Multicellularity

Multicellular organisms are those consisting of more than one cell. Two kingdoms of living organisms are strictly multicellular—the plants (Plantae) and animals (Animalia). In the kingdom fungi (Fungi), simple and complex multicellularity is generally observed, but because a few organisms in this kingdom, like yeast, are unicellular and because their evolutionary history differs from plants and animals, some scientists consider fungi clonally multicellular organisms. Two other kingdoms consist of single-celled organisms—the bacteria (Monera), which have primitive prokaryotic cells, and the protists (Protista), which have complex eukaryotic cells. Prokaryotic cells lack a nucleus and other internal cell structures and are found today only among bacteria. Eukaryotic cells, on the other hand, contain a nucleus, other complex internal cell structures called organelles (such as mitochondria, which perform respiratory functions), and sometimes chloroplasts (which contain chlorophyll and perform photosynthesis). All multicellular organisms are composed of eukaryotic cells, so the eukaryotic cell must have evolved before multicellular organisms could develop. It is generally accepted that simpler types of organisms evolved first, followed by more complex organisms.

The Multicellular Kingdoms

Plants are multicellular organisms that have chlorophyll (a green pigment used for photosynthesis), plastids (internal structures on the cell that contain chlorophyll), and a cell wall that contains cellulose. Plants are sometimes called “primary producers” because they can manufacture their own food from carbon dioxide and water through a process called photosynthesis, using sunlight for energy and producing oxygen and organic matter (carbohydrates) as by-products.

Animals are multicellular organisms that cannot produce their own food and must feed on other organisms. They are “consumers.” Metazoans have many types of cells, which are organized into tissues and groups of tissues, which form organs. There are two primary embryonic tissue layers present in all metazoans except the sponges. These are the ectoderm (outer layer) and the endoderm (inner layer). More advanced metazoans also have a third embryonic cell layer, the mesoderm, which lies between the other two layers.

Fungi (mushrooms and their relatives) possess cell walls like plants, but unlike plants, they lack chlorophyll. Although fungi appear plantlike, they cannot produce their own food because of the absence of chlorophyll, so they must feed by ingesting dead organic material or other organisms, making them consumers. Because they are neither plants nor animals, the fungi are placed in a separate kingdom (Fungi).

Advantages and Origins of Multicellularity

Multicellularity probably evolved because it gave organisms some sort of advantage, assuring them of a greater chance of survival. Multicellularity allows organisms to become larger (which helps them to outcompete other organisms and provides a greater internal physiological stability), to have a longer life (because individual cells are replaceable), to produce more offspring (because many cells can be dedicated to reproduction), and to have a variety of body plans (which permits adaptation to various modes of life or environmental conditions). Specialization of cells for particular functions allows organisms to become more efficient.

Evidence for the origin of multicellularity comes from the fossil record, studies of the organization and biochemistry of living cells and organisms, and from studies of the embryonic and larval stages of animals. During the Archean eon (between 3.8 and 2.5 billion years ago), only single-celled, prokaryotic life (bacteria-like organisms) existed on Earth. Some of the prokaryotes were photosynthetic, including the cyanobacteria (or so-called blue-green algae). Some of these prokaryotes were colonial, with cells organized into structures, such as chains, or filaments, or algal mats. These colonies differ from true multicellular organisms because they generally consist of only one type of cell rather than many types of cells. Colonies of blue-green algae formed moundlike structures called stromatolites, which were quite common during the Precambrian but are less common in the twenty-first century. Stromatolites appeared about three billion years ago but did not become abundant until about 2.3 billion years ago.

The Rise of Eukaryotic Organisms

Eukaryotic organisms appeared during the Proterozoic era. The eukaryotic cell probably evolved from prokaryotic ancestors some time before about 1.4 billion years ago. The oldest convincing fossils of eukaryotic cells are generally considered to be those from the 1.3-billion-year-old Beck Spring dolomite of California. Eukaryotic fossil cells have also been found in chert from the approximately 850-million-year-old Bitter Springs formation of Australia. The earliest eukaryotes were animal-like protozoans. This evolution occurred when photosynthetic prokaryotic cyanobacteria were ingested by protozoans and then developed a symbiotic (mutually beneficial) relationship with them. The evolution of the plantlike eukaryotes probably occurred at least 1.4 billion years ago. (This date has been suggested because primitive multicellular algae fossils are present in rocks 1.3 billion years old.)

