funding for this program is provided by annenberg media. narrator: of all the planets in our solar system, none can support complex life except earth. but the conditions on earth were not always so suitable for life, and scientists are working to reconstruct the history of two essential requirements whmake tlanet hatable. the first is free oxygen in the atmosphere. the other is a moderate and stable climate. paleontologist andy knoll has spent much of his career examining earth's rock record

to piece together the history of early life and the rise of oxygen. there's this close waltz, if you will, between the history of earth's physical environments and the history of life. narrator: geologist paul hoffman studies the time in our planet's history when the climate was much more extreme, a period called the snowball earth. hoffman: during the maximum cold of the snowball earth, you have an earth that's more like mars than it is like earth. narrator: hoffman is seeking answers to how the earth entered this snowball and, more important for us, how it returned to warmer conditions and has maintained a stable and moderate climate. both of these scientists look to the deep past for clues to the present, enriching our understanding of the one place we know in the universe where life flourishes --

eah's inedlerr of difntnima as, earth, the habitable planet. including humans, is relatively new to this planet. fossils contained in the earth's vast rock record show that the first animals emerged around 600 million years ago -- a mere 15% of earth's 4.5-billion-year history. it's likely that simple, single-celled organisms ruled alone for approximately 3 billion years. the same rocks that document this evolution of life provide information about earth's physical environment, clues to how earth managed to change from an alien world of simple microorganisms to a planet filled with complex life. andy knoll studies the early period of life,

the first 85% of the planet's history. knoll: what i really want to understand is, when are there physical events that influence the subsequent course of evolution? when are there evolutionary events that feed back onto a changed world? and in order to make that connection, we really want to think about fossils not only as objects with shape and size and sort of mechanical function, but we want to think about fossils as carriers of physiological information, and i think that's a type of information that has been underappreciated in considerations of our planet's history. but i think it's really the keystone that's going to allow us to have a much deeper understanding of how physical and biological events have interacted to bring us to our present moment.

narrator: a crucial chapter in the changing physiology of life is the rise of oxygen, as told by the signature traces that single-celled organisms leave behind in the rock record. first appearing about 2.5 billion years ago, oxygen climbed to its current level approximately 580 million years ago -- around the same time that complex animal life started to appear. knoll: it becomes pretty clear from the geologic record that animals and large plants were essentially intercalated into a biological world that was fully functioning. and in some ways, animals don't add that much to the functioning of that world. so, if you ask historically what underpins ecosystem function on this planet, the real workers that make this planet function are the bacteria. now, why should we expect

that tiny, fragile organisms like bacteria should actually leave a signature in the geologic record? and the good news is that they leave quite a signature. for example, this rock that we are looking at here was deposited about 3 1/2 billion years ago in what's now south africa. and, interestingly, it carries physical and chemical signatures of biology. it's probably a little bit difficult to see, but there are some wavy and bulbous laminations, and those are features called stromatolites. narrator: stromatolites are rock-like structures usually formed by the trapping, binding, and cementation of sedimentary grains by microorganisms. dating back 3.5 billion years, they constitute our earliest and most pervasive record

of life on earth. i think if you only had this rock to work with, you might be hesitant to even discuss whether it was evidence of life. but we can go to a number of places today, places like the bahamas, western australia, where stromatolitic structures are forming that connect this very ancient rock with the present day, where we can actually observe the processes that lead to this kind of pattern. narrator: it's believed that many ancient stromatolites were created by cyanobacteria, photosynthetic microorganisms that produce oxygen, implying that the cellular machinery for photosynthesis arose early in the planet's history. knoll: this tells us that life was present early in our planet's history, but let me show you another rock -- also 3 1/2 billion years old -- from southern africa.

and this rock looks very different. you can see here, it's bright red, and it turns out the bright red is iron oxides. and in general, this kind of banded, iron-rich rock is called iron formation. narrator: these banded iron formations, found all over the world, are further clues to the early history of oxygen. dating as far back as 3.5 billion years, they stopped forming approximately 1.8 billion years ago. continually entering the ocean through geothermal vents, iron remains dissolved until it comes into contact with oxygen. the banding of iron formations on this rock indicate that there was an iron-rich ocean with little or no dissolved oxygen and that, at some point, a pulse of oxygen became available, oxidizing the iron and causing it to settle out of solution.

