Today I have tried the following; writing non stop from start to finish a popular science article. This is not, I repeat NOT, the way to a good article, as it usually needs some polishing and some criticism and some killing of precious babies. Nonetheless, when the going gets though, the though gets going, here is the result!
During the last stages of the neolithic era – the last part of the Stone age – the gradual transition from the hunter-gatherer way of living to the start of what we now call civilization slowly changed human condition. There was agriculture and domestication of animals and concomitant settlement. There was specialization of labor, urban development and social stratification. The fertile lands around Euphrates and Tigris nourished human society and gave rise to the cradle of western civilization – a complex society where we emphasise our freedom as human beings while at the same time remain dependent on the political stability in our home lands and world wide.
However, this is not a history lesson. This is a tale about how there seems to be a penchant for cooperation among the different groups of life. And we´re not talking social structures. There will be no wolf packs, bee societies nor any examples of tree-loving fungi . Before any of the afore mentioned cooperative species evolved, something more profound happened. Single-celled organisms evolved to become multicellular. This is no cake walk. Because it involves dependency. Becoming multicelluar means identifying yourself as a part of a whole – your individuality is now of lesser importance. In fact, it is not wanted, unless it can contribute to the survival of all the other cells that constitute the multi of the organism. Now, obviously there is no you or me when we talk about single celled organism, just as there are no decision-making involved in the emergence of multi-cellular organisms. But there are advantages; reduction of predation risk and division of labor, as well as a comptetitive advantage of increase in size. Multicellularity is the work – as always – of natural selection. And time and time again, as we will see, multicellularity is selected for. As to the beginning of this text – there is a weak analogy connecting the evolution of multicellularity and the advent of civilization; cooperation and organization pays off.
Multicellularity in a broad sense simply means sticking together. Both bacteria and eukaryotes – cells with a nucleus and other cell organs in its interior – have evolved from unicellularity to multicellularity multiple times. In the eukaryote lineages there are slime molds, colony formation and of course, the plants, the fungi and us; the animals. In the bacterial lineages there are biofilms, filaments and fruiting bodies, some of which will be discussed later.
But let´s start where we all like to start; with ourselves. The animal version of multicellularity is by far the most complex. It is also multicellularity in it´s narrow sense. One cell gives rise to all of the cells in an animal body. A cell community must be able to stay together and also ensure that the right kind of cells remains next to each other. They also need to communicate so that they are all in tune with one another. If the cells loose these two important characteristics, they turn into something well known in our society today: cancer cells! Last, but not least, just as a society needs individuals with different jobs and roles, a multicellular community consists of many different cell types with specific functions. Some constitute the outer layers of a body, some make the hard structures like bone and cartilage or the carpace of crabs, lobsters and shrimps. Some build the blood vessels, other construct internal organs, some sense light, touch, sound and smell, and some make up the muscles and nervous system giving animals the ability to move. And some give rise to the egg cells and the sperm – ensuring that a new generation will develop.
The emergence of different cell types during the developmental stages of an organism is called differentiation and is a result of the genetic developmental program. Almost every cell the body of a multicellular species contain all the heritable information needed to make that organism. Differentiation is simply telling each cell which page in the cookbook, if you like, it should take it´s instructions from.
The animal lineage can trace it´s origin to the unicellular choanozoans. The ancestor of all animals, prior to the transition to a multicellular form must have looked something like a choanoflagellate, a chonozoan. At its rear end it has one flagella, a tail-like structure, surrounded by a collar made up of tiny sticky rods that makes a net used to catch nutrients. The beating flagella propel the choanoflagellate forward. Now, in one of the most basal animal groups, the sessile sponges, the cells that make nutrient-containing sea water flow into the internal canals and cavities of the sponge, have the exact shape and apperance as the choanoflagellates. These cells are appropriately called choanocytes. Their resemblance to the choanoflagellates were first described by James-Clark in 1866. With the computer programs we have today, we can also infer evolutionary relationship by looking at the genes of this species and compare it with the animal lineage. Most interestingly, the choanoflagellates form colonies. The common ancestor of both the choanoflagellates and the animal might have been a colony forming organism.
Sponges – Porifera
The beating flagellum of the choanocytes produces a current that makes water flow into the sponge
Colony forming choanoflagellate: Sphaeroeca
With the introduction of the colony forming choanoflagellates we are entering into multicellularity in it´s broad sense. Now, independently arising cells aggragate to form a structure in which they arrange themselves in a cooperative manner. The eukaryote slime mold aggregate in response to diminished food supply. It can exist as a solitary cell on decaying logs, eating bacteria. When there is no food left, they aggregate and together becomes what is called a migrating slug. When this structure reach an illuminated area , the tens of thousands og slime molds create a fruiting body, containing differentiated slime mold cells. Some make up the spore cells and others, the stalk that elevates the fruiting body. The stalk cells die, but the spore cells disperse and give rise to new myxamoeba.
The Fruiting body of Dictyostelium
Bacterias are usually thought of as the ultimate loners, but there have been a broader recognition of bacterial multicellularity in the microbiology and evolutionary biology communities. A widespread mode of bacterial multicellularity are biofilms: aggregated bacterial cells that together produce and secrete a substance in which they are embedded. Many bacterial species switch between independent life styles and sessile biofilm lifestyles depending on the resources available and other environmental factors. In streptomycetes multicellularity is initiated from a single cell; a spore. As the cell divides, the daughter cells stays connected forming network of threadlike structures called a hyphal network in the soil. A stress signal initiates differentiation and growth of aerial spore bearing fruiting bodies.
The evolution of cooperating lifeforms both in the bacterial and eukaryote lineages raises the question of what the prerequisites for a more advanced form of multicellularity are. Knowledge about bacterial broad sense multicellularity relates the animals to the rest of the three of life and gives our lineage a context in which to understand early animal evolution. How deep is the root of the genes that enables multicellularity? Or have they arisen independently at many time point during evolution? Questions like this tie together the tree of life and awakens the quriosity about what, by many, are thought of as lesser life forms. Research that focuses on the evolution of the genetic programs that increases the complexity of living forms is exciting. Working out the similarities and differences between the animal lineage and it´s closest unicellular and colony forming relatives may give scientist an idea of how and why complex multicelluar organisms appeared.
Gilbert, S. F. 2014. Developmental biology. Tenth edition.
Hickman et al. 2011. McGraw Hill’s Animal Diversity 6th Edition
van Wezel, G.P 2014. Bacterial solutions to multicellularity: a tale of biofilms, filaments and fruiting bodies. Nature Reviews Microbiology.
and some Wikipedia, as always…