The eukaryotic cell was a prerequisite for the development of multicellular organisms. The plantlike eukaryotes are considered ancestral to the multicellular algae and higher plants. The protozoans are considered the ancestors of the metazoans (animals). The first multicellular organisms may have been algae. Fossils that appear to be primitive multicellular algae are known from the 1.3-billion-year-old sedimentary rocks of the Belt supergroup of Montana, and the 800-million- to 900-million-year-old Little Dal group of northwestern Canada. Multicellular algae can be found living today in both freshwater and marine environments.

Fossil fungi first appear in the fossil record around one billion years ago. Researchers discovered microscopic fungi fossils (named Ourasphaira giraldae) in an estuary in the Canadian Arctic in 2019, which scientists assert is likely the oldest known fossil of its type. Analysis of its chemical makeup found chitin, confirming its classification. Other examples in this fossil record include filamentous fungi found in the Democratic Republic of the Congo that scientists dated to 715 million years ago. A 410-million-year-old fossil in Rhynie cherts in Scotland provided evidence of a historical plant-fungi relationship. The fossil record of fungi is lacking, but twenty-first-century findings have expanded its collection.

The Rise of Multicellular Animals

Multicellular animals evolved independently of multicellular plants, probably arising from protozoan ancestors. The oldest evidence of metazoans (multicellular animals) in the geologic record is in the form of trace fossils. Trace fossils are imprints, such as tracks, trails, or burrows made in sediment by moving animals. Over time, the sediment hardened into sedimentary rock as a result of compaction and cementation. The earliest trace fossils consist of simple trails and tubelike burrows. In some places, there is a succession of types of trace fossils, from simple tubelike burrows in older rocks to more complex structures in younger rocks. This change suggests that the evolution and diversification of increasingly complex burrowing organisms occurred during the latter part of the Precambrian. The oldest trace fossils are less than one billion years old, and many scientists believe that it is unlikely that any trace fossils exist in rocks much older than about 700 million years old. The trace fossils appear in the geologic record just before the first appearance of soft-bodied metazoan fossils. Structures that resemble trace fossils, however, have been reported from much older rocks. Among these questionable traces are the one-billion-year-old Brooksella, which resembles a jellyfish and are sometimes called star-cobbles. However, research emerged in 2023 that supported their classification as a pseudofossil rather than a trace fossil. In addition, tubelike structures from the upper Medicine Peak quartzite in Wyoming have been dated at 2 billion to 2.5 billion years, at least 1 billion years older than the oldest known metazoans. The origin of these older traces is uncertain and may be a result of inorganic processes (such as dewatering of sediment), rather than of organisms.

One possible fossil metazoan that appears to be more than 850 million years old has been reported from the Tindir group of Alaska. This fossil is less than one millimeter long and appears to be a flatworm (phylum Platyhelminthes). Both the age and identification of this fossil have been disputed, but if valid, it is a very important find because some biologists theorize that the earliest metazoans would have been primitive flatworms.

The oldest unquestioned metazoan fossils are the imprints of a diverse assemblage of relatively well-developed, soft-bodied marine animals. More than half of the organisms appear to be some type of cnidarian or coelenterate (related to jellyfishes), about 25 percent appear to be segmented worms (related to annelids), and a small percentage appear to be arthropods (related to insects, crabs, and lobsters). Trace fossils are also present. This assemblage of soft-bodied fossils is called the Ediacaran fauna. It was discovered in 1946 in sandstones of the Pound subgroup in the Ediacara Hills of the Flinders Range in South Australia. The exact age of the Ediacaran fauna is uncertain because there are no nearby rocks of the proper type for radiometric dating. The Ediacaran fauna is clearly Precambrian, however, judging from its position in the geologic sequence. The soft-bodied Ediacaran fossils are separated from the younger fossil shells of the Cambrian (570 million years old) by a thick section of unfossiliferous rock (up to several hundred meters thick). Since the 1950s, fossils similar to the Ediacaran fauna have been found in rocks of approximately the same age on virtually every continent on Earth (with the possible exception of Antarctica). In some of these other areas, it is possible to date radiometrically the rocks associated with the fauna. These radiometric dates indicate that the early metazoan soft-bodied fossils range from about 620 million to 700 million years old. It is likely that the metazoans evolved some time prior to 700 million years ago because these fossils represent well-developed, complex animals.

