German biologist Ernst Haeckel proposed the colonial theory of multicellular life in 1874. He suggested that ancestral metazoans or earlier animals originated from the symbiosis of many organisms of the same unicellular species—as opposed to the symbiotic theory, which suggests symbiosis of different unicellular species resulted in multicellularity.
The colonial theory cited the symbiotic relationship of an ancient species of unicellular flagellate protozoan as an example. This species could live on its own. It could also form a colony with other similar species through a symbiotic relationship. Over time, each cell within the colony became more specialised in structure and functions before gradually losing their individuality. The colony eventually transformed into a multicellular organism.
Several studies and scientific positions have echoed support for the colonial theory of multicellular life. Researchers have in fact used several models of unicellular species to explore and understand how a colony of single-celled organisms transitioned into a multicellular organism.
Choanoflagellates as model for colonial theory of multicellular life
Choanoflagellates are regarded as a suitable model for reconstructing the last unicellular ancestor of animals according to a review study and protocol report by Nicole King et al. This group of free-living and colonial eukaryotic unicellular organism is the closest living relative of animals. Studying the cell biology and genomes of choanoflagellates can provide new insights into metazoan ancestry and origins.
Another study by N. King with S. R. Fairclough and M. J. Dayel used the colony-forming choanoflagellate S. rosetta to understand the cell biology of colony formation and its potential to the evolution of animal multicellularity. The study revealed that rosetta colony develops through cellular division rather than cellular aggregation. In other words, the formation of a rosetta colony is dependent on cell proliferation rather than the aggregation or clumping of similar rosetta species in the immediate surroundings.
One of the implications of the aforementioned rosetta colony study is that it supports the Haeckelian view that a colony of unicellular organism transitioned to a single multicellular organism through repeated cell division. Furthermore, it is also consistent with the hypothesis that the last common ancestor of animals and choanoflagellates was capable of simple multicellularity.
The study of Anderson et al also used choanoflagellate as a model to determine the mutations that led to the evolution from unicellular to multicellular animal life. Using a technique called ancestral protein reconstruction to move backward in the evolutionary tree and trace the genetic changes that prompted single-celled organisms to evolve a protein that is critical to multicellular animal life. The technique took the researchers to 600 million years ago. They found that a single albeit random mutation resulted in the emergence of a protein that allowed an ancient unicellular organism to with other similar organism to form a colony.
A version of the aforesaid mutation now exists in all animals according to Anderson et al. They noted that this mutation has equipped animal cells with the ability to coordinate and how they divide relative to the position of their neighbours to form and maintain organised tissues.
Volvocine algae as model for colonial theory of multicellular life
The volvocine green algae or volvox is another model used for studying the colonial theory of multicellular organism. This group of algae are flagellated photosynthetic organisms that range from unicellular and multicellular forms with no cell differentiation or incomplete differentiation to multicellular forms with complete separation.
Researchers M. D. Herren et al sequenced the DNA of about 45 different species of volvox to reconstruct their family tree and determine how long ago the first colonial ancestor emerged. Results revealed that the ancestors of this group of algae diverged from unicellular and non-colony forming ancestors at least 200 million years ago or during the Triassic period. It took these algae 35 million years to complete the transition.
Throughout the course of the transition, the researchers noted that the volvox formed a multicellular colony while undergoing the cycle of cooperation, conflict, and conflict mediation. Since it diverged from its multicellular ancestors, volvox has evolved into a highly integrated multicellular organism with cellular specialization, a complex developmental program, and a high degree of coordination among cells.
What is interesting to note about this multicellular colony of volvox is that the entire constituents are not a ere cluster of cells with similar functions. Herren et al put emphasis on the fact that there is a division of labour within the colony. Some cells do the reproduction while others are involved either in producing a binding substance called extracellular matrix or in swimming and overall survival.
The study of Yuki Arakaki et al also noted that the Gonium pectorale and Volvox carteri species of volvocine generally have several common morphological features to survive as integrated multicellular organisms. These include having a rotation asymmetry of cells that enables each separate constituent to become components of an entirety or individual and cytoplasmic bridges between protoplasts in developing embryos that maintain the species-specific form of the multicellular individual before secretion of new extracellular matrix.
Further details of the colonial theory of Ernst Haeckel are discussed in his book “The History of Creation or the Development of the Earth and its Inhabitants by the Action of Natural Causes” published in 1880 in New York under Appleton & Co.
Further details of the review study of Nicole King et al are in the article “The choanoflagellates: Heterotrophic nanoflagellates and sister group of the metazoa” published in 2009 in the journal Cold Spring Harbor Protocol. Details of the study of King, Fairclough, and Dayel are in the article “Multicellular development in a choanoflagellate” published in 2010 in the journal Current Biology. Details of the study of Anderson et al are in the article “Evolution of an ancient protein function involved in organised multicellularity in animals” published in January 2016 in the journal eLife.
Further details of the study of Herren et al are in the article “Triassic origin and early radiation of multicellular volvocine algae” published in 2009 in the journal Proceedings of the National Academy of Sciences. Details of the study of Arakaki et al are in the article “The simplest integrated multicellular organism unveiled” published in 2013 in the journal PLoS One.