Objectives Describe the evidence used to determine the age of Earth. The Early Earth Understand why scientists theorize that the early Earth was hot. –zircon –asteroid –meteorite Vocabulary

In 1996, the announcement that a meteorite from Mars might contain microscopic fossils of bacteria rekindled scientific interest in the search for life elsewhere in the universe. It may be possible to identify clues to the possible existence of life on other planets through rocks from those planets. Earth’s “Birth” For about the first 4 billion years of Earth’s 4.6-billion-year existence, most of the life-forms that inhabited Earth were unicellular organisms. The Early Earth

Earth’s “Birth” There is evidence of life’s beginnings on Earth in Precambrian rocks. The Early Earth Most of Earth’s history is contained within the 4 billion years that make up the Precambrian.

How old is Earth? We know that Earth must be at least as old as the oldest rocks in the crust. The Early Earth –The age of the oldest rocks on Earth is between 3.96 to 3.8 billion years. –Evidence of 4.1- to 4.2-billion-year-old crust exists in the mineral zircon that is contained in metamorphosed sedimentary rocks in Australia. –Zircon is a very stable mineral that commonly occurs in small amounts in granite.

How old is Earth? –Meteorites have been radiometrically dated at between 4.5 and 4.7 billion years old. The Early Earth –The oldest rock samples from the Moon are approximately 4.6 billion years old. –Scientists commonly agree that the age of Earth is 4.6 billion years.

Earth’s Heat Sources Earth was most likely extremely hot shortly after it formed, and there were three likely sources of this heat. The Early Earth –The first source was radioactivity. Radioactive isotopes were more abundant during the past. One product of radioactive decay is energy, which generates heat.

Earth’s Heat Sources –The second source of Earth’s heat was the impact of asteroids and meteorites. The Early Earth Asteroids are metallic or silica-rich objects that are 1 km to 950 km in diameter. Meteoroids are small asteroids or fragments of asteroids. Meteorites are meteoroids that fall to Earth. Evidence suggests that collisions, which generate a tremendous amount of thermal energy, were much more common throughout the early solar system than they are today.

Earth’s Heat Sources –The third source of Earth’s heat was gravitational contraction. The Early Earth As a result of meteor bombardment and subsequent accumulation of meteorite material on Earth, the size of Earth increased. The weight of the material caused gravitational contraction of the underlying zones, the energy of which was converted to thermal energy. The new material also caused a blanketing effect, which prevented the newly generated heat from escaping.

Objectives Explain the origin of Earth’s crust. Describe the formation of the Archean and Proterozoic continents. –differentiation –Precambrian shield –Canadian Shield –microcontinent –Laurentia Vocabulary Formation of the Crust and Continents

Early in the formation of Earth, the planet was molten, and numerous elements and minerals were mixed throughout the magma. Formation of the Crust and Continents Over time, the minerals became concentrated in specific zones and Earth became layered. As the magma reached the surface and cooled, landmasses began to form.

Formation of the Crust When Earth formed, iron and nickel, which are dense elements, concentrated in its core. Formation of the Crust and Continents Lava flowing from the interior of Earth concentrated the less-dense minerals near the surface of Earth over time. The denser minerals, which crystallize at higher temperatures, concentrated deeper within Earth and formed the rocks that make up Earth’s mantle.

Formation of the Crust Differentiation is the process by which a planet becomes internally zoned when heavy materials sink toward its center and lighter materials accumulate near its surface. Formation of the Crust and Continents

Formation of the Crust Earth’s earliest crust most likely formed as a result of the cooling of the uppermost mantle and was similar to basalt. Formation of the Crust and Continents As sediment-covered slabs of the crust were recycled into the mantle at subduction zones, the slabs partly melted and generated magmas with different mineral compositions. These magmas crystallized to form the first granitic continental crust, which was rich in feldspar, quartz, and mica.

Formation of the Crust The formation of the majority of crustal rocks was completed by about 2.5 billion years ago. Formation of the Crust and Continents As less-dense material has a tendency to float on more-dense material, continental crust “floats” on top of the mantle below it. Basaltic crust is more dense than granitic crust, and therefore, it does not float as high on the mantle.

The Cores of the Continents A Precambrian shield is a core of Archean and Proterozoic rock that forms the core of each continent. Formation of the Crust and Continents Buried and exposed parts of a shield together compose the craton, which is the stable part of a continent. The Canadian Shield is the name for the Precambrian shield in North America because much of it is exposed in Canada.

The Cores of the Continents Formation of the Crust and Continents

Growth of Continents Microcontinents, which were small pieces of continental crust that formed during the Archean, began to collide as a result of plate tectonics early during the Proterozoic. Formation of the Crust and Continents At each of these collision sites, the Archean microcontinents were sutured or fused together at orogens. These orogens are belts of rocks that were deformed by the immense energy of the colliding continents.

