Lesson Objectives

• Describe how the early continents came together.
• Understand what was needed for the first life and the various ways it may have come about.
• Discuss the early atmosphere and how and why free oxygen finally increased.
• Know the features and advantages of multicellular organisms.

Vocabulary

• amino acid
• craton
• cyanobacteria
• eukaryote
• extinct
• greenstone
• LUCA (last universal common ancestor)
• metabolism
• microbe
• microcontinent
• nucleic acid
• paleogeography
• photosynthesis
• platform
• prokaryote
• RNA world hypothesis
• shield
• stromatolites
• supercontinent
• symbiotic

Introduction

The longest span of time is the Precambrian Era, which includes the Proterozoic, Archean, and Pre-Archean (also called the Hadean). The Precambrian began when the Earth formed and ended at the beginning of the Cambrian period, 570 million years ago. The events recounted in the previous section were all part of the earliest Earth history, the Hadean. But there was still much more to come in the Precambrian Era. The geological principles explained in the earlier chapters of this book apply to understanding the geological history of these old times (Figure below).

Early Continents

The first crust was made of basaltic rock, like the current ocean crust. Partial melting of the lower portion of the basaltic crust began more than 4 billion years ago. This created the silica-rich crust that became the felsic continents.

Cratons and Shields

The earliest felsic continental crust is now found in the ancient cores of continents, called the cratons. Rapid plate motions meant that cratons experienced many continental collisions. Little is known about the paleogeography, or the ancient geography, of the early planet, although smaller continents could have come together and broken up.

Places the craton crops out at the surface is known as a shield. Cratons date from the Precambrian and are called Precambrian shields. Many Precambrian shields are about 570 million years old (Figure below).

Geologists can learn many things about the Pre-Archean by studying the rocks of the cratons.

• Cratons also contain felsic igneous rocks, which are remnants of the first continents.
• Cratonic rocks contain rounded sedimentary grains. Of what importance is this fact? Rounded grains indicate that the minerals eroded from an earlier rock type and that rivers or seas also existed.
• One common rock type in the cratons is greenstone, a metamorphosed volcanic rock (Figure below). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones.

During the Pre-Archean and Archean, Earth’s interior was warmer than today. Mantle convection was faster and plate tectonics processes were more vigorous. Since subduction zones were more common, the early crustal plates were relatively small.

In most places the cratons were covered by younger rocks, which together are called a platform. Sometimes the younger rocks eroded away to expose the Precambrian craton (Figure below).

Since the time that it was completely molten, Earth has been cooling. Still, about half the internal heat that was generated when Earth formed remains in the planet and is the source of the heat in the core and mantle today.

Precambrian Plate Tectonics

By the end of the Archean, about 2.5 billion years ago, plate tectonics processes were completely recognizable. Small Proterozoic continents known as microcontinents collided to create supercontinents, which resulted in the uplift of massive mountain ranges.

The history of the North American craton is an example of what generally happened to the cratons during the Precambrian. As the craton drifted, it collided with microcontinents and oceanic island arcs, which were added to the continents. Convergence was especially active between 1.5 and 1.0 billion years ago. These lands came together to create the continent of Laurentia.

About 1.1 billion years ago, Laurentia became part of the supercontinent Rodinia (Figure below). Rodinia probably contained all of the landmass at the time, which was about 75% of the continental landmass present today.

Rodinia broke up about 750 million years ago. The geological evidence for this breakup includes large lava flows that are found where continental rifting took place. Seafloor spreading eventually started and created the oceans between the continents.

The breakup of Rodinia may have triggered Snowball Earth around 700 million years ago. Snowball Earth is the hypothesis that much of the planet was covered by ice at the end of the Precambrian. When the ice melted and the planet became habitable, life evolved rapidly. This explains the rapid evolution of life in the Ediacaran and Cambrian periods.

This video explores the origin of continents and early plate tectonics on the young Earth (1c): http://www.youtube.com/watch?v=QDqskltCixA (5:17).

The presence of water on ancient Earth is revealed in a zircon crystal (1c): http://www.youtube.com/watch?v=V21hFmZP5zM (3:13).

The Origin of Life

No one knows how or when life first began on the turbulent early Earth. There is little hard evidence from so long ago. Scientists think that it is extremely likely that life began and was wiped out more than once; for example, by the impact that created the Moon.

To look for information regarding the origin of life, scientists:

• perform experiments to recreate the environmental conditions found at that time.
• study the living creatures that make their homes in the types of extreme environments that were typical in Earth’s early days.
• seek traces of life left by ancient microorganisms, also called microbes, such as microscopic features or isotopic ratios indicative of life. Any traces of life from this time period are so ancient, it is difficult to be certain whether they originated by biological or non-biological means.

