Cambrian diversification of life

The abrupt appearance of a great variety of animal fossils about 544 million years ago and about three billion years after the origin of life is termed the Cambrian diversification. Study of this spectacular development provides insights into the processes of evolution.

Appearance of Complex Organisms

Although the remains of microscopic organisms are known from rocks more than three billion years old, the remains of complex animals that lived before 680 million years ago have not been discovered, while plants and animals with preserved hard parts did not become established until about 539 million years ago. The abrupt appearance of a great variety of animal fossils at the end of the Precambrian (the period ranging from 4.6 billion years ago to the start of the Cambrian period, 539 million years ago) is termed the Cambrian diversification and was spectacular radiation heralding the start of the Phanerozoic (the period ranging from the Cambrian to the Quaternary period). Notably, the date ranges for the Cambrian period have changed several times in the twenty-first century, and these dates should be considered accordingly. By the middle of the Early Cambrian time, most of the major invertebrate animal groups had appeared in the oceans, and this rapid appearance is a striking feature of the fossil record. The diversification has been documented, showing a period of about eighty million years during which the variety of life increased exponentially, suggesting that each group originated by the simple splitting into two of an ancestral group. Although there is fossil evidence of early metazoans (complex animals), it is sparse, and the reconstruction of the events during which simpler animal groups gave rise to more advanced animal groups is based primarily on comparative anatomy and embryology of modern forms.

The earliest record of metazoans occurs in the Ediacaran intervala stratigraphic unit deposited in the last 100 million years of the Proterozoic. Before that, there is only evidence of unicellular organisms. Initially, these were prokaryotic (single cells lacking nuclei), but about 1.75 billion years ago, the first eukaryotic organisms (cells with a nucleus) developed. The earliest metazoans were probably loose aggregates of eukaryotic cells that were not differentiated by function. However, the organisms found in the Ediacaran interval are already more complex. The fauna was first described from the Pound quartzite in the Flinders Ranges of South Australia. These fossils are impressions, left as the animals were stranded on mud flats and subsequently covered by sand, and they are interpreted as having been made by metazoans lacking hard parts. Many of the impressions are circular in outline with concentric or radial striations and have been considered jellyfish, though none can be tied with confidence to living organisms. Additionally, there are large (up to 1-meter-long), elongate, leaf-shaped forms considered related to modern sea pens, which are frondlike representatives of one of the coral groups. Other impressions are regarded as having been made by polychaete worms (bristle worms), while much rarer are impressions of supposed arthropods and animals of uncertain affinities.

Whether these impressions should be interpreted in terms of living jellyfish, corals, and worms has been questioned, but they are evidence of the existence of early metazoans. This fauna significantly predates the earliest Cambrian fauna and is now known from five continents. The presence of the same or similar forms as far apart as South Australia and northern Russia is striking evidence of the widespread distribution of this early shallow-marine fauna. In this fauna, scientists can glimpse a fleeting stage in metazoan diversification, a stage that may have given rise to the more diverse and abundant invertebrates of the Cambrian and later periods but that ultimately became extinct before the end of the Proterozoic.

Trace fossils also provide evidence of the early development of metazoans and can be particularly valuable, as impressions left by animals in sediment may be preserved even when the animals themselves are not. Trace fossils do not become abundant or diverse until near the Precambrian/Cambrian boundary, and most of these early traces probably resulted from the burrowing activity of soft-bodied infaunal worms. Trace fossils take the presumed origin of mobile metazoans further back into the Precambrian than do the Ediacara fauna, but only to about 100 million years before the start of the Cambrian. As the Ediacara fauna indicates that many major groups had already developed, the initial radiation of Metazoa took place earlier. The study of the relationships of modern metazoan groups can help shed light on the course of this diversification.

Levels of Organization

Several grades or levels of organization are recognized in living metazoans. The simplest forms are ones in which the cells are not separated by function. Some modern sponges can serve as examples of this grade. Most simple metazoans have cells separated by function, and the simplest grade is represented by the Cnidaria (corals and jellyfish), in which the body wall is separated into two layers: the outer ectoderm and the inner endoderm. The next grade, which includes all higher organisms, consists of forms in which the body wall is in three layers, with the mesoderm being sandwiched between the ectoderm and endoderm. In more advanced invertebrate animals, the mesoderm forms a lining to the ectoderm and overlies a fluid-filled cavity termed the coelom, which may have originally functioned as a hydrostatic skeleton but in higher organisms has also been used as space for the placement of internal organs.

