Directions: With your group, read about the many era’s of Earth. Use the information packets to fill in your outline. Once you have finished, see if you can complete the time-line from the beginning of class.
The Hadean Era And The Formation of The Earth
4.5 to 3.8 Billion Years Ago
If the history of the earth was a clock, the Hadean Time is represented by the first hour and fifty minutes. The nature of this period of the earth's history has generally been believed to be a turbulent time of extreme heat and volatile gravitational collapse. In fact, it got its name from the images we generally hold of Hades, the mythological underground. However, there are scientists that as recently as May, 2005 are presenting new evidence that the earth may have already begun to cool during this time period and that life could have been possible. Stay tuned!
Scientists have generally thought that during the Hadean era the solar system was forming out of a spinning cloud of dust and gases called an accretion disk. At the center of the cloud, heavier particles drew together through gravitational force until nuclear fusion set it ablaze in light and heat. This, of course, was the birth of our sun.
But the solar particles were not the only particles present in the accretion disk. Other particles were lumping together to form microplanets (similar to modern asteroids), larger planetesimals, and the planets of our solar system.
Scientists also believe that the Earth and other planets would have been molten at this stage of development. As the Earth cooled, the heavier molten iron sank into the core, while lighter rock rose to the surface, cooled and became the crust. The oldest known Earth rocks to date are approximately 3.8 billion years old. Meteorites and lunar rocks have been found to be approximately 4.5 billion years old. With the formation of solid rock, Earth’s geological history began.
The Archaean Era: A Cooling Crust
3.8 to 2.5 Billion Years Ago
The Archaean period was a time of continent-building and the first stages of early life. In fact, 70% of our continental land masses are formed around cores of rock, or shields, that date from this period.
With this said, let’s take a look at the various conditions that may have been in existence during this time period.
First, the atmosphere would not have been like it is today. It would have had no oxygen. Instead, it would have been filled with: Hydrogen, Methane, and Ammonia. This kind of atmosphere is called a reducing atmosphere. It is just the kind of atmosphere that could support the organic chemistry for first life. In fact, there is fossil evidence of ancient bacteria.
One type of bacteria present then were the cyanobacteria or blue-green algaes. These bacteria appear to have had a very strong cell-wall and the ability to form layers in the ancient sediments. The formations are called stromatolites. They can be found in Archaean rock formations of Western Australia.
The very oldest rocks of the Archaean are very rare, most likely because they have been changed by the pressure created by many layers or even “recycled” by being pushed so deep as to be returned to a molten state.
But the younger rocks can still be found in South Africa, Western Australia, Canada and India. These rock shields give us clues to the formation of our continents.
The forming of the continents during the Archaean probably began as lava flow under the ancient oceans. The youngest of the Archaean rock layers look like giant pillows of lava and resemble underwater lava flows from modern times.
Based on this resemblance, it seems likely that most of the continents were covered by water during the Archaean time, roughly 3 billion to 2.5 billion years ago. One can look at the Hawaiian Islands, yet underwater, to get a glimpse of modern pillow lavas.
While the Cambrian Period with its explosion of life generally marks the beginning of the Paleozoic Era and life on our planet, there are many indications that the preparation for life and the earliest forms of it all got going during the Archaean Period.
Before life forms could evolve, there had to have been chemical transformations to set the stage. The now-famous experiment performed by Miller and Urey in 1953 showed how an atmosphere of: methane, carbon dioxide, and hydrogen, exposed to heat and electrical charges would have produced organic molecules such as amino acids and simple sugars.
Even though this experiment has been questioned and refuted, many modern biochemists agree that such simultaneous events could have been the impetus behind our first life on Earth.
Amino acids would have gradually come together into increasingly more complex molecules.
The earliest life forms were anaerobic, forms that did not use oxygen to exist. In our modern world, anaerobic organisms are those that work in the process of fermentation.
These organisms lived on consumable organic matter or on other anerobic life forms. Thus they were heterotrophs, like modern fungi, that feast on organic material. The ancient heterotrophs, like fungi, were dependent on the presence of organic materials and would have eventually perished once the organic material had all been consumed.
But autotrophs appeared, saving the day! These new organisms fed on the pure energy source of the sun. They were the predecessors of our modern blue-green algae and were the cyanobacteria that formed the stromatolites.
The Proterozoic Era:
Life Gets More Complex
2.5 Billion To 543 Million Years Ago
The Proterozoic Era would have been an exciting time to be an observer of life development on planet Earth. The fiery formation processes of the Hadean and the undersea continent-building of the Archaen, now were replaced by the process of tectonics.
The plates rested on a very different magma than our modern-day plates. The plates themselves were younger and thinner (funny how that works!!!) and the magma was hotter. This would have made the magma more liquid than today, so likely the continental movement would have been faster, with collisions and fractures more frequent. A single super-continent formed. Today it is called Rodinia.
At the center of Rodinia is a baby North America called Laurentia. Its western border lies next to the infants that would grow into Australia and Antarctica , while the eastern coast is next to western Africa.
The first multi-cell organisms developed during this period. The early life that formed in the Archaean, especially the autotrophs: cyanobacteria and early plants, developed into a new type of cell as a consequence of the oxygen-rich atmosphere they had created. This cell was the eukaryote, a cell that contains a nucleus. With the onset of the eukaryote, living organisms were able to join together into groups of eukaryotic cells. Toward the end of the Proterozoic, multi-cellular algae and the first multi-celled animals were the result.
