Early earth simulations
How do we investigate what the early earth chemical reactions might have been? Georgia Mills spoke to Markus Ralser, who is unconvinved by the RNA world hypothesis...
Markus - Historically speaking, many people have tried to explain everything with the emergence of RNA and I think this is very difficult.
Georgia - Markus is unconvinced RNA could really have pushed along those early reactions needed for life - what we call metabolism…
Markus - Every cell consumes a lot of nutrients and this is very essential for life. Therefore cells and systems to convert all of these different nutrients into the molecules that your cells actually need, and that’s what we in biochemistry call metabolism. The way it works is a series of several hundred reactions that happen in all of our cells and from a lot of different angles we think that metabolism had not many origins in evolution, but probably just one. It’s very important to understand those origins of metabolism to get a feeling about which processes enabled life to emerge.
Georgia - So Markus is investigating this from the point of view of those reactions themselves, looking at just how they work…
Markus - We systematically test so we know which reactions are important for metabolism and they are typically catalysed by enzymes. A few years ago we stumbled into some reactions which looked like those enzyme driven reactions but they don’t require enzymes. Then we used analytical instruments called mass spectrometers which allow us to measure thousands and thousands of samples and they systematically test which reactions can happen in those samples.
Georgia - So one of the problems with metabolic pathways is that they need these enzymes, these proteins to help them happen. But what you’ve said is that some of these can happen without the help of proteins, they can just occur anyway. And so you’re throwing together a load of chemistry in an early Earth simulation and then just seeing which of these reactions you can recreate?
Markus - Yes, exactly. For modern life we need enzymes, but the difficulty is that enzymes themselves are made up of products of metabolism. So you end up in a chicken-egg problem and to know what was first. For a long time this was the answer of the question but then people started to see that you can do reactions as they happen in metabolism. Without enzymes, the hypothesis that the whole thing started without enzymes has gotten a huge boost.
Georgia - How do you simulate the early Earth? In my head I’m just imagining a room where you’ve made it look like early Earth but I’m assuming it’s not like that?
Markus - It’s very small volumes of liquids so we are not simulating ten thousands of litres of ocean. We simulate little droplets, 40 microlitres, 50 microlitres, and we add different components that people believe could have existed here on Earth. Those people we talked to, they obtained this information from sediments of stones that they find around the world, and they can date them to this period. Everything is preserved from the time, so a lot of the things one needs to affirm, but there is some solid evidence from geoscientists which molecules made up the early planet.
Then we had the metabolite that we know that are essential for life. We know that they had to play a role at the very early stages of life because now they are essential for life, so that’s our logical view. Then we see reactions can they do without enzymes present in those conditions, and very often we see that many of the reactions that these metabolites undergo are the ones we can find again to be essential for the life of our cells.
Georgia - Do you do anything to this simulation - do you add heat or energy in any way or do you just leave it there and see what happens?
Markus - We did different sorts of things. In the beginning we started with heat because many people think chemical reactions are accelerated by heat and this is typically true. So many reactions require a certain activation energy to happen. Meanwhile, we have expanded this and we test all sorts of different environments. One of them, for instance, is ice. So we see a surprising high reactivity of metabolic reactions to happen in frozen conditions. So there is a huge span and I would say at the moment the answer is open what the right environment was for those processes to start.
Georgia - So you’re putting these metabolites in and you’re putting them under certain conditions, and you’re finding without enzymes they can react and make what exactly?
Markus - They make other metabolites that are part of our cells. This is then not stopping at one metabolite, then one metabolite forms the next metabolite, and the next, and the next, and this results in an entire network. We can then take those networks and compare them with the networks as they happen in our cells. The closer we get, it’s more likely that we have found the origin of such a metabolic system.
Georgia - Do you think then that this metabolic reactions just starting cropping up in this early Earth and, I guess, then what?
Markus - I would say it is the other way round. In order to have life we have to have chemical reactions that happen, and life needs a fundament to build on. When you start to think about how metabolic pathways evolve, you need to have a system that produces a product. Because only if you have something that provides an advantage can the selection procedure start. Now again we have a chicken-egg problem here. If you need multiple enzymes to form a product, but you cannot form the product to select for you have nothing to start with. So we think that these chemical networks were the starting point for evolution to pick out those molecules which provide an advantage and they could select upon. So you can imagine you have a system with ten reactions and then one of them is the slowest or the least efficient reaction, and this is the reaction that evolution can improve first in order to create a product better. If you need to start to work on all ten reactions at the same time, it would be a system which would be much more likely to fail.