2015 Annual Science Report
University of Illinois at Urbana-Champaign Reporting | JAN 2015 – DEC 2015
Project 3: Theory for the Darwinian Transition
One of the key puzzles of astrobiology concerns the precision, uniqueness and rapidity of early evolution. In order for life to have evolved the main components of the modern cell as early 3.8 billion years ago, with a unique genetic code that is virtually optimal in terms of minimizing translational errors, the mode of evolution would have had to be different from the current vertical transmission of genes. We had shown in 2006 that the collective mechanism of horizontal gene transfer (HGT) is the only one capable of solving the puzzle of early evolution. The HGT means that the evolutionary process before LUCA can be thought of as a network of interactions rather than a tree, as would be the case in vertical gene transfer. The multiple connectivity of the network accelerates the evolution and allows rapid convergence to a unique, near-optimal genetic code. With all these advantages of HGT, why would it ever stop? Our project uses computer simulation of digital organisms in order to address these generic questions about the exit of life from the collective, progenote phase to the current era of vertically dominated evolution.
This project is potentially important for understanding biosignatures of life. Even on Earth, we are familiar with the tree-like structure of individual organismal lineages. If life were a network, as we believe that it once was, the usual phylogenetic pattern of individuality and species would not apply. If we encounter life on other planets, we cannot be sure if it will be in the collective (progenote) phase or the vertical-dominated phase. Thus it is interesting to understand better the inexorability and timing of the Darwinian Transition.
To address the Darwinian Transition question, we built a quasi-species model of interacting organisms that includes HGT. We ran the simulation in an environment which exhibited a “Mount Fuji” fitness landscale. We discovered that for early times, the system exhibited a progenote phase, with rampant HGT and no unique species. After some time, a transition occurred and HGT switched off, leading to tree-like vertical evolution. It is important to emphasize that the transition is spontaneous and occurs after the population of organisms have evolved their fitness. HGT is still operative, but the actual effect of it becomes minimal because the population has a whole is now near the fitness peak and the likelihood of an improved gene being transferred becomes correspondingly smaller. In other words, HGT drives itself into a regime where it is ineffective. The Darwinian Transition occurs without fine tuning or external factors being adjusted. This result contrasts with earlier theories for the Darwinian Transition which put in by hand the down-regulation of HGT as the organismal complexity increased. In summary, this calculation shows how the Darwinian Transition emerges naturally from HGT dynamics.
PROJECT INVESTIGATORS:Nigel Goldenfeld
PROJECT MEMBERS:Tommaso Biancalani
RELATED OBJECTIVES:Objective 3.2
Origins and evolution of functional biomolecules
Origins of cellularity and protobiological systems
Earth's early biosphere.
Production of complex life.
Environment-dependent, molecular evolution in microorganisms
Adaptation and evolution of life beyond Earth