this kind of rock could not, in principle, form in today's oceans because you can only transport iron through oceans when there is no oxygen. very likely, what caused this iron to change its chemical state and drop out of solution was photosynthetic bacteria. so, rocks like this tell us that for the first half of our planet's history, there was very little, if any, free oxygen gas in the atmosphere and oceans. you and i wouldn't have lasted five minutes on the early earth. narrator: some of the most revealing clues to the rise of oxygen can only be uncovered through organic chemistry. knoll's colleague, biogeochemist ann pearson,

analyzes ancient rocks to reconstruct the biology and environment of this era. the organic-rich sedimentary rocks that we can sometimes find, such as the example here, a lot of this organic matter is actually retained in its original form, which means that the molecular structure of the rock hasn't been destroyed even though this rock is well over a billion years old. and so we can identify the kinds of organisms that were present in the system, and those organisms tell us something about the environmental history of the surface of the planet. narrator: some of the compounds pearson looks for are indicators of the existence of eukaryotes, or organisms with a nucleus, the ancient relatives of today's animals and plants. to gain access to these compounds, pearson extracts fats, or lipids, from the rock sample. once the lipids are extracted, they are run through a mass spectrometer, which indicates what kinds of compounds are in the sample.

pearson: so, when we look at a mixed sample and identify what kinds of compounds we have in the sample, aass spectrum like this, and identify what kinds of compounds we have in the sample, and this is the mass spectrum of a sterol. the sterol is related to the compound cholesterol, which is something we're familiar with. and any time we see a sterol, we interpret that as meaning there was a eukaryote in the sample. and this is significant because we're able to then look for molecular fossils or biomarkers that record eukaryotes before eukaryotes became big enough to leave large, macroscopic fossils that we can then see. narrator: but sterols do more than tell us what kind of life was present. they provide evidence for its environment. this complicated maze of lines, circles, and arches is a road map of the metabolic pathways of living cells, molecular biologists' current knowledge of the sequence of reactions involved in creating compounds,

like the sterols in pearson's rock sample. the chemical pathway for sterols is on the bottom of this chart down here, and the very first step in making a sterol consists of taking a precursor molecule and adding oxygen, or o2, to it. so, when we see these sterols in the ancient rock record, we can infer that they must have been formed by this pathway that requires oxygen. so in addition to being a good marker for the presence of eukaryotes, we, also, at the same time, have a tracer, or a record, that there should have been some amount of oxygen, or o2, present in the ocean system or in the atmosphere at that time. and so these are great classes of biomarker molecules because they can tell us something both about the biochemistry of the system, because it needs oxygen, and about who lived there. narrator: ancient rocks like the 1.5-billion-year-old shale that pearson tested show a world with varied life and with some oxygen. but the conditions were far from what we would call habitable.

knoll: it's still a different world. it still is a world with much less oxygen than we have now, and we are finding out, you know -- literally, even as we're having this conversation, data sets are accumulating that tell us -- really from evidence from all over the world -- that the first rocks that record a world that's like our own, or at least similar to our own in terms of having a lot of oxygen, were deposited only about 580 million years ago. narrator: soon after oxygen reached levels near what we have today, the fossil record shows an explosion of many different mobile, macroscopic animals -- the beginning of a habitable planet. to us, at least. but how earth left one long-lived state that's alien to us

and transformed to another state that's habitable by us is still being debated. knoll: what that means is that there's something we don't know about our planet's history and therefore something that we can study and make new discoveries. if we understood everything, there would be no point in being a scientist. rather, scientists get up in the morning because there are important questions that we can't answer, and the scientific method is devising very rigorous ways of trying to answer questions about what we don't know. narrator: adding to this mystery is the fact that, during the final rise of oxygen, there were radical worldwide climate-changing events. in the country of namibia, exposed rock faces suggest processes 600 million years ago

