By Catherine Belzung

In the life sciences, systems can be described according various epistemological levels, from macroscopic to nanoscopic scales: the scale of the ecosystem, the one of an individual, that of organs, of tissues, of cells, of molecules, and so on. However, whatever the level, one can observe that these systems are generally composed of dissimilar elements that cooperate among them, favoring the complexity and the survival of the whole, and avoiding fragmentation.

For example, one can describe a biological system at the level of the ecosystem, i. e., an organized system composed of individuals from various species (animal or vegetal) that interact each with the other and with the environment. These systems are open, meaning that they receive matter and energy from the outside, with the consequence that they are subjected to challenging changes, requiring self-regulation. Each species has a precise function, complementary to the function of other species: plants can transform energy from the sun into chlorophyll but cannot move while animals can move but cannot synthetize chlorophyll. Because both collaborate, the system increases its fitness. The same diversity is found among animals and plants. For example, some plants are able to protect others from dehydration (Turner et al., 1966), which is possible only because they are different. And this heterogeneity favors the adaptation of the whole system.

Similar cooperation is found at the level of organs. For example, vertebrates are composed of various organs, such as a heart, a stomach, a brain: each has a different function that enables the survival of the whole. It is evident that none of them would be able to survive alone, and that each of them favors the survival of the other organs. The same rules can be found at the level of a single organ: for example, the brain is composed of various cells, including neurons, but also astrocytes, microglial cells, etc., and each of them is again characterized by a huge diversity: in the human brain, 145 different types of neurons can be found. The impressive capacity of these natural systems derives directly from their diversity: recent research tried to put diverse cells in an artificial dish and new properties emerged that resulted from the cooperation among them (Madhavan et al., 2018).

Finally, the same logic can be found inside cells: each one is composed of a membrane that enables delimitation from the outside, a nucleus (where information is stored), mitochondria (enabling respiration), the endoplasmic reticulum (protein synthesis). One could continue such a description at the molecular level, where each molecule is composed of different atoms, and so on.

All these very general examples illustrate that nature is heterogenous. However, heterogeneity alone is not sufficient to favor survival. It generates new properties only if these diverse parts cooperate with the others: indeed, because it is different, each element can offer something related to its specificity to the others. Of course, this cooperation is not intentional and conscious, but it is nice to change our view of nature, going beyond the current view claiming that everything is based on competition, to contemplate the diversity and mutual help that is present in it.