FIRST MULTICELLULAR ORGANISMS: ORIGIN, CHARACTERISTICS - BIOLOGY - 2025

The first multicellular organisms, according to one of the most accepted hypotheses, began to group in colonies or in symbiotic relationships. As time went by, the interactions between the members of the colony began to be cooperative and beneficial for all.

Gradually, each cell underwent a process of specialization for specific tasks, increasing the degree of dependence on its companions. This phenomenon was crucial in evolution, allowing the existence of complex beings, increasing their size and admitting different organ systems.

Colonial organisms, such as Volvox, allow us to hypothesize about the potential characteristics of ancestral multicellular organisms. Source: Frank Fox

Multicellular organisms are organisms made up of several cells - such as animals, plants, some fungi, etc. Currently there are multiple theories to explain the origin of multicellular beings starting from unicellular life forms that later grouped together.

Why do multicellular organisms exist?

The transition from unicellular to multicellular organisms is one of the most exciting and controversial questions among biologists. However, before discussing the possible scenarios that gave rise to multicellularity, we must ask ourselves why it is necessary or beneficial to be an organism composed of many cells.

Cell size and surface-volume ratio (S / V)

An average cell that is part of the body of a plant or animal measures between 10 and 30 micrometers in diameter. An organism cannot grow in size simply by extending the size of a single cell because of the limitation imposed by the ratio of surface area to volume.

Different gases (such as oxygen and carbon dioxide), ions and other organic molecules must enter and leave the cell, crossing the surface that is delimited by a plasma membrane.

From there it must spread throughout the entire volume of the cell. Thus, the relationship between surface area and volume is lower in large cells, if we compare it with the same parameter in larger cells.

A very large cell has a limited exchange surface

Following this reasoning, we can reach the conclusion that the exchange surface decreases in proportion to the increase in cell size. Let's use a 4 cm cube as an example, with a volume of 64 cm ³ and a surface area of ​​96 cm ². The ratio will be 1.5 / 1.

In contrast, if we take the same cube and divide it into 8 two-centimeter cubes, the ratio will be 3/1.

For this reason, if an organism increases its size, which is beneficial in several aspects, such as in the search for food, locomotion or escaping predators, it is preferable to do so by increasing the number of cells and thus maintaining an adequate surface for the animals. exchange processes.

Advantages of being a multicellular organism

The advantages of being a multicellular organism go beyond the mere increase in size. Multicellularity allowed the increase in biological complexity and the formation of new structures.

This phenomenon allowed the evolution of highly sophisticated cooperation paths and complementary behaviors between the biological entities that make up the system.

Disadvantages of being a multicellular organism

Despite these benefits, we find examples - as in several species of fungi - of the loss of multicellularity, returning to the ancestral condition of single-celled beings.

When cooperative systems fail between cells in the body, negative consequences can result. The most illustrative example is cancer. However, there are multiple avenues that, in most cases, manage to ensure cooperation.

What were the first multicellular organisms?

The beginnings of multicellularity have been traced back to a very remote past, more than 1 billion years ago, according to some authors (eg, Selden & Nudds, 2012).

Because transitional forms have been poorly conserved in the fossil record, little is known about them and their physiology, ecology, and evolution, making the process of constructing a reconstruction of incipient multicellularity difficult.

In fact, it is not known if these first fossils were animals, plants, fungi, or any of these lineages. Fossils are characterized by being flat organisms, with a high surface area / volume.

Evolution of multicellular organisms

As multicellular organisms are composed of several cells, the first step in the evolutionary development of this condition should have been the grouping of cells. This could happen in different ways:

Colonial and symbiotic hypothesis

These two hypotheses propose that the original ancestor of multicellular beings were colonies or unicellular beings that established symbiotic relationships with each other.

It is not yet known whether the aggregate was formed from cells with differential genetic identity (such as a biofilm or biofilm) or from stem and daughter cells - genetically identical. The latter option is more possible, since genetic conflicts of interest are avoided in related cells.

The transition from single-cell beings to multicellular organisms involves several steps. The first is the gradual division of labor within cells that are working together. Some take somatic functions, while others become the reproductive elements.

Thus, each cell becomes more dependent on its neighbors and gains specialization in a particular task. Selection favored organisms that clustered in these early colonies over those that remained solitary.

Nowadays, researchers are looking for the possible conditions that led to the formation of these clusters and the causes that could have led to their favor - compared to unicellular forms. Colonial organisms are used that might be reminiscent of hypothetical ancestral colonies.

Syncytium hypothesis

A syncytium is a cell that contains multiple nuclei. This hypothesis suggests the formation of internal membranes within an ancestral syncytium, allowing the development of multiple compartments within a single cell.

Origin of multicellular organisms

The current evidence points to the fact that the multicellular condition appeared independently in more than 16 lineages of eukaryotes, including animals, plants and fungi.

The application of new technologies such as genomics and the understanding of phylogenetic relationships has allowed us to suggest that multicellularity followed a common trajectory, starting with the co-optation of genes related to adherence. The creation of these channels achieved communication between cells.

References

1. Brunet, T., & King, N. (2017). The Origin of Animal Multicellularity and Cell Differentiation. Developmental cell, 43 (2), 124-140.
2. Curtis, H., & Schnek, A. (2008). Curtis. Biology. Panamerican Medical Ed.
3. Knoll, AH (2011). The multiple origins of complex multicellularity. Annual Review of Earth and Planetary Sciences, 39, 217-239.
4. Michod, RE, Viossat, Y., Solari, CA, Hurand, M., & Nedelcu, AM (2006). Life-history evolution and the origin of multicellularity. Journal of theoretical Biology, 239 (2), 257-272.
5. Ratcliff, WC, Denison, RF, Borrello, M., & Travisano, M. (2012). Experimental evolution of multicellularity. Proceedings of the National Academy of Sciences, 109 (5), 1595-1600.
6. Roze, D., & Michod, RE (2001). Mutation, multilevel selection, and the evolution of propagule size during the origin of multicellularity. The American Naturalist, 158 (6), 638-654.
7. Selden, P., & Nudds, J. (2012). Evolution of fossil ecosystems. CRC Press.