that created an unimaginable world, bringing new meaning to the term "ice age," where glaciers flowed worldwide and the tropics were more like the poles -- a time known as the snowball earth. hoffman: during the maximum cold of the snowball earth, the mean annual global temperature is minus-50 degrees celsius. the ocean is frozen over, and so you have a solid-surface planet. so you now have an earth that's more like mars than it is like earth. narrator: geologist paul hoffman's research centers on the question of what happened to create this planetary deep freeze, a freeze that opens a lot of questions about how the earth maintains a habitable climate even today. hoffman: this only really became a question in the middle of the 20th century, when people realized that over the 5 billion years of our solar system,

solar radiant energy has increased by almost 30%. so it's surprising, therefore, that the geological evidence indicates that the surface temperature of the earth hasn't changed very much over at least 3 1/2 billion years. so that means that there must be something internal to the earth that is self-adjusting so that the earth always maintains a stable temperature -- not unchanging, but limited in the amount of change despite this large increase in solar radiation. narrator: the earth's ability to self-adjust its temperature is linked to carbon dioxide, a greenhouse gas which captures heat that would normally radiate to outer space and sends it back to the surface. the more carbon dioxide in the atmosphere, the warmer the surface temperature. hoffman: it works in the following way.

carbon dioxide is emitted to the atmosphere primarily from volcanic activity. carbon dioxide is consumed by rock decomposition. geologists call this process weathering. and that process consumes carbon dioxide. the carbon dioxide gets dissolved in groundwater and river water, is deposited as sediment into the ocean, and ultimately sinks back into the mantle through the plate-tectonic process called subduction. it gets heated up, and decarbonation reactions take place. and it's converted back into co2, which comes back out of volcanoes. and that's the sort of complete cycle, the geological cycle, of carbon. narrator: this geological cycling of carbon from the earth's interior to the atmosphere acts as a thermostat, maintaining a stable climate on the planet.

the way the e thermostat works is the weathering reactions are themselves dependent on temperature. so, if the earth got warmer for any reason, there would be more water vapor in the atmosphere, and therefore there would be more rain. and more rain means the weathering rates would go up, and so carbon dioxide would be consumed at a faster rate. and so that would ultimately lead to a new stable climate, perhaps at a slightly higher temperature. similarly, if the earth cooled down for some reason, what would happen is that weathering rates overall would get slower. and so if the volcanic outgassing rate stayed the same, carbon dioxide would start to accumulate in the atmosphere, and that would counteract the cooling and would tend to stabilize the climate again, not at exactly the same temperature as before, but it would prevent a runaway cooling or warming.

narrator: even with the global thermostat that's built in to the carbon cycle, evidence of several past ice ages can be found in ancient glacial deposits around the world. in squantum, massachusetts, hoffman shows us features which reveal a partial ice age about 580 million years ago. hoffman: well, what we see here is a completely disorganized mixture of clay and silt and sand and stones and boulders that have all been churned and rubbed together due to glacial action. so, if you look underneath any active glacier, you see this bed of muck that's full of stones and boulders and whatnot that's getting dragged along by the flowing ice overtop, and it's a very distinctive kind of deposit. it looks like it just got dumped out of a cement mixer, okay, and that indicates that there were no waves or currents involved in the deposition of this material.

this is something that was just churning everything together, and the only process of transport that operates in that way is transport beneath a flowing glacier. narrator: in addition to indicating the presence of glaciers, the rocks can also reveal when those glaciers started to recede and the planet started getting warmer. this is shown by laminations, or layers. so, what i'm looking for here is a layer of stratified material. okay, so, this is massive till. it's somewhat churned-up. this is still pretty massive, and it's massive up there. but, okay, now, here we go. so, this is massive. a little bit of pebble layers here. and then, wow, look at this. this is all beautifully laminated and striated. so, from here right up to there is an interval in which you can see all this delicate, little lamination.