Growth of Continents Laurentia, the ancient continent which was assembled 1.8 billion years ago, would become the core of modern-day North America. Formation of the Crust and Continents

Growth of Continents Near the end of the Early Proterozoic, between 1.8 and 1.6 billion years ago, volcanic island arcs collided with the southern margin of Laurentia. Formation of the Crust and Continents The final phase of Proterozoic growth of Laurentia, the Grenville Orogeny, occurred between 1.2 billion and 900 million years ago. By the end of the Proterozoic, nearly 75 percent of present-day North America had formed.

Growth of Continents By the end of the Proterozoic, all of the major masses of continental lithosphere had formed. Formation of the Crust and Continents As the lithospheric plates moved around, they periodically collided and sutured together to form Rodinia, the first supercontinent. Rodinia began to break apart at the end of the Proterozoic and continued to do so during the Early Phanerozoic.

Growth of Continents Formation of the Crust and Continents

–cyanobacteria –stromatolite –banded iron formation –red bed Objectives Describe the formation of Earth’s atmosphere and oceans. Identify the origin of oxygen in the atmosphere. Explain the evidence that oxygen existed in the atmosphere during the Proterozoic. Vocabulary Formation of the Atmosphere and Oceans

Earth’s early atmosphere was nothing like what it is today. Formation of the Atmosphere and Oceans The oxygen that early forms of algae produced through the process of photosynthesis affected the development of life on Earth in two very important ways. –It changed the composition of the atmosphere and thus made life possible for oxygen-breathing animals. –It produced the ozone layer that filters ultraviolet (UV) radiation.

The Precambrian Atmosphere Hydrogen and helium probably dominated Earth’s earliest atmosphere but probably escaped into space due to their small masses. Formation of the Atmosphere and Oceans Gases that have greater masses, such as carbon dioxide and nitrogen, cannot escape Earth’s gravity. Considerable volcanic activity during the Early Precambrian released tremendous amounts of gases into the atmosphere through the process of outgassing.

The Precambrian Atmosphere The most abundant gases vented from volcanoes are water vapor (H 2 O), carbon dioxide (CO 2 ), nitrogen (N 2 ), and carbon monoxide (CO). Formation of the Atmosphere and Oceans Many geologists hypothesize that outgassing formed Earth’s early atmosphere. In addition, the early atmosphere most likely contained methane (CH 4 ) and ammonia (NH 3 ). Argon (Ar) also began to accumulate during the Early Precambrian.

Oxygen in the Atmosphere There was no oxygen in the atmosphere during the Precambrian. Formation of the Atmosphere and Oceans The oldest known fossils, which are about 3.5 billion years old, are the remains of tiny, threadlike chlorophyll-bearing filaments of cyanobacteria. Ancient cyanobacteria used photosynthesis to produce the nutrients they needed to survive, giving off oxygen as a waste product.

Oxygen in the Atmosphere Oxygen Producers Formation of the Atmosphere and Oceans –The abundance of cyanobacteria increased throughout the Archean until they became truly abundant during the Proterozoic. –Stromatolites, which are large mats and mounds of billions of cyanobacteria, dominated the shallow oceans of the Proterozoic.

Oxygen in the Atmosphere Evidence in the Rocks Formation of the Atmosphere and Oceans –Iron oxides are identified by their red color and provide undeniable evidence of free oxygen in the atmosphere. –Evidence indicates that there was little or no free oxygen in the atmosphere throughout most of the Archean. –Near the end of the Archean and by the beginning of the Proterozoic, photosynthesizing stromatolites in shallow marine water increased oxygen levels in localized areas, which caused banded iron formations to form.

Oxygen in the Atmosphere Evidence in the Rocks Formation of the Atmosphere and Oceans –Banded iron formations are deposits which consist of alternating bands of chert and iron oxides. –Red beds are sedimentary rocks that are younger than 1.8 billion years and rusty red in color. –The presence of red beds in rocks that are Proterozoic and younger is strong evidence that the atmosphere by this time contained free oxygen.

Importance of Oxygen Oxygen is important because most animals require it for respiration and it provides protection against UV radiation from the Sun. Formation of the Atmosphere and Oceans Earth is naturally protected from this radiation by ozone (O 3 ) molecules that are present in the lower part of Earth’s upper atmosphere. Oxygen in Earth’s atmosphere that was produced mainly through photosynthesis also contributes to the ozone layer. Nearly all the oxygen that is present was released into the atmosphere by photosynthesis.

Formation of the Oceans Oceans are thought to have originated largely from the same process of outgassing that formed the atmosphere. Formation of the Atmosphere and Oceans As the early atmosphere and the surface of Earth cooled, the water vapor condensed to form liquid water. During the Archean, tremendous amounts of rain slowly filled the low-lying, basalt-floored basins, thus forming the oceans.

Formation of the Oceans Dissolved minerals made the oceans of the Precambrian salty just as they make the oceans salty today. Formation of the Atmosphere and Oceans A recent hypothesis suggests that some of Earth’s water may have come from the bombardment of microcomets, or small comets made of frozen gas and water.

