Single-celled organisms dominated life on Earth for billions of years. The evolution to multicellular life however changed the entire landscape because it resulted in the emergence of organisms characterised by biological complexities and enormous physical appearance relative to their unicellular counterparts. Furthermore, the appearance of multicellular organisms resulted in the emergence of a new biological environment with complex ecological interactions.
Multicellularity emerged from the evolution of unicellular organisms and the emergence of the capacity of cells to specialise and arrange themselves into tissues and organs. There are many theories explaining the mechanisms by which multicellularity could have evolved. Examples of these are the symbiotic theory and the colonial theory.
Definition of multicellularity
Multicellularity is simply defined as a condition or state of having composed of more than one cell performing different functions and having different characteristics. A more specific definition however states that multicellularity is the differentiation and spatial arrangement of cell types into functioning tissues within an integrated organism.
The condition gives rise to increasing levels of complexity that results in differentiation of cellular functions and development of tissues and organs with specialised functions within the overall organism.
Central to multicellularity is the existence of several unique mechanisms that determines cellular functions necessary for tissue organisation. There are four broad processes resulting to the emergence of a multicellular organism. These are cell proliferation, cell specialisation, cell interaction, and cell movement.
In cell proliferation, a single cell divides into two leading to exponential division. Cells with different functions or characteristics subsequently emerge at specified locations. This is cell specialisation. Coordination between neighbouring cells with similar functions transpire in the process called cell interaction. In cell movement, the cells arrange themselves to form structured tissues and organs.
Advantages of multicellular organisms
There are several advantages of multicellularity. As the exponential growth in the number of cells result in bigger size beyond the surface-to-volume limits set for a single cell. Take note of the fact that most multicellular life are macroscopic in contrast to most unicellular organisms.
Because multicellularity permits a larger increase in the size of an organism, it also permits the development of higher-order structures and tissues with complex functions.
There is also a considerable effect on relative fitness due to size. Large organisms have better protection against predation. They also have better mobility that allows migration to a more favourable environment.
Another advantage of multicellularity is longer life span. A multicellular organism will continue to exist even if a single cell dies or becomes damaged. DNA duplication is also an advantage. The proliferation of cells within an organism allows faster growth and biological repair mechanisms.
The same DNA duplication allows semiconservative replication in which half of the DNA is retained while a new one is formed during cell division. From an evolutionary standpoint, this allows favourable traits to continue in the next generations while also allowing the introduction of more favourable traits resulting from adaptation. In other words, semiconservative replication allows for the conservation and adaptation of traits that would improve the survival or fitness of the organism.
Disadvantages of multicellular organisms
There are also disadvantages to multicellularity. The resulting complex organism is susceptible to varied diseases arising from the complexity of the biological composition and functions. In contrast, unicellular organisms lacked complex organ systems. This means that there are very few things can go wrong with them.
It is important to mention that unlike multicellular organisms, multicellular organisms have the ability to reproduce asexually. This gives them the advantage of not having to waste resources and energy trying to find a mate and engaging in sexual activities.
Unicellular organisms also have the ability to take energy by diffusion or osmosis. This removes the need to become mobile to look for food. Almost anything can be a potential food source for a unicellular organism.