What does a molecule need to be and do to be considered alive? The molecule must:

• be organic. The organic molecules needed are amino acids, the building blocks of life.
• have a metabolism.
• be capable of replication (be able to reproduce).

Amino Acids

Amino acids are the building blocks of life because they create proteins. To form proteins, the amino acids are linked together by covalent bonds to form polymers called polypeptide chains (Figure below).

These chains are arranged in a specific order to form each different type of protein. Proteins are the most abundant class of biological molecules. An important question facing scientists is where the first amino acids came from: Did they originate on Earth or did they fly in from outer space? No matter where they originated, the creation of amino acids requires the right starting materials and some energy.

To see if amino acids could originate in the environment thought to be present in the first years of Earth’s existence, Stanley Miller and Harold Urey performed a famous experiment in 1953 (Figure below). To simulate the early atmosphere they placed hydrogen, methane and ammonia in a flask of heated water that created water vapor, which they called the primordial soup. Sparks simulated lightning, which the scientists thought could have been the energy that drove the chemical reactions that created the amino acids. It worked! The gases combined to form water-soluble organic compounds including amino acids.

A dramatic reenactment of this experiment is performed on this video from the 1980 TV show Cosmos: http://www.youtube.com/watch?v=yet1xkAv_HY. At the end you can learn about the possible role of RNA.

Amino acids might also have originated at hydrothermal vents or deep in the crust where Earth’s internal heat is the energy source. Meteorites containing amino acids currently enter the Earth system and so meteorites could have delivered amino acids to the planet from elsewhere in the solar system (where they would have formed by processes similar to those outlined here).

Metabolism

Organic molecules must also carry out the chemical work of cells; that is, their metabolism. Chemical reactions in a living organism allow that organism to live in its environment, grow, and reproduce. Metabolism gets energy from other sources and creates structures needed in cells. The chemical reactions occur in a sequence of steps known as metabolic pathways. The metabolic pathways are very similar between unicellular bacteria that have been around for billions of years and the most complex life forms on Earth today. This means that they evolved very early in Earth history.

Replication

Living cells need organic molecules, known as nucleic acids, to store genetic information and pass it to the next generation. Deoxyribonucleic acid (DNA) is the nucleic acid that carries information for nearly all living cells today and did for most of Earth history. Ribonucleic acid (RNA) delivers genetic instructions to the location in a cell where protein is synthesized.

The famous double helix structure of DNA is seen in this animation: http://upload.wikimedia.org/wikipedia/commons/8/81/ADN_animation.gif.

Many scientists think that RNA was the first replicator. Since RNA catalyzes protein synthesis, most scientists think that RNA came before proteins. RNA can also encode genetic instructions and carry it to daughter cells, such as DNA.

The idea that RNA is the most primitive organic molecule is called the RNA world hypothesis, referring to the possibility that the RNA is more ancient than DNA. RNA can pass along genetic instructions as DNA can, and some RNA can carry out chemical reactions like proteins can.

A video explaining the RNA world hypothesis is seen here: http://www.youtube.com/watch?v=sAkgb3yNgqg. Pieces of many scenarios can be put together to come up with a plausible suggestion for how life began.

Simple Cells Evolve

Simple organic molecules such as proteins and nucleic acids eventually became complex organic substances. Scientists think that the organic molecules adhered to clay minerals, which provided the structure needed for these substances to organize. The clays, along with their metal cations, catalyzed the chemical reactions that caused the molecules to form polymers. The first RNA fragments could also have come together on ancient clays.

For an organic molecule to become a cell, it must be able to separate itself from its environment. To enclose the molecule, a lipid membrane grew around the organic material. Eventually the molecules could synthesize their own organic material and replicate themselves. These became the first cells.

The earliest cells were prokaryotes (Figure above). Although prokaryotes have a cell membrane, they lack a cell nucleus and other organelles. Without a nucleus, RNA was loose within the cell. Over time the cells became more complex.

Evidence for bacteria, the first single-celled life forms, goes back 3.5 billion years (Figure above).

Eventually life began to diversify from these extremely simple cells. The last life form that was the ancestor to all life that came afterward is called LUCA, which stands for the Last Universal Common Ancestor. LUCA was a prokaryote but differed from the first living cells because its genetic code was based on DNA. LUCA lived 3.5 to 3.8 billion years ago. The oldest fossils are tiny microbe-like objects that are 3.5 billion years old.

Photosynthesis and the Changing Atmosphere

Without photosynthesis what did the earliest cells eat? Most likely they absorbed the nutrients that floated around in the organic soup that surrounded them. After hundreds of millions of years, these nutrients would have become less abundant.

Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photosynthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts (Figure below).

In what two ways did photosynthesis make the planet much more favorable for life?

1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms.

2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out.