The coelomates (organisms with a coelom) can be divided into several major groups based on embryology, indicating the early subdivisions of the basic stock. These include a lophophorate group, encompassing Brachiopoda and Bryozoa, in which a food-gathering apparatus termed a lophophore is present; a large group including both Echinodermata and Chordata (the group that includes vertebrates); and another group that includes Arthropoda and Annelida (segmented worms). The larvae of marine annelids are very similar to those of Mollusca, which suggests that the mollusks may have split off early from the main line of descent of segmented animals. Although the time of origin of these groups is unknown, it can be concluded that coelomates were present in the Late Precambrian and that the origination of arthropods was closer to the age of the Ediacara fauna than that of the annelids or the cnidarians that form most of the population.

The Ediacara fauna vanished from the sedimentary record before the end of the Precambrian, although the presence of trace fossils indicates that metazoans were still present. The base of the Cambrian is recognized as the appearance of fossils of animals that secreted a skeleton, and this basal stage of the Cambrian, the Tommotian stage, contains a diverse fauna of small shelly organisms, many tubelike and composed of calcium phosphate. This fauna also includes sponges, brachiopods, gastropods, primitive mollusks, and hyolithids (conical shells usually considered mollusks, though their soft parts cannot be reconstructed with any degree of confidence). The best-known sequences through the Precambrian-Cambrian boundary are in Siberia, where sedimentation occurred without a major interruption through this period. In these sequences, small shelly fossils occur only rarely in the Precambrian, although they are abundant in the Tommotian, pointing to an explosive development of life in the earliest Cambrian. All modern phyla (except the Bryozoa) have a fossil record that starts in the Cambrian.

After the Tommotian stage, several groups of larger animals with hard skeletons appear. They are characteristic of later Cambrian faunas, appear very abruptly, and are fully organized and differentiated on their first appearance. One of the most conspicuous groups is the trilobites, many-legged arthropods that crawl along the sea floor, often forming conspicuous trace fossils. Brachiopods, two-valved filter-feeding organisms, are also common, and echinoderms were represented by a remarkable variety of types. Although the animals with hard skeletons were very diverse, there is also evidence that the soft-bodied faunas were equally diverse.

One indication of this diversity is provided by the fauna preserved in the Middle Cambrian Burgess Shale of British Columbia and the slightly older Chengjian fauna from China. Some Cambrian species have also been discovered in Greenland. The black shale of the Burgess Shale accumulated in an oxygen-free environment, which prevented the destruction of the animals that were washed with it. Most organisms in this fauna are nontrilobite arthropods, but there are also numerous polychaete and priapulid worms, which were already highly diversified and which might have caused the burrows found in rocks of the latest Precambrian age. One of the earliest known worm-like chordate species, Pikaia gracilens, found in this region has a rod-like primitive backbone. In addition, there are many animals of unknown affinity, often forms found only in the Burgess Shale. This fauna indicates the presence of complex communities composed of highly organized animals. The diversity in the range of feeding adaptations, for example, is quite as varied as that found in modern animals. This complex fauna is quite unlike that of the earliest Cambrian or the Ediacaran and demonstrates the explosiveness of the early radiation.

Diversification Theories

Several explanations for the sudden diversification in the Cambrian have been put forward. Environmental factors were certainly very important, and one suggestion is that the diversification may have been related to an increase in suitable environments at the end of the Precambrian. That would have been caused partly by the breakup of continental areas, which would have provided a greater extent of coastline and, therefore, a rapid increase in the availability of habitats for the shallow-marine organisms. In addition, the termination of an extensive period of glaciation that took place at the end of the Precambrian would have resulted in the flooding of coastal areas, providing increased of shallow-marine environments. The end of the glacial period would also have resulted in a warming trend that would have opened up new marine environments and possibly helped trigger the expansion of marine diversity.

It has also been suggested that an increase in oxygen levels may have contributed to the sudden development of complex metazoans. Large and thin animals such as those present in the Ediacara fauna may have been adapted to respire by diffusion in oxygen concentrations as low as 8 percent of present levels. Oxygen levels of up to 10 to 15 percent of present levels may have been present in the earliest Cambrian, which may have been sufficient to allow the increased diversification of invertebrate organisms.

Further evidence for oxygen levels is also available in the appearance of hard skeletons at the base of the Cambrian. External skeletons are useful protection, suggesting their development in response to predation. In addition, they are useful as solid surfaces for the attachment of muscles and as supports to lift organisms above the bottom. Calcium and phosphate ions are both essential to the processes within metazoan cells, and it may be that skeletal parts originated as reserves of these materials. Calcium phosphate is a hard tissue commonly found in the earliest Cambrian faunas. Organisms using calcium carbonate did not become common until the end of the Cambrian.

