2.10. Snowball Earth; It Did Freeze Over!

Loading...

From the course by University of Illinois at Urbana-Champaign

Emergence of Life

58 ratings

University of Illinois at Urbana-Champaign

Emergence of Life

58 ratings

How did life emerge on Earth? How have life and Earth co-evolved through geological time? Is life elsewhere in the universe? Take a look through the 4-billion-year history of life on Earth through the lens of the modern Tree of Life! This course will evaluate the entire history of life on Earth within the context of our cutting-edge understanding of the Tree of Life. This includes the pioneering work of Professor Carl Woese on the University of Illinois Urbana-Champaign campus which revolutionized our understanding with a new "Tree of Life." Other themes include: -Reconnaissance of ancient primordial life before the first cell evolved -The entire ~4-billion-year development of single- and multi-celled life through the lens of the Tree of Life -The influence of Earth system processes (meteor impacts, volcanoes, ice sheets) on shaping and structuring the Tree of Life This synthesis emphasizes the universality of the emergence of life as a prelude for the search for extraterrestrial life.

From the lesson

Week 2 - The Tree of Life and Early Earth Environments

The advent of life on Earth came about as a result of a remarkable confluence of physical, chemical, and biological processes, all of which were intrinsically linked to rapidly changing early Earth environments. Within this context, cutting-edge approaches in molecular phylogeny by Professor Carl Woese revealed new understandings of the emergence of life and the possible distribution of life within the cosmos. All this and more will be explored in this week's lessons!

2.1. The Scientific Inquiry of Carl Woese8:00

2.2. The Place of Carl Woese in Evolutionary Biology8:24

2.3. Revolution of the Tree of Life7:41

2.4. Evolution of the Tree of Life6:42

2.5. How Did Life Emerge?5:25

2.6. The Genetic Code and Evolutionary Theory7:23

2.7. Throwing Rocks at Earth: The Martian Meteorite10:46

2.8. Microbial Mats and Stromatolites: Laying Down On the Job6:49

2.9. Oxygenation of the Atmosphere5:15

2.10. Snowball Earth; It Did Freeze Over!8:31

Meet the Instructors

Bruce W. Fouke, Ph.D.
Director of the Roy J. Carver Biotechnology Center
Department of Geology, Department of Microbiology, and Institute for Genomic Biology

[SOUND]

[MUSIC]

So we want to consider the Earth as a series of spasmodic,

catastrophic, terrible events actually that have taken place.

And the reason we want to understand that is because those are the kind of events

that life has responded to.

The organisms that are alive at the time these events happen.

Some of them have the right characteristics,

they are able to reproduce in a certain way, they can get food in a certain way.

They are actually in an environment that was less effected than others.

But they have characteristics that they've evolved that then would

allow them to be more successful than others when the environment changes.

So one of the most striking ones, and

especially relevant to the modern day discussion in our society about global

warming is the idea that the Earth has been much warmer than it is now,

and that it was especially warm during the time of the dinosaurs in the Cretaceous

but another time period that is equally dramatic but in the opposite direction is

about 0.6 billion years ago at the end of the Proterozoic the entire Earth froze.

We had ice sheets that moved all the way from the poles down to the mid latitudes

and ended up with sea ice on the equator and we call that Snowball Earth.

Now there's discussions about Snowball Earth about whether or not it happened and

then to what degree it took place.

Some scientists now are suggesting that maybe we had a slush ball Earth where

instead of having true ice at the equators we had slushy ice at the equator but

the bottom line is that the Earth's temperature had dropped dramatically.

And in that process as the Earth cooled, then water was drawn out of the oceans and

then formed ice sheets, both on the continents but

then it spread out onto the open oceans as well.

So the Snowball Earth is dramatic, and

I want us to visit just briefly what some of the evidence for this is.

One of the things we can look for

in the geological record that are a fingerprint of a glacier?

and one of the most easy to find and

less controversial, least controversial of these, are what we call dropstones.

And dropstone is simply the idea that a piece of rock that made up

a continental land mass, was gouged and ground and

brought up into an ice sheet that formed a continent, which we call a glacier.

Then that ice moves and creeps slowly out towards the open coast and into the ocean.

And when it gets to the ocean, it breaks into pieces and forms icebergs.

So when those icebergs that have the rock material that they derive from

the eroding continent they were on when those icebergs float from the northern

latitudes into the equator then they melt.

And as they melt, this load they have of rocks,

it drops out of the bottom of the iceberg and it falls down onto the sea floor.

