There are many ways to play the game of life. The goals of this game are simple and Darwinian: survive and reproduce. The stakes are high: failure to adapt means death and extinction. A dizzying number of diverse species has emerged, each representing different bodily forms and strategies for survival and reproduction. But every player of the game, whether bacteria, insect, reptile, or mammal, needs to follow the rules.
Unfortunately, we do not have such a rulebook.
Professor Chris Kempes, a physical biologist at the Santa Fe Institute, is working to decipher the rules life must follow, which he hopes will allow us to figure out other mysteries of how life might exist (if at all) elsewhere in the universe and how we could detect them.
Nothing in life is free, including living. All living things require energy to continue to do so. The larger the organism, the more energy it needs to fuel itself in its metabolism. Could this physiological need of all life point to a universal biological rule?
To see if this could be the case, Dr. Kempes and his team examined the energy requirements of different microorganisms and how large they could be. For single-celled bacteria, among the simplest forms of life we know of, the larger they were, the more energy was devoted to growth and faster they grew. But there are trade-offs in energy for growth and maintenance. When Dr. Kempes produced a model of the data which predicted that there is a lower limit to the size of bacteria – a theoretical smallest possible bacteria. This was because at this theoretical small size, all the energy the bacteria used would need to be devoted to maintaining its current size instead of growth. In fact, in 2015, another group of researchers discovered a new bacteria species which fit the smallest size predicted in Dr. Kempe’s model.
There is also a largest possible bacterium, which interestingly is around the size of the smallest single-celled eukaryotes, another type of microorganism. There thus appeared to be a transition where bacteria could not get larger, and only eukaryotes could exist at that size. The structural difference between bacteria and single-celled eukaryotes might underlie their different size constraints. Single-celled eukaryotes are also made of one cell like the bacteria, but had additional complexity: they have more sacs of membranes within the cell which added more structure and hierarchies of function within the cell. Bacteria only have one sac for all their genetic material and ribosomes, which are the molecular machinery for building proteins, which in turn are the building blocks for cellular structure.
Biological structure is important because living things are also physical things with physical constraints. The contents of a cell, for example, cannot be bigger than the cell trying to contain it. Using experimental data measuring the volume of cellular content and the size of cells, Dr. Kempe found that there was a linear correlation between the size of bacteria and the volume of its genetic material was linear. However, ribosomes take up increasingly large proportions of the cell as bacterial size increased, until a point where they could no longer physically fit – the upper limit of bacterial size.
While Dr. Kempe’s research was only conducted in the smallest of living things, the results provide hints of the types of limitations on living things defined by energy needs and structural constraints. As the biologist Thomas Huxley observed more than two centuries ago: “Unity of plan everywhere lies hidden under the mask of diversity of structure – the complex is everywhere evolved out of the simple.” Uncovering rules of biology is a step towards simplifying the tenets of biology responsible for life’s diversity, potentially everywhere.