Jack Szostak: The Early Earth and the Origins of Cellular Life

thank you very much Laurie for the organizers for inviting me to speak here as Laurie said I want to talk about the origin of life Earth but I want to emphasize the planetary setting for the beginnings of life so what we see here is a schematic diagram of what we think a really simple early cell would have looked like and the question that we think about a lot is what kind of environments on the early Earth and when did did cells like that first emerge okay so what we see here is is there's really a lot of guesswork about the timing of the beginning of life there's really a lot we don't know we can bookend the origin of life by the time at which our planet first was formed after the moon-forming impact the planet was essentially a glowing ball of molten rock 50 or 100 million years later we think it had cooled down enough to have liquid water on the surface and sometime in the next few hundred million years the right geological settings for the chemistry that gave rise to earth emerged in series of stages so progressively more complicated organic chemistry giving rise to the building blocks of biology the nucleotides amino acids and lipids that come together to make little bits of RNA peptides and membranes eventually assembling into the first cells which long before the origin of coded peptide synthesis carried out biochemical functions using RNA molecules which can also act as catalysts of metabolic reactions and replication sometime after that we don't really know how long it took but that RNA world quickly evolved the ability to make complicated protein enzymes and to store the information required to do that in the form of DNA giving rise to the early cells with it biochemistry similar to what we're familiar with today and the process of evolution from that point forward led to the diversification of life and eventually to more complicated forms of life including all of us here today okay so what I want to do today is is talk about how our planet has changed over the four to four and a half billion years so that we can as we move back in time get a better idea of what the early Earth was like and how different it was from what we see today and so since I only have about half an hour I'm gonna have to skip over a few of the changes that our planets experienced over the last four billion years and just touch on a few highlights such as the emergence of oxygen right something pretty important for us but for much of the Earth's history there was essentially no oxygen in the atmosphere we'll go back another billion years after that before that to see the first evidence of life talk a little bit about the Earth's magnetic field which is important for protecting our atmosphere and going back even further talk about impacts of asteroids on the early Earth which had very important effects the chemistry of the atmosphere and probably played an important role in setting the stage for the emergence of life okay but let's start off with the great oxidation event which is what we call this time about 2.4 billion years ago when oxygen levels first it is too significant to see significant levels still much lower than what we see presently but enough to really completely change the nature of the of the early Earth so we we have there's so many questions about this but the main ones are you know what why did this happen why did it take so long for oxygen to emerge what happened two and a half billion years ago and I really need to thank my colleague Roger summons for help in in trying to explain this so this is a kind of a timeline of the history of oxygen or over the over the earth so for the first two billion years from four-and-a-half which is the so the first five hundred million years of Earth's history are not even shown here four billion years it was essentially no oxygen and then it looks like there were little whiffs of oxygen brief periods when small amounts may have been formed and then gone away and then all of a sudden 2.4 2.5 billion years ago there was something changed and there was a dramatic increase in the levels of oxygen and then about 600 million years ago there was another big increase up to sort of present-day levels but I want to talk about this first emergence of oxygen in Earth's atmosphere how do we know this happened and how do we study it today so one of the earliest clues was the fact that the precipitation of iron from Earth's oceans dramatically increased about that time and as you can see here there were actually mountains of iron that precipitated out of the Earth's oceans about two and a half billion years ago and you see a picture of one of the best places to see this in Western Australia and just to give you an idea of how dramatic this is how much iron came out from the reaction of oxygen produced by photosynthesis with the reduced iron in the ocean of precipitating these iron oxides here's a closer in view of one of the faces of the mountain so all of the red rock is precipitated iron here's my colleague Tanya Bozek standing in front of a cliff face of these precipitated two iron oxides and here's a closer up view where you can see the the fine laminations where we have iron oxide precipitates alternating with other other sediments so the fact that this peaked about two and a half billion years ago tells us that there was an increase in the production of oxygen that could react with iron in the ocean and precipitate these iron oxides but how do we get a closer look at this to try to start understanding why this happened how it happened in a little bit more detail and so one of the modern approaches to studying this question actually comes from a kind of surprising direction which is looking at sulfur and in particular sulfur isotopes and there's a remarkable and somewhat complicated set of phenomenon here involving the chemistry of sulfur isotopes that lets us see very accurately what happened a long time ago and the the basic idea is that after the emergence of significant levels of oxygen sulfur in the in the atmosphere so sulfur gases coming out from volcanic accelerations are oxidized and when we look at a particular aspect of the sulfur isotope fractionation