So that deposit we call a dropstone, and

it'll have the rock types of the land mass on which the original glacier was formed.

So those dropstones at the equator indicate

that we had a very much colder Earth.

Now we have other lines of evidence that has to do with the chemistry of some of

the rock deposits that had time and

some of the chemistry such as the oxygen isotopes allow us to reconstruct what

the temperature was directly of the sea water at that moment.

But, taken as a whole, the dropstones, along with this other data is indicating

that yes, we had a very cold planet, and yes we had ice sheets at the equator's,

temperate ice sheets and then ice covering the equator.

So that's one thing, good dropstones at equatorial zones in the oceans.

Now the next one is at the same time that all of us have some

strong familiarity with metal, iron ore.

Every one of us who go into a building in which there's iron gurters,

every one of us who ever rode in a car, every one of us who have ever taken

a plane ride, we're all familiar with iron and different types of ores.

So this is something that might not be a unknown to you but

you might not of thought about where some of the metal comes from that let's

just say it makes up your car.

So it turns out that during this Proterozoic Era time period, and

actually back into the Archean as well, in the deep oceans

we had fluctuations in the amount of oxygen,the amount of nutrients and

also the amount of dissolved metals, things like iron.

[COUGH] Iron become dissolved when the oxygen drops to low levels within

the ocean, whereas when the ocean becomes well oxygenated, then the iron

chemically bonds with the oxygen and it precipitates a metal ore.

So the thing that we want to think about there is that we have a fingerprint of

microbial mats and microbial activity on the deep sea floor in a very

unique kind of deposit called a BIF,a banded iron formation.

And banded iron formations are ubiquitous around the planet in these aged rocks.

Now if they're ocean rocks,

and a lot of the rocks preserved at this time were deposited in the deep ocean.

So banded iron information is composed of the following.

Imagine you're very deep down in the ocean,

far below where sunlight is able to penetrate.

So on the order of kilometers in water depth.

And on that sea floor, you have organisms that are forming these gooey-ooey

microbial mats and those organisms they don't use oxygen and

they also are not able to photosynthesize so they have to have a lifestyle and

that lifestyle for them is that they basically breathe iron.

So iron three is taking into their cells and then they use that iron to actually

generate proteins and actually create energy, ATP, the basic currency of life.

So they're going about their lifestyle just trying to survive and using the raw

materials that they can get their hands on, which in this case is iron.

So in that process of bringing iron into their cells and utilizing it,

they can drive the precipitation of the iron into a solid mineral.

And those solid minerals are forms of hematite and

curetite and some of the other iron rich minerals that we see forming.

So when you have microbial mats, and

these organisms are going about their lifestyle, they bring iron to the cell and

the end result is they precipitate the iron ores so you get a pure iron ore

that's driven by the metabolic activity of microbes that are living on the sea floor.

And these pure iron ores come out and

finally layered stratified rock deposits, called pattern iron formations.

Now they're different from sematolites in that sematolites

are cetementary rock deposits.

These are also sedimentary but they're not made out of sand grains.

They're not made out of individual grains and clay components.

They're actually composed out of metal minerals that are precipitating

out of seawater and driven by the metabolic activity of the organisms.

Now, banded iron formations are very beautiful.

Oftentimes, the iron oxidizes to a bright red.

So if you see it in a rock outcrop, it has these beautiful layers around the order of

tens of millimeters, to maybe a single millimeter and even thinner in thickness.

And you have layer, upon layer, upon layer.

And so you see these bright red layers.

That are intermixed with oftentimes a grey to a black.

And so those also have some metals in them,

but it's also other types of sediments or mineral precipitation on the sea floor.

In the end, it makes these beautiful banded iron formations, BIFs.

And the BIFs are important because they're a good indicator of ocean circulation in

the ancient oceans, and also the activity of these ancient microbes.

So we find BIF's, primarily concentrated in the Archean time period,

then up thru the Proterozoic.

So this is the type of fingerprint we see in the rock record that tells us

definitively we had a deep ocean, lack of oxygen, iron was mobile and

available to these organisms to breathe, take into their cells,the inproginis

precipitate these iron ore mats and drop a stone from a melting iceberg that's just

above it a few kilometers in water depth, and then you continue the process forward.

[SOUND]

Explore our Catalog

Join for free and get personalized recommendations, updates and offers.

Coursera provides universal access to the world’s best education, partnering with top universities and organizations to offer courses online.

© 2019 Coursera Inc. All rights reserved.

Download on the App Store

Get it on Google Play