early earth

early organisms

The first organisms to photosynthesize were cyanobacteria. These organisms may have been around as far back as 3.5 billion years and are still alive today (Figure 12.7). Now they are called blue-green algae. They are common in lakes and seas and account for 20% to 30% of photosynthesis today.

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life begins

Life probably began in the oceans. No one knows exactly how or when. Life may have originated more than once. If life began before the Moon formed, that impact would have wiped it out and it would have had to originate again. Eventually conditions on Earth became less violent. The planet could support life. The first organisms were made of only one cell (Figure 12.6). The earliest cells were prokaryotes. Prokaryotic cells are surrounded by a cell membrane, but they do not have a nucleus. The cells got their nutrients directly from the water. The cells needed to use these nutrients to live and grow. The cells also needed to be able to make copies of themselves. To do this they stored genetic information in nucleic acids. The two nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nucleic acids pass

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oxygen enters the atmosphere

Early cells took nutrients from the water. Eventually the nutrients would have become less abundant. Around 3 billion years ago, photosynthesis began. Organisms could make their own food from sunlight and inorganic molecules. From these ingredients they made chemical energy that they used. Oxygen is a waste product of photosynthesis. That first oxygen combined with iron to create iron oxide. Later on, the oxygen entered the atmosphere. Some of the oxygen in the atmosphere became ozone. The ozone layer formed to protect Earth from harmful ultraviolet radiation. This made the environment able to support more complex life forms.

continents grow

Continents grow when microcontinents, or small continents, collide with each other or with a larger continent. Oceanic island arcs also collide with continents to make them grow.

supercontinents

There are times in Earth history when all of the continents came together to form a supercontinent. Supercontinents come together and then break apart. Pangaea was the last supercontinent on Earth, but it was not the first. The supercontinent before Pangaea is called Rodinia. Rodinia contained about 75% of the continental landmass that is present today. The supercontinent came together about 1.1 billion years ago. Rodinia was not the first supercontinent either. Scientists think that three supercontinents came before Rodina, making five so far in Earth history.

early plate tectonics

Since the early Earth was very hot, mantle convection was very rapid. Plate tectonics likely moved very quickly. The early Earth was a very active place with abundant volcanic eruptions and earthquakes. The remnants of these early rocks are now seen in the ancient cores of the continents.

ancient life

For the first 4 billion years of Earth history there is only a little evidence of life. Organisms were tiny and soft and did not fossilize well. But scientists use a variety of ways to figure out what this early life was like.

early continents

The earliest crust was probably basalt. It may have resembled the current seafloor. This crust formed before there were any oceans. More than 4 billion years ago, continental crust appeared. The first continents were very small compared with those today.

multicellular life originates

For life to become even more complex, multicellular organisms needed to evolve. Prokaryotes and eukaryotes can be multicellular. Toward the end of the Precambrian, the Ediacara Fauna evolved (Figure 12.8). These are the fossils discovered by Walcott in the introduction to the next section. The Ediacara was extremely diverse. They appeared after Earth defrosted from a worldwide glaciation. The Ediacara fauna seem to have died out. Other multicellular organisms appeared in the Phanerozoic.

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life gets more complex

Eukaryotes evolved about 2 billion years ago. Unlike prokaryotes, eukaryotes have a cell nucleus. They have more structures and are better organized. Organelles within a eukaryote can perform certain functions. Some supply energy; some break down wastes. Eukaryotes were better able to live and so became the dominant life form.

instructional diagrams

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questions

The fourth supercontinent to form was

a. Pangae

-->  b. Rodinia.

c. Escherichia.

d. none of the above

Earths first crust was probably made of

a. anorthosite

-->  b. granite

c. basalt

d. peridotite

The supercontinent in question 1 formed about

a. 5.0 billion years ago.

b. 4.5 billion years ago.

c. 4.0 billion years ago.

-->  d. 1.1 billion years ago.

