Reproductive “fruiting bodies” formed by a Slime Bacteria (Myxococcus xanthus)
The Rise of Multicellular Organisms
(Essay #16, at 6-24-25)
Note: In this Essay, we used Wikipedia and Perlexity.ai for our research. We did not include traditional numbered footnotes at the bottom of this Essay. Readers can find the footnoted sources within Wikipedia’s articles or within Perplexity.ai’s question-and-answer sections, all of which are available online. — JXM
While many parts and events are missing to form the complete story of abiogenesis (the origin of life on Earth), we have a more complete understanding of the story of the evolution of multicellular organisms.
Here are 6 key information sources that Perplexity.ai shared about the evolution of multicellularity:
- Grosberg, R. K., & Strathmann, R. R. (2007). “The Evolution of Multicellularity: A Minor Major Transition?,” Annual Review of Ecology, Evolution, & Systematics, 38, 621-654.
- Herron, M. D. (2016). “Origins of Multicellular Complexity: Volvox and the Volvocine Algae,” Molecular Ecology, 25(6), 1213-1223.
- Ratcliff, W. C., Denison, R. F., Borrello, M., & Travisano, M. (2012). “Experimental Evolution of Multicellularity,” Proceedings of the National Academy of Sciences, 109(5), 1595-1600.
- Brunet, T., & King, N. (2017). “The Origin of Animal Multicellularity and Cell Differentiation,” Developmental Cell, 43(2), 124-140.
- Lyons, N. A., & Kolter, R. (2015). “On the Evolution of Bacterial Multicellularity,” Current Opinion in Microbiology, 24, 21-28.
- Bonner, J. T. (1998). “The Origins of Multicellularity,” Integrative Biology: Issues, News, and Reviews, 1(1), 27-36.
The above sources can provide detailed information on the environmental factors, protective strategies, and evolutionary processes involved in the rise of multicellularity.
Cyanobacteria
Cyanobacteria, also known as blue-green bacteria and blue-green algae, are a phylum of bacteria that obtain their energy through the process of photosynthesis. The ability of cyanobacteria to perform oxygen-producing photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to the endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors through endosymbiosis, a process in which one single-celled organism engulfs another single-celled organism of a different species. (See LibreTexts: Biology, 8.9A: Cyanobacteria.)
Microscopic photo of Cylindrospermum, a filament-shaped species of cyanobacteria.
Some Very Early Cyanobacteria Lived in Colonies
Many ancient cyanobacteria lived in colonies, long before the evolution of plants. Their colonial lifestyles provided ancient cyanobacteria with various advantages, including protection and improved resource sharing, allowing them to thrive in diverse environments and play a crucial role in shaping Earth’s early ecosystems and atmosphere. Evidence from fossil records and scientific studies shows that these microorganisms formed various types of colonial structures listed below: (perplexity)
Stromatolite mineral formations on a beach in Australia.
Stromatolites: Ancient cyanobacteria created layered sedimentary structures called stromatolites by trapping sediment and precipitating minerals. These structures date back to the Early Archaean period, over 3 billion years ago.
Microfossils: Fossils of colonial cyanobacteria have been discovered in Precambrian rocks, including examples from the Bitter Springs chert in Australia, dating back approximately 850 million years.
Planktonic colonies: Ancient cyanobacteria, similar to modern Microcystis, formed colonies ranging from 0.1 to over 3.3 mm in diameter. These colonies can be subspherical, elliptical, or have more complex shapes, such as peanut, reniform, or treelike structures.
Terrestrial colonies: Recent studies have identified fossils of cyanobacteria that colonized land as early as 407 million years ago. For example, Langiella scourfieldii formed colonies in soils, freshwater, and hot springs during the Early Devonian period.
Today, the size of cyanobacteria colonies ranges from just a few cells to several thousand cells, depending on the species. Their colonial lifestyle provides various benefits to cyanobacteria, including protection and improved resource sharing among cells.
The Beautiful, Yellow-Orange Slime Mold
This section about the Yellow-Orange Slime Mold is repeated from our Essay, “Nature Creates Increasingly Complex Systems.” (See that Essay for footnotes.)