At present, an atmospheric oxygen level of at least 16 percent is required before marine organisms can secrete calcium carbonate skeletons, suggesting that increased oxygen levels in the Cambrian may have contributed to the development of diverse organisms at that time. It appears, therefore, that various environmental factors—including an increase of shallow-marine areas, a warming trend, and higher oxygen levels—may have contributed to the explosive Cambrian diversification of invertebrate organisms.

Study of Fossils

The evidence for the early evolution of complex organisms rests on the fossils present in the sediments. Studies of these remains give scientists insights into how early life developed. However, preserving organisms is an unusual occurrence and particularly rare when the organisms are completely soft-bodied. Preservation potential is enhanced by the presence of some hard parts, like a skeleton or shell, that resist erosion and decay. The potential for preservation can be enhanced if the organism lives in an environment where sedimentation is taking place, thus improving the chances of incorporation in the sediment. Because of these constraints on preservation, scientific knowledge of shallow-marine faunas through time is much greater than knowledge of faunas in other environments.

The earliest metazoan faunas are those from the Ediacaran. These organisms were soft-bodied, and their preservation is an extremely uncommon event. The fossils consist of natural molds and casts formed when the animals were covered by drifting sands in nearshore environments. Scientists can study the specimens directly, as they are normally clearly exposed on the bedding planes of the sediments when the rocks are split. In some cases where only molds are present, a latex cast may be made to facilitate detailed study. Preserving the early Cambrian fauna in the Burgess Shale is rather different, and a greater variety of techniques can be used in its study. The specimens are compressed into thin films of carbon and are exposed on the bedding planes of the shale when it is split. In many cases, the splitting results in breakage through the specimen, so parts of the original adhere to two separate rock slabs (termed the part and counterpart).

Such compression means that in complex animals such as arthropods, delicate preparation may be necessary to remove surfaces so that other surfaces and structures may be examined. Such preparation is conducted under a binocular microscope, and small engraving tools and needles are used to remove the matrix. Interpretation of the specimens involves producing drawings and photographs, and the Burgess Shale material provides special problems here, as the specimens are preserved as black carbon films on a black shale. Ultraviolet light may enhance the reflectivity of the specimens relative to the surrounding matrix. They may also be photographed under a liquid (water or ethyl alcohol), as this process will also enhance differences between the specimen and the matrix. Drawings are normally made using a camera-lucida attachment on a binocular microscope. This attachment allows the scientist to look through the microscope to see both the specimen and their drawing hand and pencil, thus enabling the tracing of the specimen to produce an accurate illustration.

The study of data as a whole requires analyzing large amounts of information so evolutionary trends can be assessed and periods of rapid diversification or higher-than-normal extinction can be recognized. These studies depend on the original description and identification of the fossils, but manipulation of the data is accomplished by computers.

Principal Terms

Coelomate: an organism possessing an internal cavity termed the coelom

Eukaryote: a cell that has a nucleus surrounded by a well-defined membrane; the type of cell present in metazoans

Metazoan: a grade of organization of living organisms in which the cells are specialized for various functions and cooperate for the good of the whole organism

Phanerozoic: the period from 544 million years ago to the present, during which sediments accumulated containing obvious and abundant remains of animals and plants

Precambrian: the period that includes nearly 90 percent of geologic time, ranging from 4.6 billion years ago, when the Earth formed, to 544 million years ago, when the Cambrian period started

Trace fossils: traces of animal activity, such as burrows or trackways, preserved in the sediment

Bibliography

Benton, M. J. Extinctions: How Life Survives, Adapts and Evolves. Thames & Hudson, 2023.

"Cambrian Period." National Geographic, www.nationalgeographic.com/science/article/cambrian. Accessed 20 July 2024.

Davis, Josh. "The Cambrian Explosion was Far Shorter than We Thought." The Natural History Museum, 2019, www.nhm.ac.uk/discover/news/2019/february/the-cambrian-explosion-was-far-shorter-than-thought.html. Accessed 20 July 2024.

Glaessner, Martin F. The Dawn of Animal Life: A Biohistorical Study. re-issued, Cambridge University Press, 2010.

Gould, Stephen Jay. Wonderful Life: The Burgess Shale and the Nature of History. Norton, 2007.

Hou, Xian-Guang, et al. The Cambrian Fossils of Chengjiang, China. 2nd ed., Blackwell Publishing, 2017.

Prothero, Donald R. Bringing Fossils to Life. 3rd ed. McGraw-Hill, 2013.

Stearn, C. W., and R. L. Carroll. Paleontology: The Record of Life. reprint. John Wiley & Sons, 2008.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed., Brooks/Cole, Cengage Learning, 2016.

Zhang, Xingliang, and Degan Shu. "Current Understanding on the Cambrian Explosion: Questions and Answers." PalZ, vol. 95, no. 4, 2021, pp. 641-660. doi.org/10.1007/s12542-021-00568-5.