READ: Histroy of Life on Earth

Lesson Objectives

• Know that geologists and paleontologists use evidence to determine the history of Earth and life on Earth.
• Know that geologists and paleontologists measure the radioactivity in certain rocks to determine the age of the earth and fossils.
• Know that the earth is between four and five billion years old.
• Know that scientists need to know what the environment (what chemicals were around, the temperature, etc.) was like on Earth billions of years ago to know how life formed.

Vocabulary

Cambrian explosion

A sudden burst of evolution that may have been triggered by some environmental changes that made the environment more suitable for a wider variety of life forms.

extinct

Something that does not exist anymore; a group of organisms that has died out without leaving any living representatives.

mass extinction

An extinction when many species go extinct during a relatively short period of time.

radiometric dating

A method to determine the age of rocks and fossils in each layer of rock; measures the decay rate of radioactive materials in each rock layer.

stromolites

Fossils made of algae and a kind of bacteria; some of the oldest fossils on Earth.

Introduction

It is no surprise that people have wondered about the age of the earth, how it was formed, and how life began on Earth for hundreds, even thousands, of years. Try to imagine how ancient philosophers tried to explain the history of the earth and life. Many people used mythology or cultural beliefs to explain elaborate stories about how and when the earth formed.

The past two to three hundred years has been an exciting time for geologists, paleontologists and other scientists who are trying to trace the history of the earth. What was once a hobby, studying land forms and fossils has become a science that is revealing the history of the earth and life on Earth.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Age of Earth

During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that confirmed that the earth is very old, far older than the 6,000 years that some leaders had claimed. Their evidence included:

• Fossils of ancient sea life on dry land far from oceans: This supported the idea that the earth changed over time and that some dry land today was once covered by oceans.
• The many layers of rock: When people realized that rock layers represent the order in which rocks and fossils appeared, they were able to start to trace the history of the earth and life on Earth.
• Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of the earth’s surface. This supported the idea of an older Earth.

Radiometric Dating

During the past one hundred years, geologists and paleontologists have been able to delve even deeper into the earth’s history with new tools of science. The most convincing method, calledradiometric dating, was developed more than one hundred years ago. Rocks are made up of minerals. Scientists found that they could measure the age of rocks by measuring the radioactivity of certain minerals in rocks. Geologists and paleontologists still use variations of radiometric dating to determine the age of fossils and rocks today (Figure below).

Over 4 Billion Years

The earth is at least as old as its oldest rocks. The oldest rock minerals found on Earth so far are zircon crystals that are at least 4.404 billion years old. These tiny crystals were found in the Jack Hills of Western Australia. Since the earth is at least as old as the oldest minerals found on Earth, geologists estimate that the minimum age of the earth is 4.404 billion years.

Likewise, the earth cannot be any older than the solar system. The oldest possible age of the earth is 4.57 billion years old, the age of the solar system. Geologists and geophysicists based the age of the universe on the age of materials within meteorites that are formed within the solar system.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Origin of Life on Earth

There is good evidence that life has probably existed on Earth for most of Earth’s history. Some of the oldest fossils of life forms on Earth are at least 3.5 billion year old fossils of blue green algae found in Australia (Figure below).

The next step is to determine exactly how life formed billions of years ago. First, scientists need to know what the environment was like 3.5 to 4 billion years ago; they need to know what kinds of materials were available then that could have been involved in the creation of life. Scientists believe the early earth contained no oxygen gas, but did contain other gases, including nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide and probably a few others.

Life from Random Reactions

Today, we have evidence that life on Earth came from random reactions between chemical compounds that formed molecules; in a series of random steps, these molecules created proteins and nucleic acids (RNA or DNA), and then cells. We know that the ingredients for life (the building blocks of life), were present at the beginning of Earth’s history. Some chemicals were in water and volcanic gases. Other chemicals would have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions and lightening. Keep in mind that this process may have taken as much as 1 billion years. Our understanding of how life originated on Earth is developing gradually (Figure below).

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Geologic Time Scale

Geologists and other earth scientists use geologic time scales to describe when events occurred throughout the history of Earth. The time scales can be used to illustrate when both geologic events and events affecting plant and animal life occurred. All of the earth events we see happening today, such as earthquakes, volcanic eruptions, and erosion, have happened throughout history. Past catastrophic events, such as asteroids and comets also hit the earth long before humans evolved.

The geologic time scale in Figure below illustrates the timing of events such as:

• earthquakes
• volcanic eruptions
• major erosion
• meteorites hitting Earth
• the first signs of life forms
• mass exterminations

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Evolution of Major Life Forms

Life on Earth began about 3.5 to 4 billion years ago. The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria. The first multicellular organisms did not appear until about 610 million years ago in the oceans. These of course would be eukaryotic organisms. Some of the first multicellular forms included sponges, brown algae, and slime molds.

Many of the modern types of organisms we know today evolved during the next ten million years in an event called the Cambrian explosion. This sudden burst of evolution may have been triggered by some environmental changes that made the environment more suitable for a wider variety of life forms.

Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods (insects and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago.

Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still prokaryotes – small, single celled organisms. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the earth no longer exist.

The basic timeline of Earth is a 4.6 billion year old Earth, with (very approximately):

• about 3.5 - 3.8 billion years of simple cells (prokaryotes)
• 3 billion years of photosynthesis
• 2 billion years of complex cells (eukaryotes)
• 1 billion years of multicellular life
• 600 million years of simple animals
• 570 million years of arthropods (ancestors of insects, arachnids and crustaceans)
• 550 million years of complex animals
• 500 million years of fish and proto-amphibians
• 475 million years of land plants
• 400 million years of insects and seeds
• 360 million years of amphibians
• 300 million years of reptiles
• 200 million years of mammals
• 150 million years of birds
• 130 million years of flowers
• 65 million years since the non-avian dinosaurs died out
• 2.5 million years since the appearance of Homo
• 200,000 years since humans started looking like they do today
• 25,000 years since Neanderthals died out

Mass Extinctions

Extinctions are part of natural selection. Species often go extinct when their environment changes and they do not have the traits they need to survive. Only those individuals with the traits needed to live in a changed environment survive (Figure below).

Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and major earthquakes changed the environment. Scientists have been looking for evidence of why dinosaurs went extinct over fairly short periods. Many scientists are examining the theory that a major cataclysmic events, such as an asteroid colliding with Earth, may have caused the extinction of dinosaurs 65 million years ago (Figure below).

Since life began on Earth, there have been several major mass extinctions. If you look closely at the geological time scale, you will find that at least five major massive extinctions have occurred in the past 540 million years. In each mass extinction, over 50% of animal species died. The total number of extinctions could be as high as 20 mass extinctions during this period.

The fossil record tells the story of these mass extinctions: millions of species of fish, amphibians, reptiles, birds, mammals, mosses, ferns, conifers, flowering plants, and fungi populated the seas and covered the Earth - as continents crashed together and broke apart, glaciers advanced and retreated, and meteors struck, causing massive extinctions. Two specific extinctions occurred at the end of the Permian period and when the dinosaurs went extinct.

At the end of the Permian, an estimated 99.5% of individual organisms perished. Several factors may have contributed, and one factor relates again to the supercontinent Pangaea. Marine biodiversity is greatest in shallow coastal areas. A single continent has a much smaller shoreline than multiple continents of the same size. Perhaps this smaller shoreline contributed to the dramatic loss of species, for up to 95% of marine species perished, compared to “only” 70% of land species. Although the exact cause remains unknown, fossils clearly document the fact of Earth’s most devastating extinction.

The dramatic extinction of all dinosaurs (except those which led to birds) marked the end of the Cretaceous period. A worldwide iridium-rich layer, dated at 65.5 million years ago, provides evidence for a dramatic cause for their ultimate extinction. Iridium is rare in the Earth’s crust, but common in comets and asteroids. Scientists associate this layer with a huge crater in the Yucatan and Gulf of Mexico. A collision/explosion between the Earth and a comet or asteroid could have spread debris which set off tsunamis, altered the climate (including acid rain), and reduced sunlight 10-20%. A consequent reduction in photosynthesis would have caused a drastic decrease in food chains, leading to the extinction of the dinosaurs. The fossil record obviously depicts the presence of dinosaurs on Earth, and the absence of dinosaur fossils after this extinction event demonstrates the relationship between the fossil record and evolution.

After each mass extinction, open ecological niches are quickly filled by other species. This is well documented in the fossil record. This episodic speciation following an event such as a mass extinction also shows the relationship between evolution and the fossil record.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Lesson Summary

• During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that confirmed that the earth is very old.
• Fossils of ancient sea life on dry land far from oceans supported the idea that the earth changed over time and that some dry land today was once covered by oceans.
• The many layers of rock represent the order in which rocks and fossils appeared.
• Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of the earth’s surface.
• Radiometric dating allows scientists to measure the age of rocks by measuring the radioactivity of certain minerals in rocks.
• The oldest rock minerals found on Earth so far are zircon crystals that are at least 4.404 billion years old.
• Some of the oldest fossils of life forms on Earth are at least 3.5 billion year old fossils of blue green algae found in Australia.
• Scientists believe the early earth contained no oxygen gas, but did contain other gases, including nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide and probably a few others.
• Geologists and other earth scientists use geologic time scales to describe when events occurred throughout the history of Earth.
• The geological time scale of Earth's past is organized according to events which took place during different periods on the time scale.
• Life on Earth began about 3.5 to 4 billion years ago.
• The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria.
• The first multicellular organisms did not appear until about 610 million years ago in the oceans. Some of the first multicellular forms included sponges, brown algae, and slime molds.
• Plants and fungi appeared roughly 500 million years ago. They were soon followed by arthropods (insects and spiders).
• Amphibians evolved about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago.
• Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the earth no longer exist.
• Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and major earthquakes changed the environment.
• There have been at least five major massive extinctions have occurred in the past 540 million years.
• In each mass extinction, over 50% of animal species died.