so we see a very flat signal going back to about 2.4 billion years ago and then before there was oxygen the chemistry of the sulphur isotope fractionation was different and we see this big variation in this particular form of fractionation called mass independent fractionation and that variation is a sign that before that date there was essentially no oxygen in the early Earth's atmosphere and so the way that this is studied though involves really a lot of effort you have to go to remote areas of the earth where we can find these old rocks where they have not been hugely altered by high temperatures and pressure over time and then you actually have to set up huge drilling operations to get cores so we can get clean samples of these early rocks and so you can see a drilling operation up there and then these core samples are carefully logged and accumulated and then shipped off to laboratories where people can look at the sulfur isotopes and really try to get a detailed picture of the emergence of oxygen which now looks like it happened in a very narrow time window only a few million years oxygen levels dramatically and permanently rose to a much higher level completely changing the nature of the environment on the earth and setting the stage for the emergence of different kinds of of life ok so let's go back now another billion years to look for the first really clear signs of life on the ancient earth this is even harder to find because you know rocks from three-and-a-half billion years ago are quite rare quite difficult they're only found in a few places on the modern earth and to find rocks of that age that are well-preserved and that still can show us signs of early life is is even more challenging but we can see in in in a few places on earth are the the fossilized remnants of bacterial mats in the form of stromatolites and so again my my colleague Roger Simmons helped a lot in putting together some of these slides so what you can see here is that one of the best places to see these remnants of early life is in Western Australia and what we can see are these kind of dome-like structures which are the remnants of bacterial mats that have grown in shallow marine environments about three and a half billion years ago so I just want to show a few pictures of what these look like so here you see a particularly nice example and so the layers here represent successive layers of bacterial growth in in a shallow environment and here's another example these don't like structures we think are the remnants of early bacterial life and here's a view from the surface now looking down on a set of these stromatolites one of the things that I think's really amazing about this is it just a few hundred miles from where we see these ancient fossilized remnants of early life you can actually find stromatolites today in this area on the coast of Western Australia called Shark Bay and here are a few pictures of what stromatolites look like when they're growing in a shallow marine environment today so they form these structures which are growing up and they they kind of look just like rocks in the water but they're actually covered with bacterial mat which grows and trap sediment and as as you go year by year these structures go-girl lapse so this is what we think life might have looked like on the early earth roughly three and a half billion years ago going back even earlier than that it gets harder and harder to find traces of life and those earlier traces become increasingly controversial there's a lot of open questions a lot of arguments about whether singing life or or not but we can look at other aspects of the early planet that we think were important for setting the stage and one of those is is Earth's magnetic field perhaps surprisingly so probably none of you use compasses anymore to navigate around our planet but the Earth's magnetic field is is it's actually really important to us today not for navigation but for protecting our atmosphere and so the question here is you know did Earth always have a magnetic field that would protect the atmosphere did it start up at some point so there's a lot of open questions here again so I just want to show a few images so these are courtesy of my colleague Clare Nicholls at MIT who's studying the the origin of our magnetic field in the in the circulation which it comes from the circulation of the molten iron in Earth's core and what Claire and her colleagues are trying to do is is see how far back can we find evidence for the Earth's magnetic field now why this is important is important as shown here so on the top image you see what happens when you have a planet like Mars that doesn't have a strong magnetic field so then the solar wind can can come along and an impact directly on the atmosphere stripping away molecules and and basically resulting in the loss of almost all of the atmosphere underneath you see how our planet Earth is protected from the solar wind by the magnetic field so the charged particles of the solar wind are deflected and then by and large never get to actually hit our atmosphere and blow it off into space something for which we should be very grateful so how do we find evidence for the the early magnetic field and so again you have to find the oldest rocks which is hard to do but it turns out there is some very ancient and quite well-preserved rocks in an area of West southwestern Greenland and and in the sort of farthest most remote aspect of this there are rocks about 3.7 billion years old and I just want to show a few pictures to show both how how challenging and and interesting and exciting is to to go to places like that so that particular site you can only reach it by helicopter and this little Hut here is the only place for dozens of kilometres around where you can stay so all the scientific teams that go to study these ancient rocks stay here and then they have to hike for miles and miles to find sites like this where you can actually get information about the Earth's magnetic field about 3.7 billion years ago and so what you need are sites like this where you have ancient rock on both sides and you see this this dike in the middle is magmatic intrusion that came up and pushed apart the