In a famous book called « Wonderful life », Stephen Jay Gould explored how, some 500 million years from now, the development of the arthropods body plan has gone through an incredible variety of shapes and functional appendices in a relatively short span of time. This Cambrian explosion, illustrated through arthropod fossils, is a milestone in Gould’s punctuated equilibrium theory of evolution. As if when something new happens in life and there is room for it, this innovation tends to go through all kind of experiments before it stabilizes in a few enduring, dominant or specialized forms. For instance, insects body plan is now massively dominant among terrestrial arthropods and the looser pancrustacean model is dominant at Earth’s scale. Creation as the starting point of evolution: a shocking statement in the face of classical gradualism.
Now imagine that, in a much older time, something similar happened with unicellular organisms in the field of symbiotic association. An explosion of symbioses. Of course, we don’t have many fossils to rely on for supporting this hypothesis. But we have DNA and protozoan examples. From DNA, we know that the mitochondria, our respiratory organelles, but also chloroplasts, responsible for photosynthesis in vegetal cells, are “tamed” symbionts that our eukaryotic ancestors acquired some 2 to 1 billion years ago. And from existing protozoans, we know that unicellular eukaryotes are experts in building vital symbioses mostly with bacteria, who provide them with energy that they metabolize using various sources in their environment, such as light and sugar of course, but also organic acids, sulphide and others. And primary metabolism is not the only benefit that protozoans obtain from their bacterial symbionts, they also acquire protection against grazing and parasites, through a wide variety of metabolites, namely antibiotic substances (or “bio-controlling” molecules, to put it more generally).
Whether the raising of symbiosis model and their subsequent stabilization is a homolog to the Precambrian explosion is something that should be discussed and a theory that may have many flaws. To begin with, the “explosion” may be a wrong label for something that would have happen over a rather long timespan. However, it makes little doubt that symbioses played a key role in the evolution and endurance of eukaryotic cells and that a lot of them have been experimented long before plants and metazoans develop on their own and already ancient endosymbioses. Hence another interesting question: that of the emergence of multicellularity as a stabilisation in intercellular signalling and recognition, first experimented within and between species.