The Slime Mold lives part-time as individual, single-celled organisms. But when these separated cells are stressed by a lack of surrounding food, they will join together and function as a single multicellular creature! Then, when plentiful food is found, the cells will disaggregate and resume living independently once again.
The acclaimed science writer Steven Johnson begins his fascinating book Emergence: The Connected Lives of Ants, Brains, Cities, and Computer Software, unfortunately known as the Slime Mold (specifically, the Dictyostelium discoideum). Typically growing on rotting bark and leaves in a forest, this common variety of slime mold looks like a patch of rust-orange lichen. It is actually a simple organism without intelligence or a central nervous system, closely related to the organisms collectively classified as fungi.
Yellow-Orange Slime Mold growing on a forest floor — species: Dictyostelium discoideum.
When food supply, temperature, and humidity are favorable, the Slime Mold lives as separated, individual cells. But when conditions are strained, these cells coalesce to form a single, larger organism. Time-lapse photography shows that the organism then crawls very slowly along the forest floor in a unified search for food. When its environment improves, the cells separate and become solitary foragers once again!
The Slime Mold Can Form a Coordinated Colony called a “Grex.”
A slime mold is not always a “grex,” but it can form one during certain stages of its life cycle. A grex, also called a pseudoplasmodium or, informally, a “slug,” is a specific multicellular stage in the life cycle of some slime molds, particularly the cellular slime molds like our Dictyostelium discoideum. A grex forms when individual amoebae aggregate in response to depleted food supplies. This multicellular assembly has a definite anterior and posterior, can respond to environmental stimuli, and has the ability to migrate. The grex stage is an intermediate step between the individual amoebae and the formation of a fruiting body (sorocarp). It’s important to note that not all slime molds form a grex. Plasmodial slime molds, for example, form a large multinucleate cell called a plasmodium instead of a grex. The grex is characteristic of cellular slime molds, which spend most of their life as individual amoebae and only come together to form the multicellular slug when conditions necessitate.
Scientists have even shown that a unified slime mold patch like ours can locate a food source by extending out “arms” to explore (through trial and error) and find (by a process of elimination) the shortest route through a laboratory maze. At the right time, the grex will also send up “fruiting bodies” containing spores for reproduction.
The orange-colored Slime Mold is an excellent example of a Complex System. It demonstrates the key principle that simple parts interacting together can achieve goal-seeking behavior without any direction from the central control of a brain, or even of a central nervous system. The slime mold does not even have central “pacemaker” cells. Their cooperative behavior simply emerges, as cells pick up and follow scent trails of the pheromone acrasin secreted on the ground by other cells that have already found good food. (The same thing happens with colonies of ants, termites, and bees, but in those cases, the individual creatures each have primitive central nervous systems.)
The formation of the larger, grex slime mold is a prime example of emergence and self-organization in complex systems. It is a “bottom-up” (i.e., not centrally directed) creation of a new and different whole out of the interaction of simpler, identical parts. The self-organization process of the slime mold involves communication (via pheromone trails), just as exploring worker ants communicate as they mark out the way to a good food source for the columns of ants who follow after them.
Note: An excellent, recent short film called “The Insane Biology of: Slime Mold,” produced by Real Science, is now on YouTube. It presents a host of newly discovered capabilities of the Slime Mold.
Multicellularity has evolved independently at least 25 times in eukaryotes (cells that have a nucleus and organelles) and also in some prokaryotes (cells that lack a nucleus or organelles), such as cyanobacteria and myxobacteria. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida (green algae plus land plants), once for animals, once for brown algae, three times in the fungi (in chytrids, ascomycetes, and basidiomycetes), and perhaps several times for slime molds and red algae. The first evidence of multicellular organization, which occurs when unicellular organisms coordinate their behaviors and may be an evolutionary precursor to true multicellularity, comes from cyanobacteria-like organisms that lived 3.0–3.5 billion years ago.
Multicellularity has evolved independently in various species across different lineages of life. Here are some examples:
> Volvocine algae: Multicellularity has evolved at least twice in this group. The genus Volvox and its relatives showcase a range of multicellular complexity, from single cells to colonies with thousands of cells.