The earliest life on Earth

-->  a. may have been wiped out more than once

b. got its nutrients from photosynthesis

c. passed genetic information using amino acids

d. all of these

Early Earth had

a. many volcanoes.

b. high temperatures.

c. abundant earthquakes.

-->  d. all of the above

How do cells make copies of themselves?

-->  a. Nucleic acids pass on genetic information

b. Using their metabolism

c. By combining cells to become multi-cellular

d. None of these

Which of the following is true?

a. Prokaryotes and eukaryotes are both only single celled.

b. Prokaryotes are only single-celled; eukaryotes are only multicellular.

c. Prokaryotes are single-celled or multicellular; eukaryotes are only multicellular.

-->  d. Prokaryotes and eukaryotes both are single-celled or multicellular.

Earths earliest life forms

-->  a. consisted of one cell.

b. could breathe oxygen.

c. lacked a cell membrane.

d. none of the above

The earliest organisms to photosynthesize

a. first appeared about a billion years ago.

b. went extinct millions of years ago.

-->  c. are still common in lakes and seas.

d. two of the above

Continents form when

a. seafloor spreading creates them

b. Earth melts and then re-solidifies

-->  c. microcontinents or island arcs collide

d. none of these

Eukaryotes first evolved about

a. 4.5 billion years ago.

b. 3.5 billion years ago.

-->  c. 2.0 billion years ago.

d. 0.5 billion years ago.

Which of the following types of organisms evolved first?

a. multicellular organisms

b. Ediacara fauna

-->  c. cyanobacteria

d. eukaryotes

Prokaryoes are more common than eukaryotes.

a. true

-->  b. false

DNA is short for deoxyribonucleic acid

-->  a. true

b. false

Rodinia was the first supercontinent.

a. true

-->  b. false

Early in Earth history mantle convection was super fast.

-->  a. true

b. false

The evolution from prokaryotes to eukaryotes to multi-cellular organisms took a few million years.

a. true

-->  b. false

Nucleic acids in living things include DNA and RNA.

-->  a. true

b. false

Continental crust first appeared on Earth about 2 billion years ago.

a. true

-->  b. false

The first continents on Earth were very large compared with those today.

a. true

-->  b. false

Pangaea was the first supercontinent to form on Earth.

a. true

-->  b. false

Life may have originated on Earth more than once.

-->  a. true

b. false

The first oxygen on Earth combined with iron to form iron oxide.

-->  a. true

b. false

Some of the oxygen in Earths early atmosphere became ozone.

-->  a. true

b. false

The first photosynthetic organisms were most like modern E. coli.

a. true

-->  b. false

There are no longer any prokaryotes living on Earth.

a. true

-->  b. false

Ediacara fauna evolved toward the end of the Precambrian Era.

-->  a. true

b. false

first organisms to make food by photosynthesis

a. nucleic acid

b. oxygen

c. eukaryote

d. prokaryote

-->  e. cyanobacteria

f. ozone

g. photosynthesis

type of organism that contains a nucleus in its cell(s)

a. nucleic acid

b. oxygen

-->  c. eukaryote

d. prokaryote

e. cyanobacteria

f. ozone

g. photosynthesis

process that added oxygen to Earths early atmosphere

a. nucleic acid

b. oxygen

c. eukaryote

d. prokaryote

e. cyanobacteria

f. ozone

-->  g. photosynthesis

gas that protects Earth from harmful radiation

a. nucleic acid

b. oxygen

c. eukaryote

d. prokaryote

e. cyanobacteria

-->  f. ozone

g. photosynthesis

type of organism that lacks a nucleus in its cell(s)

a. nucleic acid

b. oxygen

c. eukaryote

-->  d. prokaryote

e. cyanobacteria

f. ozone

g. photosynthesis

organic compound that stores genetic information

-->  a. nucleic acid

b. oxygen

c. eukaryote

d. prokaryote

e. cyanobacteria

f. ozone

g. photosynthesis

waste product of photosynthesis

a. nucleic acid

-->  b. oxygen

c. eukaryote

d. prokaryote

e. cyanobacteria

f. ozone

g. photosynthesis

diagram questions

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