so, this wasn't deposited underneath ice. this had to have been deposited underneath water. but if you look closely, there are stones that are scattered all through here. there's a big one. oh, there's a beauty right over there. look at this one. here's a round stone of granite. another little one there. there's one. and they're just sitting in this fine, laminated silt and mud. okay, so this finely laminated stuff must have been deposited just by slow settling in very calm water -- a fine-grain material. and so the problem is, how do these outsized stones get there? and the almost only way you could imagine these stones coming is from having been floating in ice. and then as the ice melts, the stones drop down out ofhe icebergs and, plunk, plop down on the seafloor.

and so this interval here represents a retreat of the ice. narrator: features like those found in squantum are also found in namibia, showing a period of glaciation approximately 600 million years ago followed by a warming period, where the glaciers melted away. hoffman: the reason why the rocks are so interesting there is that we know that namibia was the warmest part of the ocean at that time, or one of the warmest parts, and it was glaciated. and not only was it glaciated. there were no mountains there. these are not mountain glaciers. these were glaciers that formed at sea level in the warmest part of the ocean. okay, so, there you're front and center with the main paradox of these glaciations -- that you had glaciers at sea level in the warmest parts of the world, which implies that the rest of the world was glaciated, as well. narrator: evidence for two distinct snowball periods,

one 710 million years ago and another 635 million years ago, is found not just in the glacial deposits of namibia, but in glacial deposits worldwide. through paleomagnetic testing, these deposits have all been placed near the equator, painting the picture of a snowball earth where all of the continents are in the tropics. at this point, all of the land on the planet would be where it is warmest and wettest, leading to more weathering. this elevated weathering would absorb more and more carbon dioxide over time, cooling the earth. as temperatures get colder, the area of ice and snow at the poles becomes greater and greater. the expanses of ice at the poles will increase the planet's reflectivity, or albedo, cooling it even more. hoffman: so, presumably, the area that's covered by ice and snow gets greater.

and as a result, the albedo becomes higher. more of the radiation is being reflected, less is being absorbed, and so there's an additional cooling effect. and so, therefore, the feedback becomes stronger and stronger, and there becomes a point where the feedback is self-sustaining. you can't stop it. that's the tipping point. and once you go beyond that point, the advance of the ice would occur very rapidly, and the entire tropical ocean, which is half the surface area of the earth, would become ice-covered in a matter of months to years. so that would be a catastrophic advance of the ice in the final stages. okay, so there's an instability. there's a tipping point beyond which the ice can't be stopped. narrator: but if our planet fell into a deep freeze, how did it ever escape?

the escape is plate tectonics and, in particular, the way plate tectonics drives the carbon cycle. if a snowball occurred, weathering rates would be extremely slow, but plate tectonics would continue. so volcanoes continue to pump co2 into the ocean water and into the atmosphere. so what happens is that slowly, over time, the carbon dioxide and maybe other greenhouse gases in the atmosphere build up and create a stronger and stronger greenhouse effect. and ultimately, according to theory, the greenhouse effect due to the accumulation of carbon dioxide becomes so strong that it's able to counteract the high albedo of the ice-covered earth and precipitate what is believed to be a violent deglaciation, or meltdown, of all the ice

under the influence of an enormously elevated carbon-dioxide load in the atmosphere. narrator: the timing of these radically different climates occurs intriguingly close to the beginning of what is known as the cambrian explosion, when complex life on earth began. this closeness in timing has led to questions about the possibility that the snowball earth and its greenhouse aftermath may have been somehow linked to the sudden appearance of many new kinds of animals in the fossil record. hoffman: there were stresses associated with the glaciation. there were obviously stresses associated with the greenhouse aftermath. so you could imagine that that would be an environment in which there would be strong, selective pressure, and, therefore, that might be an incentive for evolutionary change. but the arrival of multicellular animals is not just a change.

this is a biological innovation. this is a change to a world in which you had organisms which achieved a level of complexity and behavior that had never been seen previously. we don't have any idea at this point why a snowball glaciation and its greenhouse aftermath might have created a circumstance that would have been selectively favorable for this great change in the course of biological evolution, but the coincidence in timing is tantalizing. narrator: while the question of how complex life emerged on our planet persists, the work of scientists like paul hoffman and andy knoll brings us closer to the answer, shedding light not just on how earth became habitable, but on how it remains habitable today.

[ birds squawking ]

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