Biology's Biggest Mystery: The Origin of Life

Where is the world's oldest fossil, and how can we be sure it's really evidence of life? Georgia Mills spoke to Dominic Papineau from UCL, whose interest in life elsewhere in the Universe is taking him deep into the past...

Dominic - I’m going about it the only way that technologically we currently can, although we do have rovers that are capable on Mars to directly search for signs of life and there’s also the SETI programme. My approach is mostly looking at the very distant past on the early Earth and trying to identify signs of life and understand the environmental conditions where these microbial communities were living because we’re obviously talking about microorganisms. At that time there were no trees, fungus, or other animals or this microscopic life that we see today wasn’t there in most of the Archean probably.

Georgia - So you’re searching for something very, very small - how do you even start?

Dominic - You need to target, I think, the right rocks. You can narrow it down to some of the oldest rock sequences our there and this rock sequence that I’ve been at in 2008 in northern Quebec, the Nuvvuagittuq supercrustal belt, is known to be at least Eoarchean in age since 2004. The Eoarchean is the first time period this sedimentary rock record appears.

Georgia - Was it case of hey, these rocks are really, really old, let’s have a look in them, and then you’re just hoping you might find something?

Dominic - Not quite. The Nuvvuagittuq supracrustal belt is about 3 kilometres in length and the banded iron formation unit that is smack in the middle of this has been known for more than a decade now. I was studying banded iron formations so I was interested in these particular biffs as well because these are very ancient. They’re as old as those I was looking for from Isoa and the island of Akilia in Greenland, and that was going to be another example of eoarchean banded iron formations. So I went there in the hope of finding similar things as what other people had found and I myself had found. Even better I found the red banded iron formations with concretions so ‘eureka.’

Georgia - Is the combination of the fact that they were very old and the type of rock that made them of interest to you?

Dominic - Yes, exactly.

Georgia - When you found something, what did you find - what did it look like?

Dominic - I was there specifically for the banded iron formation so I walked a long strike back and forth. It was striking to me that at one particular site, very localised, there were outcrops of this red banded iron formation. When I was walking on it I was carefully looking at the rock and I could see these rounded spherical structures which I had seen in the Hammersley before.  So I knew that these concretions had something, so I sampled them and that’s exactly where we found the microfossils.

Georgia - You’ve got a selection of these rocks in your cabinet in your office. They’re very pretty but I would never expect they contained any kind of life.

Dominic - If you go to the Museum of Natural History, you can see very often that marine animal fossils are preserved in these rounded bits of rock. They’re called concretions; they’re called limestone concretions. They have various names and the reality is that in geology we don’t really have a good explanation for how they formed these particular structures. But we know that in the recent young geological record that they’re associated frequently with animal fossils.

Georgia - Can we go and have a look at your fossils then?

Dominic - Sure… This is what we see.

Georgia - Oh Wow!

Dominic - So that’s what you say - oh wow.

Georgia - So this is a slice of the rock from your office - a very thin slice under a microscope.

Dominic - They’re a little thicker than the usual ones so we’re experimenting with different techniques now. Yes, this is what these things look like.

Georgia - Id’d have a photo of this in my house. It’s a bit like Jackson Pollock or something - a sort of white background and then little flecks of red and black.

Dominic - And there’s a twisting like a spiral.

Georgia - Oh yeah. That’s like a corkscrew.

Georgia - In the slide, amongst the flecks of red you can clearly see a black spiraling tendril. Dominic suggests this could have been made by a microscopic creature, maybe similar to modern day bacteria. But could these shapes simply be caused by geological processes?

Dominic - So far we’ve had one substantial criticism and that is that there are these so-called chemical gardens that can grow tubular structures. Other such non-biological experiments have been successful at growing these filaments that are sometimes twisted so they have morphologies that look like they could potentially be analogues to features that we observe. But if we just consider this kind of comparison based on the morphology, this is one of the main reasons why there’s been so much controversy in the past. It’s because there's very little lines of evidence. But if you consider the bulk of our lines of evidence - we have twelve independent lines of evidence - so I think this is bullet proof.

Georgia - The twelve lines of evidence are the various shapes and chemicals found in the rock, The patterns, the radioactive isotopes, they’ve all been associated with life. Some on their own can appear without the presence of life but, all together, Dominic is confident he has the oldest evidence of life on Earth. Which is pretty exciting...

Dominic - I celebrated with my wife. But when my student, Matt Dodd, took these images and did the due diligence, and he catalogued the images and mapped everything that he was looking at as I instructed him, he brought to me this very compelling images. At some point he brought me this fossil with the twisted stalks and these were very compelling in my opinion. So this was ‘yay- we have the oldest fossils!’. It’s exciting but, ultimately, it’s what it implies with respect to the rapidity that life arises on the planetary surface. The conditions in which it arises has implications for evolutionary biology because many of these microfossils that we find are similar to microorganisms that are living today in these kinds of environments. There’s a continuity in biology that tells us that some organisms haven’t changed much over all this time so it has implications for the origins of life. It has implications for exobiology. I think it’s an important discovery but it’s one that’s going to be followed by many more to come.

