[MUSIC] So far in this class we've talked about the origin of the universe. The origin of the elements, the origin of stars, of our solar system, and of the Moon and the Earth. The next logical step, of course, is the origin of life. This is one of the most important questions in theology, philosophy, science, and maybe even just around the dinner table. When, where, and how did life begin? Now, I will begin with the short answer, we don't know yet. But just because we don't have all the answers does not mean that we don't have any ideas. The next two videos are, in a way, a progress report. We'll look at how far we've gotten in our understanding of the origin of life, and where the holes are in the picture. Now, of the three origin questions, when, where, and how did life begin, the how is by far the hardest to answer. And in general I like to approach problems by tackling the easy parts first. It gives us a little confidence before we face the hard part. So let's start with the when and the where and see how far we can get. Of course we can see simply by looking outside that life is abundant on Earth today. We find it in almost every environment we look at, including the deepest part of the ocean in the Marianas Trench more than 10,000 meters below sea level, in rock fractures more than a kilometer underground, and nearly boiling hot springs and in polar ice. Life today is diverse and constantly evolving, from more primitive to more advanced organisms. It's reasonable to assume that like geological processes, the law of uniformitariasm applies to biological processes. If life is present and constantly evolving on modern Earth, it must have existed in the past. And that past life must of also been constantly evolving from less complex to more complex organisms. Now just as the rock record is the key to understanding the geological history of the Earth, the fossil record is the key to understanding the evolutionary history of life. The simplest definition of a fossil is that it is an imprint of past life in the rocks. So if we want to know when life began, we just need to look back into the rock record until we find the oldest imprint of life. Right? The rocks older than this last imprint will be prebiotic, that is, planet Earth before life, and we can use those rocks to interpret what the prebiotic environment was like, or in other words where life originated. Well, to focus our search, we should first consider what it actually is that we're looking for, that is what counts as evidence for life, and what types of rocks we might expect to find it in. So why don't you take a stab at it first? What do you think we should look for? If you said yes to everything, then you're absolutely right. Each of the items we listed was an example of a fossil. A fossil is any record of past life, and it can take on many forms. The most direct evidence of course is to find the physical remains such as the skeleton of a dead or extinct animal just like you see here. But an imprint of an organism or an imprint of an organism's activity that gets preserved in the rock record also counts as a fossil. Just like shoe prints of people who strolled along the beach earlier today tell us that there was living organisms here earlier that are not necessarily here now. Imprints of ancient animals, such as footprints, or burrow trails, or scratch marks are useful indicators of life. These footprints are simply very recent examples of trace fossils. Back at the Natural History Museum, we have an excellent collection of ancient examples of trace fossils of which you will likely see many specimens in the coming weeks as you learn more about life evolution. But trace fossils do not always have to be a physical imprint of past life. Something that could be touched. A trace fossil can also be a chemical remnant of past life. Thermodynamically speaking, it's possible to think of life as a catalysts for chemical reaction. One that takes advantage of the potential energy of a chemical system that is out of equilibrium. An excellent example of a chemical fossil of life is the oxygen that you and I breathe. The free oxygen that makes up about 20% of the modern day atmosphere, and is a necessary resource for our survival is only present as a byproduct of oxygenic photosynthesis, plants breaking down carbon dioxide to form organic matter and free oxygen. So knowing that we can use fossils, trace fossils, and chemical fossils as evidence of past life, where should we be looking? What types of rocks might contain ancient evidence of life? As you might have deduced, the most likely places to find evidence for life in the rock record is by looking in rocks that formed at the earth surface, where life is most abundant. Looking at our rock cycle, this is primarily sedimentary rocks. Clastic rocks, where grain sizes are small such as in fine sandstones, siltstones, and mudstones are good places to find physical imprints of past life. Shelly fossils, like the ones we looked at in the sandstone go back as far as the beginning of the Precambrian, about 452 million years ago and they're often found in classic sedimentary rocks. Chemical sedimentary rocks that form by precipitation of calcite or silica from sea water are also good places to look, because the sediments that form these rocks are limey muds or siliceous oozes that accumulate on the sea floor. And they are easy to imprint on, so it's possible that organisms that don't have any hard parts like bones or shells could still leave a fossil. And they often form in warm environments with lots of sunlight, which at least on the modern Earth, is an ideal place for life to thrive. Arguably, there are fossils and trace fossils of single-celled organisms that are as old as 3.5 billion years. These fossils are primarily either imprints of microbes, preserved in chemical sedimentary rocks called chert that is formed from the crystallization of silica-rich ooze. Or they're trace fossils of algal mats that form distinct sedimentary structures called stromatolites. And