Multicellularity is a form of life where an organism consists of more than one cell. All species of animals, land plants, and most fungi are multicellular, an arrangement that allows for the specialization of different cells to perform specific functions. The evolution of organisms composed of many cells from single-celled ancestors stands as a major transition in the history of life, fundamentally altering Earth’s biodiversity.
The Unicellular World
For the first few billion years of life on Earth, the planet was inhabited exclusively by single-celled organisms like prokaryotes and the first eukaryotes. The earliest evidence of primitive multicellularity is from cyanobacteria-like organisms that existed 3 to 3.5 billion years ago. This long period of unicellular dominance took place in an environment with an atmosphere largely devoid of free oxygen.
Around 2.4 billion years ago, the Great Oxidation Event began, triggered by the photosynthetic activity of cyanobacteria. This event increased oxygen concentrations in the atmosphere and shallow oceans. While toxic to many anaerobic organisms, this shift also created the environmental pressures and opportunities for more complex life to evolve.
The Evolutionary Leap
The shift to a multicellular existence was driven by selective advantages. One primary pressure was predation; in a world of single-celled predators, being bigger made an organism more difficult to consume. Experiments with green algae have shown that they can evolve to form multi-celled colonies that are invulnerable to attack due to their increased size.
Another advantage was efficiency in resource acquisition. Groups of cells could anchor themselves in nutrient-rich locations, outcompeting smaller, free-floating organisms and providing a more reliable source of sustenance.
This grouping of cells also opened the door for functional specialization. When cells live together, they can divide labor, with different cells taking on distinct roles. This division of labor is a foundational step toward the development of specialized tissues and organs.
Essential Biological Innovations
For single cells to successfully form a cooperative, multicellular organism, a specific set of biological tools had to evolve. These innovations allowed cells to stick together, communicate, and take on specialized roles.
- Cell adhesion molecules evolved to act as a form of molecular glue. These proteins, such as cadherins in animals, allow cells to bind to one another and form stable, cohesive groups instead of drifting apart after division.
- Cell-to-cell communication systems were required to coordinate activities. Signaling pathways enabled cells to send and receive messages, allowing them to act as a unified entity and coordinate everything from metabolic activity to collective movement.
- Cell differentiation, or specialization, was another major innovation. This process allows a single organism to develop different cell types with distinct jobs, like nutrient absorption or reproduction, which dramatically increases the organism’s overall efficiency.
- Programmed cell death, also known as apoptosis, is the ability for cells to die in a controlled manner. Apoptosis is used to sculpt tissues during development, eliminate damaged or infected cells, and maintain a balance of cell populations.
The genes for many of these systems, including communication and apoptosis, have been identified in the unicellular relatives of animals, indicating their ancient origins.
Pathways to Complexity
The most widely accepted model for the transition to multicellular life is the Colonial Theory. This theory suggests that multicellular organisms arose from colonies of genetically identical single-celled organisms. Initially, these cells would have come together in a simple group, but over evolutionary time, they became more cooperative and interdependent.
An excellent living example is found in choanoflagellates, the closest known single-celled relatives of animals. These aquatic microbes can live as individuals, but many species form simple colonies when dividing cells fail to separate completely.
Within these simple colonies, the first steps toward specialization can be observed. Studies show that some cells may have better access to nutrients based on their position, creating selective pressure for cells to adopt different roles. Over vast timescales, this process of increasing cooperation and specialization could lead to an integrated, complex multicellular organism.
Evidence in the Tree of Life
Multicellularity is not a single event but a strategy that life has adopted on multiple, independent occasions. The eukaryotic tree of life reveals that complex multicellularity evolved at least six separate times: in animals, fungi (twice), brown algae, red algae, and green algae (which gave rise to land plants). This independent emergence demonstrates that it is a successful evolutionary solution to certain environmental challenges.
The fossil record provides tangible proof of this ancient transition. Among the earliest potential examples are coiled filaments called Grypania, found in rocks dated to around 2.1 billion years ago. The oldest undisputed fossil of a complex multicellular organism is a red alga, Bangiomorpha pubescens, from rocks dated to about 1.2 billion years ago. Bangiomorpha is significant because it shows clear evidence of specialized cells, including a holdfast structure for attaching to a surface and reproductive spores, making it the oldest known organism to exhibit such complexity.