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.

Could hydrothermal vents be where life originated? Dr Nick Lane told Georgia Mills about the striking similarities between vent pores and cells...

Nick - Essentially, these vents microporous labyrinths of interconnected pores, and the pores have got very thin inorganic walls around them. ‘That’s what the barrier is, the walls around these pores, and the pores may be a few micrometres across, and perhaps just a micron thick. So they’re bigger than cells but we’re getting down to a cell-like size and it has the same structure as cells, that’s what’s intriguing about it. You essentially have a pore with a barrier around it that’s equivalent to the cell membrane and, effectively, a charge on that barrier. So we have a kind of topology which is equivalent to cells.

Georgia - Right. You think this kind of structure, the fact that it’s similar to cells and it’s got this barrier like a cell would have a membrane, means that it’s possible these were where life started?

Nick - That’s my view. You would find lots of people who would sweep that aside.

Georgia - Nick believes that we might be missing something when running simulations of how these first organic molecules were created. And that missing piece of the puzzle, is looking at how cells do it today, which is through using carbon dioxide and hydrogen for energy, powered by an electrical charge on their membranes.

Primitive creatures called methanogens could provide a clue...

Nick - The simplest cells like methanogens, for example, they live from carbon dioxide and hydrogen, so just gases. The kind of gasses that just bubble out of the ground. In fact, hydrogen gas just bubbles out of hydrothermal vents. They don’t react easily, hydrogen and CO2; if they did we could probably fix global warming by stripping CO2 out of the air, and we could fix the energy crisis by making synthetic gasoline. So I’m sure people have been looking at this behind closed doors for a long time.

But this is what methanogens do, and to get them to react they use some of the simplest, most ancient proteins that we know about. Things like ferredoxin, which are iron sulphur proteins and the cofactor that does the chemistry is, in fact, a very small mineral made of iron and sulphur. These are the kind of minerals that form spontaneously in the vents we’re interested in.

The other thing that the methanogens do is they use this electrical charge on the membrane, as well as these iron sulphur proteins to drive the reaction between hydrogen and carbon dioxide. That’s what I think could happen spontaneously in the vents because those natural structures, barriers including these iron sulphide minerals are natural proton gradients that are equivalent to the charge on the membrane. So the prediction is that it ought to drive that reaction and, if it does, then we can look for organic molecules being formed that way.

Georgia - So the hydrothermal vents provides us with a network of pores, similar to cells, a natural charge on the barriers, which could drive reactions, and they’re rich in the chemicals which are used by all cells today to grow. So can we recreate this environment to look for these reactions?

Nick - We would love to do that. The issue, of course, is that these vents can be 60 metres tall. Today, the few that we know about, come in field which can be kilometres square across, and we think 4 billion years ago that they could have spread all across the entire sea floor. So the whole planet becomes a chemistry lab from that point of view. And there’s millions of years to spare, and there’s high pressures, and high concentrations of hydrogen, so to try and do it in the lab is a joke in the sense that we cannot possibly simulate very much of that.

But what we can simulate is the structure. In effect, what you have barriers containing iron sulphur minerals and those are semiconducting, which means electrons can pass across this barrier. One one side we have a simulated an alkaline hydrothermal fluids, the kind of things that we see in the vents today down at the bottom of the oceans.

On the other side, what we think would have been an acidic early ocean, mildly acidic - maybe Ph6 or something - with lots of carbon dioxide in it. The idea is that that difference in Ph (difference in acidity) will help drive that reaction transferring electrons across the barrier. So that’s the kind of  experiment that’s relatively easy, conceptually at least, to set up in the lab. The reality is that even very simple systems turn out to be more complex than you can really control.

Georgia - Have you had any results yet?

Nick - It looks as if we are able to make simple organic molecules, and from those simple organic molecules it’s relatively easy to go on and produce more complex ones like, for example, ribose or  deoxyribose, which are the sugars used in DNA and RNA. There’s so many steps, that we’re really probing the ones that seem easiest for us to do at any one time rather than the most coherent set of rational steps, we’re doing the ones we can actually do.

So there’s big gaps everywhere and we’re trying to repeat a lot of what we’ve already done. I suppose the normal scepticism is if you can’t repeat everything with different people doing the same experiments then you begin to worry that someone did it wrong, or there was some reason why it’s not believable. You realise when you’re looking for a tiny amounts of trace substances that are actually found on everybody’s breath, for example, you’ve got to be really careful. It’s very easy to make a fool of yourself by thinking you’ve made something when it turns out that you didn’t at all. It as just on someone’s breath.