Oxygen in the Atmosphere Oxygen Causes Change Formation of the Atmosphere and Oceans –The Precambrian began with an oxygen-free atmosphere and simple life-forms. –This oxygen added by cyanobacteria not only enabled new life-forms to evolve, but it also protected Earth’s surface from the Sun’s UV rays. –Oceans formed from abundant water vapor in the atmosphere and possibly from outer space. –Earth was then a hospitable place for new life-forms to inhabit.

–amino acids –hydrothermal vent –prokaryote Objectives Describe the experimental evidence of how life developed on Earth. Distinguish between prokaryotes and eukaryotes. Identify when the first multicellular animals appeared in geologic time. Vocabulary Early Life on Earth –eukaryote –Varangian Glaciation –Ediacara fauna

Origin of Life on Earth Fossil evidence indicates that life existed on Earth about 3.5 billion years ago. Early Life on Earth Earth probably could not have supported life until about 3.9 billion years ago because meteorites were constantly striking its surface. This places the origin of life somewhere between 3.9 and 3.5 billion years ago.

Origin of Life on Earth Experimental Evidence Early Life on Earth –Molecular biologists in the 1920s also suggested that an atmosphere containing abundant ammonia and methane but lacking free oxygen would be an ideal setting for the “primordial soup” in which life may have begun. –Stanley Miller and Harold Urey set up an apparatus that contained a chamber filled with hydrogen, methane, and ammonia to simulate the early atmosphere. –Sparks from tungsten electrodes simulated lightning in the atmosphere.

Origin of Life on Earth Experimental Evidence Early Life on Earth –Their atmospheric chamber was connected to a lower chamber that was designed to catch any particles that condensed in the atmospheric chamber. –Only one week after the start of the experiment, the lower chamber contained organic molecules such as cyanide (CN), formaldehyde (H 2 CO), and four different amino acids. –Amino acids are the building blocks of proteins, the basic substances from which life is built.

Origin of Life on Earth Experimental Evidence Early Life on Earth –Continued experiments showed that 13 of the 20 amino acids known to occur in living things could be formed using the Miller-Urey method. –Further experiments demonstrated that heat, cyanide, and certain clay minerals could cause amino acids to join together in chains like proteins. –Miller and Urey demonstrated that however life first formed, the basic building blocks of life were most likely present on Earth during the Archean.

Origin of Life on Earth The Role of RNA Early Life on Earth –The nucleic acids RNA and DNA are the basic requirements for reproduction, an essential characteristic of life. –In modern organisms, DNA carries the instructions necessary for cells in all living things to function. –RNA ribozymes, unlike DNA, can replicate without the aid of enzymes, and may have been the first replicating molecules on Earth. –An RNA-based world may have been intermediate between an inorganic world and the DNA-based organic world that followed.

Origin of Life on Earth Hydrothermal Vents and the Beginnings of Life Early Life on Earth –Life on Earth may have originated deep in the ocean, near active volcanic seafloor rifts. –Hydrothermal vents are the openings where hot water rises and is expelled from the ocean floor. –All of the energy and nutrients necessary for the origin of life are present at these deep-sea hydrothermal vents. –Some scientists hypothesize that during the Archean, near hydrothermal vents, amino acids joined together on the surfaces of clay minerals to form proteins.

Proterozoic Life The only evidence of life-forms that existed before the Proterozoic is the fossilized remains of unicellular organisms called prokaryotes. Early Life on Earth –A prokaryote is an organism that is composed of a single cell, which does not contain a nucleus and is the simplest kind of cell. –A eukaryote is an organism that is composed of a cell or cells that contain a nucleus.

Proterozoic Life The Varangian Glaciation was a widespread glaciation event that occurred between 800 and 700 million years ago that played a critical role in the extinction of many members of a group of possible eukaryotes, the acritarchs. Early Life on Earth Shortly after the ice retreated toward the poles, 700 million years ago, multicellular organisms first appeared in the fossil record.

Ediacara Fossils Fossils collectively referred to as the Ediacara fauna are the impressions of soft-bodied organisms that were discovered in Late Proterozoic rocks in the Ediacara Hills of southern Australia. Early Life on Earth

Ediacara Fossils It is generally agreed that these fossils represent animals that were composed of different types of eukaryotic cells. Early Life on Earth Scientists are unsure, however, whether the Ediacara fauna are relatives of modern animal groups or whether they were completely different types of organisms. The Ediacara fauna seem to provide fossil evidence of an ancestral stock of complex Proterozoic animals.

Ediacara Fossils Some scientists consider the similarity in shape to animals in other phyla coincidental and that the Ediacara fauna represents a virtual dead end. Early Life on Earth Ediacara fossils have been found in all parts of the world. These organisms seem to have flourished between 670 and 570 million years ago until an apparent mass extinction.