flanking rocks and why that's important is shown here because you can get samples of those rocks all the way across and what we're looking for is that the older rocks on the sides should have magnetic field and one orientation and the as the hot magma intruded it would destroy that in the in the adjacent rock and when it cooled down you would see Meg magnetic orientation in a different direction from the direction of the Meg of the Earth's field at that point if instead if you sample these rocks and the magnetic field is the same all the way across that's a sign that it's been over printed by some more recent event and so to actually do that you actually have to go there and what you see here is clear he's postdoc doing the hard work of drilling out these course and her postdoc advisor professor ben weiss is carefully watching her to make sure that things are done properly that clears doing the hard work okay so so the results are still coming in from that but this is the kind of thing that people are doing to try to study the earth as early as we can so what can we learn trying to go back even further and think about the atmosphere on the on the on the very young earth so the current view of the Earth's atmosphere is not that it was highly reducing that but it was probably composed mostly of kind of neutral molecules like nitrogen slightly oxidizing molecules like co2 traces of hydrogen so here this neutral term two mildly reducing early atmosphere but one of the important things that's been recognized recently is that impacts of asteroids or comets could completely change the nature of the atmosphere for short periods of time by which I mean maybe a million years or so so we can have a fluctuating chemistry of the of the Earth's early atmosphere and I need to thank my colleague David Catlin for help in with the following slides so if we look at Earth today and look at the the gases that are being emitted from from volcanoes what we see is that most of that volcanic gases water and co2 and there are just traces of mildly reducing gases like carbon monoxide and hydrogen and similarly if we look at outgassing from hydrothermal vents below the ocean there's their lower temperature there's more hydrogen that hydrogen is generated from reactions of water with with rock a so called serpentinization reaction which you may hear about a lot more from nick laying our next speaker so what do we know about how this might have changed back in time and this is again a very open question a lot of work going on a lot of arguments and controversies but there is growing amount of data suggesting that as you go back in time the Earth's mantle and the gases being emitted were somewhat more reducing so we would expect to see more hydrogen coming out into the into the early atmosphere but still giving us only a kind of mildly reducing atmosphere and then the interesting thing is when you have an impact from an asteroid it can completely change the atmosphere for a short period of time so for example giving in the first few hundred thousand years a lot of methane photochemical haze but even after the haze is gone there would be synthesis of cyanide in the atmosphere significant amounts of hydrogen and methane for quite some time okay so how do we know that there were early that there were impacts and and and how can we look at that and this brings up this phenomenon of late accretion which is or the late veneer the idea that about 0.1 to 1% of the Earth's mass it created after the moon-forming impact and it brought in elements that we don't expect to see on the surface of the earth so these elements are the precious metals things like platinum palladium osmium rhenium that should have gone with the iron into the Earth's core what we see very surprisingly is that in the in the Earth's mantle and also the mantle of Mars we see levels of these so-called highly sideral file elements that are maybe a hundred times more than what are predicted based on the segregation of the elements with the iron into the Earth's core so where did this extra material come from it had to have come from impacts of asteroids onto the surface of the earth after the core had formed and this has actually given rise to really interesting question and controversy that's being discussed today which has to do with the ratio of these elements on the earth and on the moon and so the observed ratio of these elements is that the earth has a hundred - maybe a couple of thousand times more on its surface than what we see on the moon but how can you explain that because if you're thinking about impacts the area of the cross-sectional area of the earth is only about fourteen times that of the moon so there seems like Earth has more than than it should have so there are various models - to explain this one of the models that's been put forward recently is that this is really a stochastic event that there was one big impact that brought everything to the earth and the moon just didn't happen to get that another well there are several other models but one that I kind of like is that when impactors hit the moon they vaporize and a lot of that material is lost and so this is a kind of histogram of expected impact velocities of meteorites hitting the earth or the moon and and so what you see is only a small fraction of them would hit the moon with a low enough velocity that the material would be retained most high velocity impacts lead to so much vaporization that material is lost from the moon whereas impacts that hit the earth was it with it's much higher gravity all of that material is is retained and so if you put these things together what people like Sarah Stewart saying is that there actually is no discrepancy but you know this argument and this question continue today okay so this early impact history is really important because it affects the atmosphere and it affects the delivery and synthesis of cyanide so why is cyanide so important we actually think that somewhat ironically cyanide is the best material to use to make the building blocks of biology because it can be made in the atmosphere it can be brought to the surface it can be accumulated in probably different ways but this model from John Sutherland