Predicting the evolution of biospheres – Tim Lenton (U Exeter, UK)
All our knowledge about biospheres is based on Earth. So, Earth system scientist Tim Lenton (University of Exeter, UK) simulated ‘virtual biospheres’ to increase our understanding of the interaction between life and planet. Lenton is inspired by Lovelock’s Gaia hypothesis and wants to know what selection mechanisms could help life to shape the biosphere.
There are different gases present on a planet with life, compared to a lifeless planet. Biospheres need to recycle materials to ‘build bodies’, as Lenton put it. He showed ‘one pot’ simulations in which nutrient recycling loops appear in a robust way. By coupling flasks in the simulation, he observed how communities that regulate their environment tend to spread at the expense of those that degrade theirs. This works in particular with heterogenous environment variables. However, on Earth many variables like oxygen or carbon dioxide concentrations are well-mixed. In this case, other mechanisms should drive the interaction between life and the biosphere.
At the planetary scale, sequential selection could occur when life affects the environment. If this leads to a stable situation, it will persist. But often, unstable situations occur, leading for instance to a snowball Earth. This ‘resets’ the system and as long as some life persists it will eventually lead to new effects on the environment and the chance of a stable outcome.
The simulations show different types of outcomes: a situation where life always survives, a ‘bottleneck’ scenario with early abrupt dying or long-term survival, a critical scenario where random extinction times are found and an outcome where life always perishes. This could provide some guide to what we may expect to detect on potentially habitable exoplanets around other stars.
As for the future of Earth, Lenton noted that humanity causes changes to the environment, but is now able to observe the consequences and alter them, thus creating self-aware planetary self-regulation.
Synthesis and experimental evolution of an artificial cell model – Tetsuya Yomo (ECNU, CN)
Molecular biologist Tetsuya Yomo of East China Normal University took up the challenge to build a life-like network from biomolecules. He used a system where RNA and RNA replicase were present in liposomes. When he added other liposomes with energy and building blocks, they fused and produced new RNA strands. Fission of the vesicles (using a freeze-thaw method) produced a new ‘generation’. In this system, the size of the vesicles remained stable over some ten cycles.
Conducting the daily passage experiment of more than 500 generations with mutations during RNA replication, Yomo studied whether his system would show Darwinian evolution. Short strands of RNA evolved to act as ‘parasites’, as they are copied faster. But the parasites were suppressed over several generations, as the affinity of the RNA replicase for the short strands diminished. Thus, there was Darwinian evolution present. Yomo concluded that a simple proto-cell of less than 10 µm diameter could have evolved a gene replication network and show life-like characteristics.
Evolution on a changing planet – Marcel Visser (NIOO-KNAW, NL)
Once evolution has started, is it possible to predict its course? This question was tackled by evolutionary ecologists Marcel Visser (Netherlands Institute of Ecology, NIOO-KNAW, the Netherlands) and Jacintha Ellers (Free University Amsterdam, the Netherlands). In their talk, they also discussed three misconceptions about evolution. The first is that evolution is something from the past. ‘Our planet changes, so life changes’, Visser explained. Long term studies of the great tit show that in response to climate change, the birds start breeding a week earlier than 25 years ago.
So can evolution come to the rescue of species faced with global warming? Visser: ‘Misconception number 2 is that evolution is too slow. But that is not always the case.’ He gave several examples of species adapting to e.g. urbanization or climate change. However, there are also many species which don’t seem to adapt. The big question is whether we can predict this. ‘That is one of the game changer questions in the Origins Center’, explained Visser. This question is a dot on the horizon they hope to reach in perhaps 20 years.
Evolution acts on the phenotype, but it changes the distribution of genotypes. This brought Visser to misconception number 3: that genotype predicts phenotype. In fact, the path from genes to phenotype is very complicated and many environmental factors play a role. In the great tits mentioned above, the genotype shapes the phenotype much more in warm years than in cold years.
One thing we do already know is that we can speed up evolution by increasing variability in the population. The classic method is connecting populations, but other, albeit less practical, approaches are inducing non-genetic evolution (e.g. by training birds to fly certain migration routes) or genetic modification by using Crispr/cas9 (e.g. via gene drives).
The timescale of evolutionary history – Phil Donoghue (U Bristol, UK)
Finally, paleobiologist Phil Donoghue (University of Bristol, UK) presented data on when the last common universal ancestor (LUCA) of all current life must have lived. The fossil record won’t answer this question, so Donoghue used the molecular clock, based on mutations in genes that were present in LUCA. He used a panel of 29 nine genes common to all life, analyzed in 102 species. He factored in the fact that the rate of mutations is not always constant over time, lineage and genes. The genetic data were combined with ‘hard dates’ for the last sterilization event on Earth, first dated life forms and the oldest known eukaryotes.
The result is that Donoghue placed LUCA at 4.5 billion years BP, the Archaea/Bacteria split at 3.4 billion years and Eukaryotes at 1.8 billion years. ‘These are fairly robust results’, he concluded. However, his results did contain a few surprises: the Great Oxidation Event (GOE), traditionally linked to the rise of photosynthetic cyanobacteria was no longer connected to this evolutionary innovation. Also, in his data the diversification of Eukaryotes is placed long after the GOE. As he stated at the beginning of his talk, the time scale of evolution and the evidence for the start of life on Earth are still in flux.