The oldest known skeletonized fauna (animals with shells or other hard parts), Coronacollina acula, appeared around 550 million years ago. The skeletonized faunas are represented by microscopic scraps, cones, tubes, and plates made of calcium phosphate or a hard organic material called chitin. It is not known exactly what types of organisms produced these skeletal remains, but the tiny fossils are so diverse and complex that it is assumed that the organisms must have had a long history of evolution during the Precambrian. The origin of skeletons was advantageous to marine organisms because hard parts provide protection against predators as well as the mechanical functions of support and muscle attachment.

Theories of the Origin of Multicellular Life

There are a number of theories that explain the origin of multicellular life. Most of the theories are derived from studies of various types of cells and living organisms, including advanced protozoans, early developmental stages (embryos), and larval stages. Four types of cells are central to these theories, and they are grouped into two categories: motile (capable of movement) and nonmotile (not capable of movement). The motile protists include flagellate cells (those with a whiplike “tail,” or flagellum) and amoeboid cells (those such as Amoeba, which move by pseudopodia, or fingerlike extensions of the cell membrane). The nonmotile stages include coccine cells (those with many nuclei, sometimes called multinucleate cells) and sporine cells (those that divide and stick together to form multicellular aggregates).

There are many theories that have been proposed to explain the origin of plants, fungi, and metazoans (animals). Formerly, it was thought that plants evolved from prokaryotic algae (cyanobacteria, or blue-green algae), but it is more likely that plants arose from a eukaryotic ancestor, such as a flagellate cell. Flagellate algae are similar to flagellate protozoans, but it is not certain whether the algae evolved from the protozoan or vice versa. The presence of plastids (such as the chloroplasts that contain chlorophyll used for photosynthesis) may be the key feature separating plants from protozoans, fungi, and animals. According to a theory proposed by Lynn Margulis, plastids evolved from prokaryotic blue-green algae that were captured by eukaryotic cells. The sporine cell is another possible ancestor of the plants. Sporine cells appear to have had the capacity to evolve beyond the colony level and to produce complex tissue-level green algae and higher plants.

Fungi used to be considered plants that had lost (or never evolved) chlorophyll. The discovery of a single-celled stage with flagella among the more primitive fungi, however, suggests that fungi probably evolved from protozoans. Scientists assert that around 1 billion years ago, animals and fungi evolved from an organism with flagella while plants evolved from a single-celled prokaryote.

The ancestral multicellular organisms, which gave rise to all the more complex living animals, are all extinct. The simplest multicellular animal living today is the sponge (phylum Porifera). The sponges are not considered to be ancestors of the more complex animals because their body organization and developmental history are very different. Sponges have no tissues, mouth, or internal organs. Instead, they consist of an aggregate of flagellate and amoeboid cells (and a few other types) roughly arranged in layers. The sponges may have evolved independently from the other metazoans. Sponges are classified as a distinct side branch of the animal kingdom (Parazoa), with a primitive multicellular grade of organization (no tissues). The remaining multicellular animals are grouped into the Eumetazoa.

Theories of Metazoan Origin

Several theories have been proposed to explain the origin of the metazoans. These theories can be placed into the following categories: evolution from single-celled protozoans, evolution from colonial protozoans, evolution from multinucleate coccine cells as a result of the development of internal cell boundaries, and evolution from sporine cells. There are several versions of each of these theories, and there is no general agreement on which theory is best. Some researchers promote the colonial theory as the most widely accepted theory, whereas others claim no longer to take it seriously. Most experts agree that evolution of metazoans from colonial protozoans would seem to be easier than evolution directly from a single cell. Multicellularity may have arisen independently several times in several different ways.