> Fungi: Complex multicellularity in the form of fruiting bodies (e.g., mushrooms) has evolved independently on approximately 12 separate occasions in fungi.
> Algae: Red, brown, and green algae have each independently evolved their own multicellular forms over the past billion years.
> Bacteria and Archaea: Some species in these domains have developed simple forms of multicellularity, although each cell typically functions as an individual.
> Dictyostelids and Myxobacteria: These organisms exhibit a form of temporary multicellularity triggered by environmental changes.
> Animals and plants: While these groups each made the transition to multicellularity only once, they represent major independent evolutionary events.
> Eukaryotes: Multicellularity has evolved independently in at least 16 different eukaryotic lineages, including animals, plants, and fungi.
These examples demonstrate that the evolution of multicellularity is a recurring phenomenon in the history of life, occurring through various mechanisms and in response to different selective pressures across diverse taxonomic groups.
Why Did Multicellularity Evolve?
Several factors may have contributed to the evolution of multicellularity:
- Predation: Cells may have evolved aggregation as a defense mechanism against predators.
- Functional specialization: Group living enabled the division of tasks among cells, thereby increasing overall efficiency.
- Environmental cue integration: Multicellular organisms could better process and respond to environmental signals. [perplexity]
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Chart showing the position of Fungi on the Tree of Life
Theories of Multicellular Origins
> Aggregation Theory: This hypothesis suggests that function-specific cells aggregated into a slug-like mass called a grex, similar to what slime molds do today.
> Coenocytic Theory: A primitive cell may have undergone nuclear division without cell division, forming a coenocyte, which later developed membranes around each nucleus. A coenocyte is a body of algal or fungal cytoplasm containing several nuclei, enclosed in a single membrane. A coenocyte has multiple nuclei in a single mass of protoplasm. This happens when nuclei divide without the division of the cytoplasm of the original cell, as seen in many fungi.
> Incomplete Division Theory: As unicellular organisms divide, daughter cells may have failed to separate completely, resulting in a conglomeration of identical cells.
> Symbiotic Theory: Different species of single-cell organisms may have formed symbiotic relationships, eventually leading to the incorporation of their genomes into one multicellular organism.
> Endosymbiotic Theory: This symbiosis entailed the complete engulfment of one organism by a larger “host” organism of a different species. (“endo” means “taking in.”)
Colonial Theory: Symbiosis among organisms of the same species may have contributed to the evolution of multicellular organisms.
An Important Example of the Endosymbiotic Theory:
Mitochondria have a matrix & external membranes, like the free-living (proteobacteria from which they likely derived.
Mitochondria were most likely engulfed by ancient cells in a process known as endosymbiosis. This event occurred approximately 1.45 billion years ago when a larger cell engulfed a smaller, free-living bacterium. The engulfed bacterium, likely a purple non-sulphur bacterium, eventually evolved into what we now know as mitochondria. The endosymbiotic theory, which explains this process, suggests that a proto-eukaryotic cell engulfed a proto-mitochondrion, which then evolved into an organelle within the larger host organism. This event marked a significant milestone in the development of fully formed eukaryotic cells. The integration of mitochondria into host cells was a gradual process that occurred over millions of years. It involved the development of complex protein-import machinery and the insertion of protein carriers into the inner membranes of the engulfed bacteria. This allowed the host cell to extract energy from the endosymbiont, creating a mutually beneficial relationship. Evidence supporting this theory includes:
- Even today, Mitochondria contain their own DNA and reproduce independently within the host cell.
- Mitochondrial DNA shares a structural resemblance with bacterial DNA.
- Mitochondria have proteins in their outer membranes that are similar to those found in bacteria.
- Cardiolipin is found in both mitochondrial and bacterial cell membranes.
This endosymbiotic event was crucial in the evolution of full-fledged eukaryotic cells and has significantly influenced our understanding of eukaryote origins.
Here are two other examples of endosymbiosis, these from the lyrical prose of D. Lewis Thomas, author of “Lives of A Cell: Notes of a Biology Watcher:”
“…green plants could not be green plants, or green, without their chloroplasts, which run the photosynthetic enterprise and generate oxygen for the rest of us. As it turns out, chloroplasts are also separate creatures with their own genome, speaking their own language. And we carry stores of DNA in our nuclei that may have come in, at one time or another, from the fusion of ancestral cells and the linking of ancestral organisms in symbiosis.”
