Snowball Earth

The snowball earth theory says that at least once during the earth's history, the entire earth was covered with ice. The end of the last period of world glaciation was 550 million years ago, and marked the beginning of the age of plants and animals. As evidence, scientists have found rocks that look like drop stones in ancient rock layers close to the equator. Drop stones are stones dropped by glaciers when they melt. The snowball earth is fascinating because it says that our planet has gone through extreme climate changes throughout its history.

Usually we say that Venus, which is about the same size as earth and is closer to the sun, is "too hot" for life to exist. Mars, which is smaller than the earth and farther way from the sun, is "too cold." The earth is "just right."

However, the snowball earth theory says that every planet is constantly subject to tug of war between the frozen cold of space and the inferno of magma within.

• The temperature on the Earth's surface is the result of many factors:
• The shape and location of the continents
• The amount of CO2 in the atmosphere
• The reflective properties of the surface - water, snow, rocks, plants

 The following three BBC videos explain the evidence for Snowball Earth. It is an exciting story of scientific detective work, and how scientists build on the work of others to better understand our past and our future.

 

Precambrian Era

Although it does not appear so large in the table, the Precambrian period actually made up 88% of earth's history. We know little about it, because there are relatively few rocks or fossils from that time.

In the 1800's, it was believed that life did not begin until the Cambrian era. Now we know that during long Precambrian period, microscopic life developed the biochemical and cellular foundations of life used by plants and animals

Cyanobacteria

Cyanobacteria are bacteria that a capable of photosynthesis. They are are also called blue-green algae, but this name is misleading, since they are not true algae, but bacteria. The name "cyano" means blue, and refers to their blue-green color. Cyanobacteria are one of the most successful life forms that has ever existed on earth.

Small is beautiful 

Cyanobacteria are a very ancient form of life, and existed more than 2.8 billion years ago. They are responsible for the modern world that we know today. In the early earth's atmosphere, carbon dioxide levels were 10-100 times higher than they are today, while oxygen levels were much lower than today. As cyanobacteria grew abundant, they reduced the amount of carbon dioxide in the atmosphere, and increased the amount of oxygen. Other organisms that could not tolerate the presence of oxygen became extinct, or were confined to restricted ecological niches. For example, the bacteria that causes tetanus, Clostridium tetani, is anaerobic and can only survive in deep wounds where there is no oxygen.

Cyanobacteria might have caused global glaciation or "snowball earth". As the amount of carbon dioxide in the atmosphere decreased, the earth became colder. The longest and most severe period of glaciation the earth has ever known was the Huronian glaciation 2.1-2.4 billion years ago. However, this period of extreme climate change may have been the stimulus for the formation of the first multicellular life forms.

What do cyanobacteria look like? 

Some forms of cyanobacteria can change color according to the light. In red light they produce more phycocyanin, and in green light, they produce more phycoerythrin. Thus, they appear green in red light, and red in green light. This helps them get the most energy from the available light.

Cyanobacteria clump together in colonies of various shapes. Some form filaments, others form sheets, clumps or globes. Here are some pictures of cyanobacteria.


http://www.abc.net.au/science/news/stories/s1441707.htm


http://serc.carleton.edu/details/images/2698.html

http://www.mbl.edu/news/press_releases/images/cyanobacteria2_brmma.jpghttp://www.mbl.edu/news/press_releases/images/cyanobacteria2_brmma.jpg

http://www.mbl.edu/news/press_releases/images/cyanobacteria_brmma.jpg

http://commons.wikimedia.org/wiki/File:Cyanobacteria_guerrero_negro.jpg


http://biodidac.bio.uottawa.ca

  http://www.botany.hawaii.edu/faculty/webb/BOT311/Cyanobacteria/CyanoHyellaStella300Crop.jpg

Stromatoliths are large clumps of cyanobacteria. Stromatoliths are among the oldest fossils, and still exist today in Australia.
 
http://www.nirgal.net/life_seek.html


Cyanobacteria Shapeshifters

Cyanobacteria change form as environmental conditions change.

• The normal photosynthetic cells are called vegetative cells.
• When conditions are unfavorable, cyanobacteria survive as spores, that are resistant to harsh conditions.
• When oxygen levels are low, some types of cyanobacteria can form heterocysts, which can turn nitrogen gas into ammonia, nitrites or nitrates. Heterocysts have thick walls which contain the nitrogen reducing enzymes.
• Cyanobacteria can form mobile filiaments called hormogonia, that can travel and form new colonies. The cyanobacteria in the hormogonia are thinner than vegetative cyanobacteria.

Where are cyanobacteria located?

Just about everywhere! Cyanobacteria live in the ocean. They also live in fresh water ponds, lakes, streams and rivers. They live in damp soil, and in deserts after a rain. They even in Antarctica, and in glaciers, forming "green snow" in the spring. One of their most unusual habitats is the fur of sloths in South America. Cyanobacteria form a symbiotic relationship with fungi to form lichens. Lichens are able to survive on rocks, and are among the first inhabitants of new land.

  http://www.sheridanmedia.com/news/community-colleges-quotpartnershipsquot-and-lichens13308

Descendants of cyanobacteria may also be living in every green leaf on the planet. It is thought that the chloroplasts in all green plants and red algae were originally cyanobacteria that were engulfed by other microoganisms. This symbiotic relationship proved beneficial to both the cyanobacteria and the hosts, producing the green planet that we enjoy today.

