9.1- Earth Forms and Life Begins

About Earth:● Earth was formed about 4.6 billion years ago!● The first life on earth appeared about 4 billion years ago

○ Life started out as small, single-celled organisms● Life on Earth has evolved from these single-celled organisms to

where we are today.○ This has taken a very loooooong time

● Many events have happened throughout Earth’s history○ Continents moved (drifted)○ Meteorites○ Ice age

● These events have caused extinctions, and new organisms have formed.

Earth in a DayIf you think about the formation of Earth and Earth’s history, humans have only appeared in the last minute of a 24 hour day!

The Evolution of Life on Earth

The Fossil RecordFossils are remains of organisms- fossils provide evidence that an organism lived long ago (plants or animals)

Fossil record - provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.

This is a fossil of an extinct lion species

The Age of a Fossil

• In order to become a fossil, the organism needs to be buried and then compressed over time where it can harden.

• Fossils are found mostly in sedimentary rocks (layered rock) – prevents damage.

• Fossils can also be frozen in glaciers or tree resin• Footprints are also considered fossils

Do you think it’s easy for an organism to become a fossil?

FossilsFossil Found in Amber (recent)

Dating Fossils

Once we find a fossil, we have to figure out how old it is:

● Relative dating --- Based on where the fossil is found. Layers are younger at the top and older at the bottom.

Dating Fossils● Absolute Dating- Gives an approximate age of the fossil (in years).

○ Radiometric dating --- test that uses radioactive isotopes in the rock to determine the age.■ Determines the amount of carbon-14 in the fossil

(less carbon = an older fossil)

Carbon Dating animation

Carbon Dating: How Does it Work

Molecular ClocksDetermine how long ago different species split from a common ancestor.

● Similar species have a recent common ancestor, until something happened to cause the organism to split into different species.○ A molecular clock determines how long ago that happened.

● For example- mice and rats have a more recent common ancestor (and are therefore more similar) than mice and dogs.

Molecular ClocksWe can look at differences in DNA sequences of organisms to determine how long it has been since the two species have split.

● The more differences in DNA sequences, the further apart a common ancestor between the two species is.

● In the table we can see that humans and chimpanzees are more closely related than humans and chickens.

Organism Similarity with Human DNA (percent)

Chimpanzee 98%

Mouse 85%

Chicken 60%

Fruit Fly 44%

Geologic Time ScaleDivides Earth’s history into sections based on major events and changes. (like a calendar)

How Earth FormedHow the Earth Formed

● Earth formed about 4.6 billion years ago!● When the Earth formed, it was extremely hot.● Over time, the Earth cooled and formed a crust (a layer of solid rock).

Volcanoes released gases.● An atmosphere of gases (including ammonia, methane, water vapor,

and carbon dioxide) formed.● As the Earth continued to cool, the water vapor turned into rain.

○ This rain formed the oceans we have today.○ It took a long time for organisms to appear in these oceans.

The First Organic MoleculesOrganic molecules are the building blocks of life. Without organic molecules, organisms could not exist.

● Scientists believe organic molecules developed about 4 billion years ago.● We are still searching how the first organic molecules developed. Some ideas

include:1. Lightning created chemical reactions that developed organic molecules2. Active volcanoes provided the materials for organic compounds to

develop

Which Organic Molecule Came First?The organic molecules are:

● Carbohydrates● Proteins● Lipids● Nucleic acids (DNA & RNA)

Each of these organic molecules are important, so which one came first?

RNA World HypothesisScientists think that RNA may have developed first.

● RNA can carry instructions● Some RNA can carry out chemical reactions (doing the jobs that

proteins would do)

RNA World

The First CellsSomehow, a membrane grew around RNA, forming the first cells

● We’re not sure how this membrane formed

LUCAEventually this one cell (or group of cells) developed into an organism.

This cell (or group of cells) is called the Last Universal Common Ancestor (LUCA).

● LUCA is the first organism to develop on Earth.● It developed about 3.5 billion years ago.● LUCA is prokaryotic (meaning it did not have

a nucleus or other organelles)

Photosynthesis & Cellular Respiration● Cells started out as heterotrophs (they were able to get energy from other

molecules around them).● Photosynthesis evolved about 3 billion years ago.

○ Organisms could use sunlight to make food○ These organisms are called autotrophs○ As a result, these organisms put a lot of oxygen into the atmosphere

● The “oxygen catastrophe” that the autotrophs created caused many organisms to die out (they were not used to the oxygen in the air)

● Cellular respiration allowed cells to use oxygen to get more energy.

Evolution of EukaryotesEukaryotes evolved about 2 billion years ago

Endosymbiotic theory:

● Large cells ate smaller cells● These small cells lived within the large cells and later evolved into organelles

○ Mitochondria○ Chloroplasts

Eventually, new adaptations would allow these eukaryotic cells to develop into the organisms we know today.

Arsenic- New Biochemicals for Life?After researching and studying different elements, scientists have discovered that it is possible that life can grow on other plants using different elements than the ones that Earth started with.

This is controversial, but scientists continue to search for life on other planets.