shows how if cyanide rains into surface ponds or lakes the circulation of the water through hot rocks brings up iron which can react with cyanide generating ferrocyanide salts which could accumulate over geological periods of time giving a concentrated reservoir of cyanide minerals which subsequently could be processed to give all of the molecules that we know that we need to build life lipids nucleotides amino acids and peptides so we don't have time to go through all of the chemistry that has been worked out in recent years but these are long complicated synthetic pathways and to make something like that plausible on the early Earth you can't expect to just mix a bunch of stuff together and have the molecules of life emerge right it has to go step by step and so one of the important realizations has been that intermediates on the way to complicated molecules like nucleotides can sometimes crystallize out of solution as a way of purifying intermediates that can happen spontaneously on the early Earth so here's a beautiful example of an intermediate on the way to the synthesis of nucleotides that spontaneously crystallize I say leaving all the impurities and side products in solution okay so once those molecules have formed we think that they could self assemble into primitive cells or protocells such as shown here which have basically a membrane boundary and caps awaiting short bits of RNA so the question that we've been thinking about a lot is what environments could support the origin and and nurture the the growth and replication of such primitive forms of cellular life so the model that I like many of my colleagues like but certainly not everyone is that this happens on surface environments where molecules can be made that can be concentrated you have sources of energy like the ultraviolet light from the Sun and you have a fluctuating temperatures that can help to drive the cell cycle and so we see environments like that on the modern earth so for example in like Yellowstone there are hydrothermal vents in this shallow lake that emit streams of hot water that we think could drive the primitive cell cycle so the idea would be in an environment like that these primitive cells could be in a cool environment most of the time every now and then they get swept up in one of these plumes of hot water coming from these shallow hydrothermal vents that could lead to rearrangement of RNA structures and influx of of nutrients into these primitive cells so in order to get a better feeling for those kinds of environments and for the range of environments that we would see in such early volcanic regions one of the things that we've been doing in our origin of life group at Harvard is is going on field trips with our students to learn about the Earth's environments and so so here's a picture that just says three of our graduate students on the left is Zoe who's an astronomy student in the middle is Yaya who is a planetary scientist and then on the right is Lydia who is a chemistry student who works on the origin of life in my lab and so we're all going to visit these environments to try to learn about the environments from what we can see on the monitor is to get a better feeling for what the range of environments on the early Earth and so just show you a few images of some of the things that we saw when we visited Yellowstone last year there are geysers or hydrothermal pools that have these amazing mineral precipitates forming on the edge and on the surface here's some of our colleagues trying to sample life from a hot spring pool in Yellowstone here's an amazing example of terraced outflow from a different hydrothermal pool so there's an amazing variety of environments out there and I think it's important to think about the early Earth in the same way that there were lots and lots of different environments over the whole surface of the planet and this lets us think about what kinds of environments like that could have nurtured early Earth the earth also has impact craters which we think are another very nice environment for the origin of life we haven't been able to visit any yet but I hope to do that in the future so let me just end with a related question which is you know is there life out there is there only life here on our planet or is the universe filled with life we know now from the Kepler space telescope that almost all stars have planets there are more planets than stars and what we've seen from some of the Kepler results is that there are many planets that we know of now that are in the habitable zone around their star so there are planets that could have liquid water on the surface here is a planet which is so far the one that's closest to Earth slightly larger but similar orbit could have water on the surface and the question that we're thinking about now that's transformed the field of astronomy is you know do these planets have life or not and essentially this comes down to questions of chemistry chemistry as the atmospheres of these planets and and so we hope to know in the coming decades and and it's it's actually the students here who will probably be the ones to figure this out you know is actually the question of whether there is life out there can we get evidence of life from looking at the atmospheres of these other planets so by studying the chemistry and the geology of our planet and beginning to think about those questions for planets orbiting other stars we hope to get a better idea of how life started both here and whether it has started many other places in the universe so I just want to end by thanking so many of my colleagues who have helped to educate me about astronomy and geology and chemistry and so on many of them listed here but also the students who've worked with me in my lab for the past 20 years studying the questions of the chemistry of how life got started so I need to thank all of them and thank you for listening thank you [Applause] you

Channel: MoleCluesTV

Views: 6,906

Rating: 4.6965518 out of 5

Keywords: molecular, frontiers, Molecular Frontiers, Origin of life, Jack Szostak, RNA world, Harvard Medical School, In vitro Evolution

Id: h1KqvoLEj7c

Length: 32min 12sec (1932 seconds)

Published: Thu May 16 2019