid increase in diversity. Glaciers may have also released nutrients into the ocean, creating conditions that supported the Cambrian explosion.
Recent research suggests that volcanic activity along mid-ocean ridges caused a sudden increase in ocean calcium levels, enabling marine animals to form hard parts like shells. Another possibility is that erosion from mountain ranges, such as the Transgondwanan Supermountain, released calcium into the ocean. Evidence of this mountain range is found in East Africa today.
Phosphorus levels in the Ediacaran oceans were much lower than in later periods, which may have delayed the development of organisms with phosphorus-based shells.
In 2014, studies using boron isotopes found that ocean alkalinity increased before the Cambrian period. This likely made it easier for organisms to form calcium carbonate, helping them develop hard parts like shells and exoskeletons.
Some theories suggest that small changes in how animals develop from embryos to adults could lead to large differences in their final forms. Genes called Hox genes control which body parts develop in specific areas. For example, one Hox gene might cause a body part to become a limb, while a different gene in the same area might cause it to become an eye. This system allows for a wide range of body types from a limited set of genes. However, it is unclear why the development system itself would cause such a rapid increase in diversity. Evidence shows that many of the genetic tools needed for the Cambrian explosion were already present before the Cambrian period.
A theory suggests that the physics of development changed when simple multicellular life first appeared. This created new conditions where physical processes could interact with genes that had previously been used by single-celled organisms. This may have led to the development of complex body structures like layers, segments, and appendages through self-organization.
Another possibility is that horizontal gene transfer—where genes are shared between unrelated species—helped organisms quickly gain the ability to form hard parts, such as shells. Evidence suggests that a key gene for this process was originally transferred from bacteria to sponges.
Some theories focus on interactions between different species. For example, changes in the food chain, predator-prey competition, or coevolution (where species influence each other’s evolution) may have driven the rapid increase in diversity and complexity during the Cambrian period. These theories explain why diversity and variety increased so quickly, but they do not fully explain why the Cambrian explosion happened when it did.
Relationship with the Great Ordovician Biodiversification Event
After a mass extinction at the boundary between the Cambrian and Ordovician periods, a new diversification of life forms occurred. This event, called the Great Ordovician Biodiversification Event (GOBE), is often viewed as a continuation of the Cambrian explosion. However, recent research suggests that the Cambrian explosion and the GOBE may not have been two separate events, but rather part of one long period of evolutionary change. Analysis of data from the Geobiodiversity Database (GBDB) and Paleobiology Database (PBDB) did not find strong evidence to support the idea that these two events were distinct.
Some scientists have proposed that a time called the Furongian Gap, which may have occurred between the Cambrian explosion and the GOBE, was a period with lower biodiversity. However, studies of fossil sites in South China, such as the Guole Konservat-Lagerstätte, show that the Furongian period was instead marked by rapid changes in life forms. These findings have made the idea of a Furongian Gap highly debated among researchers.
Uniqueness of the early Cambrian biodiversification
The "Cambrian explosion" can be seen as two stages in which animals expanded into new environments: first, an increase in variety as animals explored the Ediacaran seafloor, and later, a second increase in the early Cambrian as they moved into the water column. The speed of new species forming during the Cambrian period was greater than any other time for marine animals. This event affected all animal groups for which Cambrian fossils have been found. Later events, such as the diversification of fish during the Silurian and Devonian periods, involved fewer types of animals and mostly similar body structures. Although the recovery from the Permian-Triassic extinction began with a similar number of animal species as the Cambrian explosion, this recovery created far fewer completely new types of animals.
The event that caused the early Cambrian diversification created many new ecological niches that had not been available before. Once these niches were filled, there was little room for such widespread diversification to happen again, because competition became strong in all environments and existing species had an advantage. If many niches had remained empty, different groups of animals would have continued to change and become distinct enough for scientists to recognize them as separate phyla. When niches are filled, species tend to stay similar to each other even after they evolve separately, because there are fewer chances for them to change their lifestyles or body forms.
There were two similar events in the evolution of land plants: after a long but hidden history beginning about 450 million years ago, land plants quickly diversified during the Devonian period, about 400 million years ago. Additionally, flowering plants (angiosperms) appeared and rapidly spread during the Cretaceous period.