Massive outsourcing of energetically costly amino acids at the origin of animals

Animals are generally capable of synthesizing eleven amino acids, while the remaining nine, often referred to as essential, have to be acquired through diet. This has a profound impact on animals by defining their ecological lifestyle and evolutionary trajectory. Recent phylogenomic studies reveal that this phenotype is a result of gene losses that occurred at the root of animal tree. However, it remains unclear which selective forces, if any, were directing this far-reaching metabolic simplification event. Here, we show that the essential amino acids are energetically far more expensive to synthesize than non-essential ones and that animals, by outsourcing their production, significantly removed constraints on their usage. This suggests that the selective pressures on energy management drove this metabolic outsourcing, which in turn allowed animal genes to evolve more freely through protein sequence space.