Earth’s Third Atmosphere

The addition of oxygen is what created Earth’s third atmosphere. This event, which occurred about 2.5 billion years ago, is sometimes called the oxygen catastrophe because so many organisms died. Although many species died out and went extinct, this event is also called the Great Oxygenation Event because it was a great opportunity. The organisms that survived developed a use for oxygen through cellular respiration, the process by which cells can obtain energy from organic molecules.

What evidence do scientists have that large quantities of oxygen entered the atmosphere? The iron contained in the rocks combined with the oxygen to form reddish iron oxides. By the beginning of the Proterozoic, banded-iron formations (BIFs) were forming. The oldest BIFs are 3.7 billion years old, but they are very common during the Great Oxygenation Event 2.4 billion years ago (Figure below). By 1.8 billion years ago, the amount of BIF declined. In recent times, the iron in these formations has been mined, and that explains the location of the auto industry in the upper Midwest.

With more oxygen in the atmosphere, ultraviolet radiation could create ozone. With the formation of an ozone layer to protect the surface of the Earth from UV radiation, more complex life forms could evolve.

Early Organisms

What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobacteria (Figure below). Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years.

Modern cyanobacteria are also called blue-green algae. These organisms may consist of a single or many cells and they are found in many different environments (Figure below). Even now cyanobacteria account for 20% to 30% of photosynthesis on Earth.

Cyanobacteria were the dominant life forms in the Archean. Why would such a primitive life-form have been dominant in the Precambrian? Many cyanobacteria lived in reef-like structures known as stromatolites (Figure below). Stromatolites continued on into the Cambrian but their numbers declined.

Eukaryotes

About 2 billion years ago, eukaryotes evolved. Eukaryotic cells have a nucleus that encloses their DNA and RNA. All complex cells and nearly all multi-celled animals are eukaryotic.

The evolution of eukaryotes from prokaryotes is an interesting subject in the study of early life. Scientists think that small prokaryotic cells began to live together in a symbiotic relationship; that is, different types of small cells were beneficial to each other and none harmed the other. The small cell types each took on a specialized function and became the organelles within a larger cell. Organelles supplied energy, broke down wastes, or did other jobs that were needed for cells to become more complex.

What is thought to be the oldest eukaryote fossil found so far is 2.1 billion years old. Eukaryotic cells were much better able to live and replicate themselves, so they continued to evolve and became the dominant life form over prokaryotic cells.

Multicellular Life

Prokaryotes and eukaryotes can both be multicellular. The first multi-celled organisms were probably prokaryotic cyanobacteria. Multicellularity may have evolved more than once in life history, likely at least once for plants and once for animals.

Early multicellular organisms were soft bodied and did not fossilize well, so little remains of their existence. Multicellular organism will be discussed in the lesson, History of Earth’s Complex Life Forms.

Lesson Summary

• After partial melting of the original basaltic crust began, silca-rich rock formed the early continental crust.
• The oldest felsic continental crust is found in cratons. A craton found at the surface is a shield; a sediment covered craton is a platform.
• Precambrian rocks help scientists piece together the geology of that time.
• The continents formed as cratons collided with microcontinents and island arcs to form large continents.
• Rodinia was a supercontinent composed of Laurentia and other continents.
• Snowball Earth may have occurred during the late Precambrian and its end may have led to the explosion of life forms that developed during the Ediacaran and Cambrian.
• Amino acids were essential for the origin of life. They link together to form proteins.
• RNA may have been the first and only nucleic acid at the beginning of life.
• A cell needs a way to replicate itself, a metabolism, and a way to separate itself from its environment.
• An atmosphere that contains oxygen is important because of the ozone layer and cellular respiration.
• Multicellular organisms evolved long after prokaryotes evolved and they may have evolved more than once.

Review Questions

1. What is the difference between a craton, shield, and platform?

2. If a rock contains rounded grains of sediments, what can you tell about that rock?

3. What does a greenstone indicate about the plate tectonic environment in which it formed?

4. What happened to all of the heat Earth had when it formed?

5. What was Laurentia and what lands was it composed of? What happened to it?

6. How was Rodinia like Pangaea?

7. What were the possible sources of amino acids on the ancient Earth?

8. What was the significance of the Miller-Urey experiment?

9. What is the RNA world hypothesis and why is it called that?

10. What is the difference between prokaryotes and eukaryotes?

11. What was LUCA? Is LUCA still alive?

12. Why are banded-iron formations important?

13. Why were cyanobacteria important in the early Earth?

14. How are eukaryotes thought to have originated?

Further Reading / Supplemental Links

Read about the oldest material yet found in the Solar System: http://news.nationalgeographic.com/news/2010/08/100823-oldest-solar-system-two-million-years-older-science/.

Points to Consider

• What would life be like on Earth if there were no free oxygen?
• Why did it take so long for eukaryotes or multicellular organisms to evolve?
• How did the evolution of life affect the non-biological parts of the planet?