Rethinking the Last Universal Common Ancestor of Life: Network Convergence and the Root of the Tree

The tree of life is rooted at the "origin of life." One model holds that core biochemistry, which includes the genetic code, the ribosome, biopolymer backbones, and amino acid and nucleotide monomer alphabets, was inherited vertically from a single origin of life. In this model, core biochemistry is a frozen accident that reflects prebiotic chemistry. In an alternative model explored here, life arose across diverse planetary environments and generated diverse biochemistries that competed and cooperated. These biochemistries converged through selection driven by the "network effect." The network effect conferred greater fitness on participants in increasingly dominant biochemistries: the more extensive the adoption of a biochemistry, the greater the benefits for systems using it. In this model, the evolution of core biochemistry was driven, in part, by compatibility, integration, and coordination. The last universal common ancestor (LUCA) in this model represents a diffuse tipping process-where biochemical convergence reached critical mass. LUCA is a process of convergence rather than a specific organism or collection of organisms. At the tipping point, the biosphere committed to the transition from competing biochemical platforms to a universal standard. After the tipping point, biological innovation exploded, with fixed core biochemistry. This model makes testable predictions: core biochemistry should show evidence of evolutionary optimization rather than frozen accidents; core biochemistry should show molecular entanglement that reflects incremental coevolution; and biosynthetic pathways should differ from prebiotic chemistry. These predictions appear to be supported by observations.