(Bantam Books, 1974, pp. 2-3.)
Reasons (Pressures) For Evolving Multicellularity
The rise of multicellularity was a complex process driven by various environmental and cellular factors, occurring multiple times throughout Earth’s history. This transition laid the foundation for the incredible diversity of life we observe today. [perplexity/]
> Multicellularity provided better protection from predators and infections: Interior cells were shielded by the outer cells.
> Multicellular aggregates evolved because they performed certain functions more efficiently than single cells. For instance, in chemotaxis (the movement of cells in response to chemical stimuli), groups of cells are better at following chemical gradients than individual cells.
> Multicellularity enabled the evolution of specialized cells for various functions within an organism. Humans today have very different, but cooperating nerve cells, muscle cells, light-sensitive cells, odor-sensitive cells, blood cells, bone cells, and many others.
> Multicellularity was an early step in Nature’s Continuing Creation of Biological Complexity.
> Predation: Multicellular aggregates may have evolved as a response to predation.
> Chemotaxis: Multicellular groups perform movement toward food & water more efficiently than single cells, providing a selective advantage.
> Periodic environmental changes can influence the evolution of cellular adhesion.
> The “Snowball Earth Hypothesis:” The Cryogenian period, known as “Snowball Earth,” may have been a catalyst for the evolution of complex multicellular life. The period between the Sturtian and Marinoan glaciations allowed planktonic algae to dominate the seas, paving the way for rapid diversification of both plant and animal lineages.
> Multicellular Magnetotactic Prokaryotes (MMPs). MMPs are a unique class of bacteria that exhibit multicellularity with no observed unicellular stage. They are able to orient and move in response to magnetic fields.
> Mutations in the ace2 gene, preventing daughter cell separation after division, drove multicellularity in multiple fungal lineages that diverged 300mm years ago.
> Simple multicellular structures evolved in response to predation after approximately 750 generations in experimental settings.
Recent Experimental Insights
Recent experiments have shown that multicellularity can evolve relatively quickly under laboratory conditions. This demonstrates the potential rapidity of such evolutionary transitions.
> Unicellular yeasts were induced to grow into large multicellular clusters over the course of nearly two years, forming branching structures visible to the naked eye.
> In Schizosaccharomyces pombe yeast, multicellular clusters with a cube-shaped organization rapidly evolved, displaying key hallmarks of an evolutionary transition in individuality.
> “A newly discovered bacterium, the “HS-3,” forms layer-structured colonies with liquid crystal properties.
> “Evolved multicellular clusters:” Experiments with Escherichia coli have shown that bacteria can evolve to form macroscopic multicellular clusters through genetic assimilation.
These examples demonstrate that multicellularity in bacteria is diverse and can arise through various mechanisms, including clonal growth, aggregation, and evolutionary adaptations to environmental pressures. Multicellularity can evolve relatively quickly under certain selective pressures, providing valuable insights into this major evolutionary transition.
Mechanics of Achieving Multicellularity
Multicellularity evolved independently in at least 16 different eukaryotic lineages, including animals, plants, and fungi, over 600 million years ago.
This transition from unicellular to multicellular life occurred through two main mechanisms:
- Clonal development: Multicellularity arises through serial cell division without separation of sister cells.
- Aggregation: Separate cells converge and adhere to each other.
Aspen Trees Grow in Colonies – but Share ONE Root System
A Colorado Aspen Colony in the Autumn
Most Aspen trees grow in large “clonal colonies.” All the trees in such a colony are genetically identical, derived from a single seedling, and spread by means of root suckers. New tree stems (tree trunks) in the colony may appear up to (100–130 ft) away from the parent tree. Each individual tree can live for 40–150 years above ground, but the shared root system of the colony is very long-lived. In some cases, the root system can live for thousands of years, sending up new trunks as the older ones die off above ground. These may not seem to be colonies of cells… but all the trees do share a single root system… so isn’t an Aspen Grove ONE organism?
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J.X. Mason. ContinuingCreation.org