this is one here. Well, these look like strong evidence of life going all the way back to the mid-Archean age. For many of the specimens, their validity as fossils remains contentious. And the reason is, that while many of these features or structures look like they are formed biologically and they very likely did, there is still potentially abiotic ways in which they could have formed. So it is hard to conclusively identify them as fossils. Now, you're going to learn a lot more about these putative fossils in an upcoming lesson about the microbial world of the Precambrian. But in the meantime, let's consider the last form of evidence for life, chemical fossils. This type of fossil has the advantage that it does not necessarily have to occur in a well-preserved sedimentary rock. Chemical traces of life, while likely forming near Earth's surface like other fossils, can survive deformation and in some cases even metamorphism. That's good for us because when it comes to the oldest rocks on Earth, they're all metamorphic. So that's pretty much what we have to work with. You might remember that there are very few places on Earth where ancient rocks formed in the first billion years of Earth history are visible at the surface. One of these places is a strip of land that covers part of Greenland and northeast Canada called the North Atlantic Craton. And while most of this Craton is made up of granites that have been metamorphosed, just like the Acasta gneiss. There is one place that is really interesting for looking at the origin of life. And this is a place called the Isua Supracrustal Belt. It's located right about here, just at the edge of the Greenland ice sheet. Isua is a package of rocks that formed at the Earth's surface between 3.7 and 3.8 billion years ago. This rock package is a mix of sediments and volcanic rocks that are the oldest surficial rocks on Earth. Now unfortunately, these rocks have had quiet a history since they were deposited. They were buried and metamorphosed at temperatures of at least 500 degrees C. And in some places, magma intruded into them further metamorphosing them and forming pockets of intrusive igneous rocks. But other little pockets of this suite of rock are actually extremely well preserved, despite the high metamorphic temperatures. This is a piece of rock from Isua that is called a metasediment, that is a clastic sedimentary rock that formed in shallow ocean water, and has since been metamorphosed. And what makes this rock so interesting is that it has these very black layers. In fact, this piece is almost entirely black and the black layers are there, because they're very rich in small grains of the mineral graphite. Now, graphite is a mineral that is made entirely out of the element carbon, and you can see this image taken of a thin section of the rock how much graphite is in here. All of those little black flacks. Now there are only two ways that graphite can form. It can come from an inorganic source of carbon from deep in the Earth and we call this as Primordial carbon and it is what is released during volcanic eruptions. Or, it can come from the metamorphosis of buried organic matter on the ocean floor. Of course, the fact that this graphite is in a metamorphic sea floor sediment is pretty suggestive that it came from the latter source, but an even better way to test where the carbon came from is to look at its carbon isotope composition. Carbon has two stable isotopes, 13C and 12C, and different reservoirs of carbon. That is different systems or rocks or organisms that contain carbon, have a unique ration of the 13C to the 12C. Now this Delta value of 13C over here is just a standard way to measure carbon isotopes of something. It compares the ratio of 13C to 12C in this item that you're interested in to the same ratio in a standard with known quantities of 13C and 12C. Really the only thing you need to know when you are looking at the Delta values of stable isotopes, is that the more negative the number, the less of the heavy isotope, in this case 13C, there is. Primordial carbon, that is the carbon that comes from vulcanism, it has a delta 13C ratio of about minus 5 per mil. Biogenic carbon has much lower delta 13C value. On average it is around minus 26 per mil, and this is because organic carbon is produced by photosynthetic organisms that convert the CO2 that is dissolved in seawater into organic matter. And the enzyme that does the work in the first step of the Calvin cycle, where CO2 is dissociated to carbon via rubisco catalysis, really, really, really likes using carbon-12 more than carbon-13 due to the different vibrational energies of the two isotopes. And that means that the carbon that is the end product of photosynthesis has a lot less 13C in it than the original reservoir of CO2 from which the carbon came which is the dissolved CO2 in the ocean. Thus, that reservoir becomes enriched in 13C relative to its starting composition. In essence, if the graphite in Isua came from primordial carbon, it would have a delta 13C isotope value of around minus 5 per mill. If it comes from life, it will have a much lower delta 13C value. To tell us more about the graphitic rocks that were discovered in the Isua Supracrustal Belt, we turn to Professor Minik Rosing, who discovered these ancient sediments and described them in a seminal paper in Science in 1999. Since 1980 he has been doing field work in the region and during the 2015 field season he took a moment to show us this incredibly famous rock outcrop. So we are now at Isua in West Greenland and we are standing on some of the oldest rocks on Earth, and these are the oldest sedimentary rocks that carry a memory of the surface conditions on Earth at the time when they formed 3,800 million years ago. And then the rock we're standing at here is a marine sediment. It formed at the bottom of the ocean. You can see some layering in color. And these layers were, of course, deposited horizontally at that time. But since it has been displaced and deformed by