The colonial theory suggests that the metazoans evolved from flagellate or amoeboid protozoans that lived together in colonies, much like the modern green alga Volvox, which is shaped like a hollow sphere. From an original hollow spherical form, the shape of the ancestral metazoan changed as an indentation or invagination formed in the side. The indentation became larger, producing a double-walled “cup” (envision pushing one’s thumb into the side of a deflated ball until that side becomes nested into the other side of the ball, forming a cuplike shape). The double-walled cup shape is referred to as a diploblastic body plan, meaning two layers of body tissue. These two layers are the ectoderm (outer layer) and the endoderm (inner layer). This process of indentation to produce a diploblastic (double-walled) form occurs in the embryos of many animals. The jellyfish are a good example of animals with a diploblastic body plan. Nearly all groups of animals have ectoderm and endoderm (except the sponges), suggesting that nearly all groups of animals are related. Because the jellyfish (phylum Cnidaria) have the simplest body plan, they are believed to be the most primitive. The diploblastic ancestral form has been called a gastrea. Ernst H. Haeckel, a prominent nineteenth-century German biologist who studied animal embryos, believed that all bilaterally symmetrical animals evolved from a gastrea.

A second theory for the origin of metazoans suggests that the ancestral form was a bilaterally symmetrical animal resembling a flatworm. Some scientists believe that the complex organs and organ systems of metazoans are beyond the evolutionary potential of flagellate and amoeboid cells. The flatworm may have evolved from “cellularization” of a multinucleate coccine cell (formation of cell membranes around each of the nuclei) or from clumping of sporine cells. Most of the cells in the metazoans are sporine cells that stick together to form multicellular aggregates. Sporine protozoans do not exist, so it is hypothesized that sporine ancestors of the metazoans must have evolved from “preprotozoans.” These hypothetical ancestors may have been solid balls of cells resembling the early stages of many embryos. At some point, the exterior cells may have developed (or redeveloped) flagellae and become specialized for locomotion, and the interior cells may have become specialized for digestion and reproduction. Such colonies of cells would have resembled the larval (immature) form of cnidarians, called a planula larva, and, hence, they are called planuloids. Planuloids are believed to have given rise to two groups of metazoans: the cnidarians (jellyfishes and their kin) and the flatworms. Primitive flatworms are believed to have been ancestral to all other bilaterally symmetrical metazoans.

Evidence from Rocks

Theories to explain the origin of multicellular life have been developed by biologists as a result of studies of various types of cells and living organisms, including advanced protozoans, early developmental stages (embryos), and larval stages.

Geologists (scientists who study rocks) and paleontologists (scientists who study fossils) use a variety of techniques that they use to search for evidence of life in Precambrian rocks (older than 570 million years). These include searching for fossil remains and chemical analysis of organic residues that are probably the breakdown products of once-living organisms.

The first step in the search for Precambrian life is to locate rocks of the proper age. Geologic maps exist for virtually all parts of the world. From an examination of these maps, it is possible to identify areas that contain rocks of the proper age. (The age of a rock is determined by radiometric dating.) Age, however, is not the only consideration. For fossil remains to be preserved, the rocks must also remain little altered from the way they were originally deposited. Metamorphism (geologic alteration caused by heat and/or pressure) has deformed many Precambrian rocks to the extent that any fossils that may have been present can no longer be recognized.

Assuming that undeformed rocks of the proper age can be located, the search begins for fossil remains. However, most Precambrian rocks are not fossiliferous. Precambrian multicellular fossils are found in only a few places in the world. In Australia, soft-bodied Precambrian metazoan fossils are restricted to a few thin layers of sandstone in a sequence of Precambrian rock more than one thousand meters thick. In most places in the world, however, there is a thick section of unfossiliferous rock separating the Precambrian metazoan fossils from the shelly faunas in the Cambrian rocks. This unfossiliferous sequence of rock is an interval for which there is little or no information on the types of life that exist.

Before multicellular organisms appeared (prior to perhaps one billion years ago), only microscopic, single-celled organisms existed on Earth. Microscopic fossils of single-celled organisms are found by careful examination of fine-grained, dark-colored rocks, such as black cherts. The black color of the rocks commonly indicates the presence of carbon, which is present in all living organisms and may be preserved in some fossils. Very thin slices of rock are prepared and mounted on glass slides so that the organic matter can be studied. These slices of rock, called thin sections, are so thin that light can pass through them, and they are examined with a microscope. Much of the carbon in these rocks is present as amorphous (indistinct or shapeless) patches, but in some places, microscopic structures appear to be the fossilized remains of single-celled organisms. Pieces of rock can also be prepared for examination using a scanning electron microscope. The search for microfossils is difficult and painstaking. Among the problems involved are the possibility of contamination by modern-day organic matter in the laboratory and the possibility that the microscopic structures may really be inorganic in origin.