Timeline of Life

The history of life on earth is divided into four major eras.

During the Precambrian era, which actually lasted for almost 90% of the whole time that life has existed on earth, the only forms of life were microscopic single-celled bacteria and eukaryotes, and some early multi-cellular life forms.

The Paleozoic era was life before the dinosaurs - invertebrates, early forms of fishes, amphibians, and early reptiles and mammal-like forms.

The Mesozoic era was the age of dinosaurs.

The Cenozoic era is the age of mammals that has emerged after the dinosaurs became extinct, and continues to the present time.

In the chart below, you see the geological time periods, how long ago they existed, the animals and plants that emerged during that time period, and on the right, major continents during that period of time.

How Old Is The Moon?

'The moon is about the same age as the earth, about 4.6 billion years old. The "giant impact" theory says that a planet larger than Mars struck the earth. Most of the other planet and a large part of earth vaporized. Debris from the great explosion circled the earth and eventually coalesced into the moon.

Maybe before the moon formed, earth had rings like Saturn does today!

The moon is covered with impact craters.

The moon's surface shows striking contrasts of light and dark. The light areas are rugged highlands. The dark zones were partly flooded by lava when volcanoes erupted billions of years ago. The craters appear dark because sometime after they were formed, the basins were flooded with lava. That means that the craters were formed a long time ago, when the moon still had a liquid core.

The Far Side

The earth has about 80 times more mass than the moon. The pull of the earth's gravity has slowed down the rotation of the moon, so that the same side of the moon always faces the earth.

So human beings did not see the far side of the moon until the first pictures were sent back from an unmanned probe in 1959.

The far side of the moon is has many more craters than we see on the "earth side". One theory is that there was less lava flow to cover the craters on the far side of the moon. There was more lava flow on the near side of the moon because earth's gravity pulled the molten core off center. Learn more about this theory at: http://www.msnbc.msn.com/id/40135552/ns/technology_and_science-space/

Of course the obvious reason why there are fewer craters on the near side of the moon is that the near side of the moon received some shielding from the earth.

How old is the Earth?

The earth is about 4.54 billion years old. The entire solar system is not much older. It is about 4.567 billion years old. The oldest rocks on earth that we have found are small zircon chrystals from Jack Hills, in Western Australia. These rocks are 4.404 billion years old.



You can find out how this date was arrived at if you go to http://en.wikipedia.org/wiki/Age_of_the_Earth

What happened next?

At first the earth was a molten ball of gas and liquid rock. Gradually, it began to cool and the crust hardened. We do not know exactly when the crust solidified, but the best guess is in 10's of millions of years.

Where did the oceans come from?

Water comes from two sources: From molten rock within the earth, and from meteorites outside the earth. We do know that the oceans formed about 3.8 billion years ago. By the time the oceans formed, the crust was cool enough that the water did not boil off.

When did life appear?

We don\'t know exactly when life first developed on earth. The oldest fossil microorganisms we have found are 3.5 billion years old.

Did life come from outer space?

Mars is smaller than the earth, and further away from the sun. It cooled off faster than the earth. Early in its history, Mars had liquid water and was probably a hospitable place for life. It is possible that life originated there before conditions were suitable for life on earth. But could life have hitched ride on a meteor to earth? In the early solar system, there were huge numbers of meteorites striking all the planets. Many meteorites from Mars fell on earth. Go to
http://discovermagazine.com/2001/aug/featmars
to learn more.

Recently a bacterium survived six years in outer space. No one knows how much longer it could have survived in outer space. So it is possible that life started on Mars.

When did life arrive on land?

The first life lived only in the sea. The land was sterile. The next step was surviving along sea coasts, where organisms had to endure drying out when the tides went out. This early life was mounds of bacteria called stromolites. The stromolites may have been capable of photosynthesis.

These are modern day stromatolites:

http://commons.wikimedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg


Photosynthesis

Photosynthesis produced a revolution on earth. For the first time, life could get energy from the sun, instead of depending on minerals from the earth. Photosynthesis had a down side. It produced oxygen, which was poisonous to many organisms that lived at that time. Only organisms that could cope with oxygen would thrive in this new world. On the plus side, oxygen produced ozone, which is essential for protection again solar and cosmic radiation. We now believe that bacteria grew on land in "organic mats" as long as 2.6-2.7 billion years ago.
http://www.space.com/searchforlife/life_origins_001205.html

The first eukaryotic organisms appeared 1-2 billion years ago. Eukaryotic organisms are one-celled like bacteria, but have a more complex internal structure than bacteria. Eukaryotic organisms have a separate nucleus. They also have mitochondria, which can use oxygen to produce energy. Mitochondria were probably separate organisms that developed a symbiotic relationship with the host microorganism. This relationship was so successful that it is found in all higher organisms today. The first highly organized multicellullar plants and animals first appeared about 600 million years ago.These trilobites lived about 300-500 million years ago.



Here is a video that explains in another way how old the earth is: (there is a short ad at the end of the video)

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