movements in Earth. And then, you can see there are two different rock types, you have the grey rock type here and the black rock type here. The grey is from a submarine mud avalanche that rolled down some slope on the ancient sea floor and settled on the floor, maybe within a minute or less, and still preserved like this. But the dark layers are formed by very slow sedimentation of clay on the ocean floor. And, as a rule of thumb, it takes about a thousand years to deposit one millimeter of clay out on the ocean, so there's many, many, many, years of deposition here. They are really interesting because they represent everyday conditions on the surface of Earth 3,800 million years ago. And one of the more profound things that we can see, immediately actually, is that this is sediment that was deposited under the ocean. So we didn't know until we found Isua that there was an ocean 3,800 million years ago, so that's one thing that's really interesting. But if you look closer at the dark layers, you can actually see that you have a very sharp boundary between the mud avalanche deposit and the shale deposit that was formed from clay settling very slowly in the ocean floor. You can also see within the black layers there are very fine colorless layers. And the colorless layers are the same materials as this and they represent very small batches of material that rapidly sedimented on the ocean floor. And when we have a rapid sedimentation, there would be very little carbon, because there's no time for a lot of organisms to live and die and settle with the sediment. Whereas in the slowly sedimenting layers, there would be time for a lot of life in the upper part of the ocean to live and die and settle with the sediment, so you've got a lot of carbon into the sediment. And the fact that you have the preservation of these colorless and the black layers preserved on a millimeter and sub-millimeter scale is proof that the carbon was a part of the original sediment, and not something that came in by later processes. So, this is really really essential that the structure of the sediment and the chemistry is preserved. By combining the observation of the primary structures with the chemistry of the sediments we can reconstruct the environment and say it was a volcanic environment with a volcano somewhere nearby in deep water. The grey material represents some probably destabilized volcanic ash deposited on the flank of the volcano that slumped into the ocean and settled as the gray stuff. And this is the quiet times. Maybe millions of years of quiet time between eruptions, where we just had the background sedimentation of clay and dead organisms. So that's really interesting because carbon in Earth surface environments is usually always connected to oxygen, either as carbon dioxide in the atmosphere or as carbonic acid in the sea or as carbonate-like chalk in the rocks. But carbon in nearly always caught up with rocks then, and never just free carbon. And one of the few mechanisms that convert carbon dioxide into something that's like elemental carbon is life. So when we see the black color here, it kind of gives us a hint that life might have existed on Earth that far back in time. So obviously we took some of these rocks back, looked at them and found out there's carbon. And then, tried to figure out is there a way to ascertain that this is really traces of life. Carbon has two stable isotopes. Carbon 12 and carbon 13. They occur on Earth in a given set ratio that we know and we can recognize in all these environments. But when living organisms take carbon dioxide out of the air and form organic matter from it, they do it more easily with carbon 12 than carbon 13 and over time the organisms themselves built a deficit in carbon 13 that we can recognize. If you go down to the sea today and scoop up some algae or whatever, they would have a deficit of 26, 27 per mill of carbon 13. And if you take the carbon out of these rocks and analyze it, turns out you have exactly the same deficit in carbon 13, which proves that the carbon in these rocks were derived from living organisms. And the only way you can explain the amount of carbon in these rocks is that the organisms that made it were already able to perform photosynthesis and harvest energy from the sun and use it for their purposes. This is a very difficult and advanced strategy for organisms to do. So by implication, it would mean that life must have been on Earth a long, long period prior to the deposition of the sediment. And one could speculate that life had existed since 4.4 billion years ago when the first ocean formed on Earth. So, therefore we can call this rock the birth certificate of life on Earth, because it's actually the first evidence we have for the presence of life on Earth. Now, what does this mean? It means that the oldest rocks we have on this planet that come from the surface of the Earth contain strong chemical evidence that life was both abundant and complex enough to have evolved photosynthesis at this time. That means that life had to have formed well before the Isua rocks were formed. And as you know, we don't have any rocks older than these other than the intrusive igneous rocks that make up the Acasta gneiss and the Jack Hills zircons. So even by looking at the oldest rocks we have, we still cannot say when precisely life arose, although it is clear that it was well before the sediments in Isua were formed, so probably within the first 500 million years of Earth history. But because that is in this time period called the age before the rocks we have no pre-biotic rocks to look at. No rock record to tell us the where part of the question, the where life was formed. Which means that in order to get closer to understanding where and how life originated, we will have to do the same thing that we did to understand how earth originated. We consider what we know of the physical, chemical and biological processes that control life and try to deduce a plausible scenario for its origin on Earth. [MUSIC]