Chemical tests are used to search for the products of biological activity, which may be preserved in rocks. In principle, rocks that have been influenced by biological activity should contain certain characteristic isotopic ratios. There are a number of problems inherent in searching for organic residues. Organic material may have been preserved in the rock, but it could easily have been altered subsequently by heat and pressure or by circulating fluids. In addition, circulating fluids can contaminate the rocks by introducing organic material from much younger rocks.

Multicellularity in the Evolutionary Process

Studying the origin of multicellularity helps one to understand the conditions that led to the evolution of plant and animal life on Earth. As one begins to understand how multicellular life evolved, one may begin to wonder about why it was such a slow process. It is known that Earth formed about 4.6 billion years ago, the first cells appeared about 3.5 billion years ago, but the first multicellular life did not appear until approximately 1 billion years ago. In other words, it took more than 3.5 billion years for multicellular life to develop. More than three-quarters of Earth’s history had passed before multicellular life ever appeared.

One may also wonder about the conditions that promoted the origin of multicellular life. Of all the planets in the solar system, Earth seems uniquely suited to life. Two of the most important factors involved are the presence of liquid water (which requires a specific temperature range) and the presence of an oxygen-rich atmosphere. None of the other planets in this solar system has either of these two characteristics. The Earth originally did not have liquid water or an oxygenated atmosphere. Geologic evidence suggests that Earth’s early atmosphere was the result of volcanic outgassing and that it consisted of gases, such as carbon dioxide, carbon monoxide, ammonia, methane, hydrogen sulfide, nitrogen, and water vapor. As the planet cooled from its original molten state, the water vapor in the atmosphere condensed to form liquid water, which fell to Earth as rain and accumulated to form the oceans, rivers, and lakes. There is abundant geologic evidence that Earth’s early atmosphere lacked the free oxygen that is breathed today. In the absence of free oxygen, chemical evolution in the oceans or lakes led to the formation of organic compounds, or what has been called the “primordial soup.” The first living cells, the prokaryotes, evolved in this organics-rich water. As time passed, some of the early prokaryotic cells became photosynthetic, which allowed them not only to produce their own food from water and carbon dioxide but also to produce oxygen as a waste product. Oxygen was toxic to these early organisms. To survive, the cells had to develop a mechanism to adapt to the presence of increasing levels of oxygen. The buildup of oxygen led to the development of the ozone layer in the atmosphere and to the appearance of the eukaryotic cell. As the percentage of oxygen in the atmosphere increased, it is believed that some threshold level was reached, and it became possible for the environment to support multicellular organisms. That allowed a rapid diversification of life on Earth.

Hence, it appears that multicellular life on Earth appeared as a result of some prehistoric accident that resulted in global atmospheric change—the buildup of a toxic waste product (oxygen) as a result of photosynthesis by early life-forms. One might also speculate on the possible global effects of the increasing waste products that humans are now producing. The thinning of the atmospheric ozone layer is but one manifestation of the way that life is presently changing Earth’s fragile environment. Knowing that the formation of the ozone layer was probably essential to the appearance of multicellular life on Earth, it is alarming to speculate on the consequences of its destruction. Life as humans know it depends on an earth with environmental conditions in a precarious balance.

Principal Terms

Ediacarian (Ediacaran) Fauna: A diverse assemblage of fossils of soft-bodied animals that represents the oldest record of multicellular animal life on Earth

Eukaryotic Cell: A cell that has a nucleus with chromosomes and other complex internal structures; this is the type of cell which makes up all organisms except bacteria

Fossils: The remains of ancient life preserved in sediment or rock

Multicellular Organisms: Organisms consisting of more than one cell; there are diverse types of cells, specialized for different functions and generally organized into tissues and organs

Precambrian Eon: The earliest chapter of Earth’s history, covering the time interval between the formation of Earth, about 4.6 billion years ago, and the beginning of the Cambrian period, about 570 million years ago

Prokaryotic Cell: A primitive cell that lacks a nucleus, chromosomes, and other well-defined internal cellular structures; only members of the kingdom Monera (such as bacteria) are prokaryotic cells—all higher organisms have eukaryotic cells

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