The Paleozoic Era:
The Beginning Of An Explosion Of Life Forms
The Paleozoic Era is the beginning of an explosion of life forms. The Cambrian Explosion marks the era with thousands of new life forms in the ancient seas. The first fish appear, although there is still no life on land.
Life In The Seas
It is this characteristic, life in the ancient sea, which distinguishes the Paleozoic Era from all others. You see, the autotroph’s makeup didn’t just add oxygen to the atmosphere. It was an entirely new sort of cell structure: one that had a cell membrane or wall (or both) that enclosed a central nucleus. Today we call this a eukaryotic cell.
These new cells could now join together to do specialized tasks, the kind of specialization needed to make the tissues of plants and animals. Now species could exist that responded to the diversity of their watery environments. Some would live on the bottom sediments, burrowing with tube-shaped bodies or walking with many tiny legs. Others would live in the ocean column, developing fins for effective swimming..
New Phyla (categories of plants and animals)
Many phyla were represented in the Ordovician part of the Paleozoic Era. In fact all of the phylum in today’s world were represented during the Paleozoic Era. Some species that began in the Ordovician Period are: mollusks, corals, worms, primitive fish, and starfish.
Life Moves Onto Land
Plants and animals first moved onto land in the Silurian Period. But they didn't really flourish there until the Carboniferous Period when huge forests covered the land. Fish learn to adapt to live in rivers and streams. These forests were so dense and covered so much of the earth that oxygen was 35% of the atmosphere compared to present day levels of 21%.
It is this high oxygen content that is believed to be the cause of the gigantic arthropods that lived in these forests. A dragonfly called Meganeura had a wingspan 70 cm! The centipede like Arthropleura could reach a length of 1.8 meters and giant scorpions grew over 50 cm long. Of course there were an abundance of insects of sizes we would find unremarkable as well.
The high oxygen content may have made it easier for the amphibians that also left the water during the carboniferous period. During the carboniferous period, reptiles appear on land. These four legged creatures called tetrapods lived at the waters edge. The tetrapods became very successful evolving many different forms some of which grew quite large over 2 meters in length! They became the dominant life forms in the lakes, rivers and swamps of the Carboniferous Period. The first reptiles showed up late in the Carboniferous Period.
Life on our planet wasn’t exactly like that of today, but the similarities had arrived. Earth’s miracle of life had begun and was taking hold in ancient seas of the Paleozoic.
Near the end of the Paleozoic Era, during the Permian period, was an event called the Permian mass extinction. At this time about 95% of all living things on earth became extinct!
The Mesozoic Era:
The Age of Dinosaurs
The Mesozoic Era begins where the upheavals of the Permian Extinctions end. A mass extinction at the end of the Permian Period had eliminated most of the species of life that had existed throughout the Paleozoic Era. Sometimes called the Age of Dinosaurs because this era becomes dominated by dinosaurs and reptiles. During the Triassic Period, dinosaurs, small mammals, and crocodiles appear.
Toward the end of the Paleozoic Era the land that would become Europe and Asia slammed into North America. By the time of the Mesozoic Era Pangea the super continent had formed. It was roughly the shape of a “C”. The huge land mass protected the Tethys Ocean which lay across tropical latitudes. Pangea and the Tethys were ringed by the Panthalassic Ocean. The shifting plates allowed for the opening of the Atlantic Ocean 130 million years ago.
Climate During The Mesozoic
The temperatures, both on land and in the ocean, were much higher than during the Paleozoic, and climates were more tropical in nature. Despite this, the seas were lower, leaving different types of land masses for life to deal with. Over all the Mesozoic Era was dryer than in the Paleozoic Era. There were more deserts and less marshland.
Within the three periods of the Mesozoic Era ( Triassic, Jurassic and Cretaceous) there were times of wide temperature and seasonal variation.
Life Recovers From The Permian Extinctions
It took most of the first and second periods of the Mesozoic, the Triassic and the Jurassic periods, for the diversity of species to recover and achieve some balance. While plant species had survived somewhat better than animals over the Permian Extinction, new types of plants developed to survive the changing conditions.
The warmer drier conditions of the Mesozoic required new reproductive methods in plants. During the Jurassic period, the most common land plants are ferns, palm like trees called cycads, and grasses. Their reproductive methods allowed for good protection of the spores or seeds that would have to get through periods of drought before growing into the infant plant.
The survivors of the Permian Extinction had very little competition. Corals, mollusks and fish dominated the life in the oceans. Some Reptiles took to the water to become the first air breathing hunters in the oceans.
The Rise of The Reptiles and Dinosaurs
The animals that developed in the Mesozoic needed new body types to survive the extremes of temperature and moisture. Amphibians developed respiratory mechanisms that allowed them to live in or out of the water for extended periods of time. But it was the reptiles that were better adapted to the warmer dryer conditions. They developed thick, leathery skin on both their own bodies and their eggs. The reptiles thrived, dominating the landscape in both size and numbers. The dinosaurs evolved from these reptiles. During the Jurassic and Cretaceous Periods the dinosaurs ruled the earth.
Both plants and animals reached giant proportions during the Mesozoic. During the 180 million years of the Era, reptiles lived on land, in seas, and in the air. Small mammals, although not significant during the time, did exist during this era.
Mass Extinction Ends The Mesozoic Era
Another mass extinction occurred at the end of the Cretaceous Period, bringing an end to the dinosaurs and the tropical forests. This extinction, while not as broad and devastating as that at the end of the Permian, had the effect of eliminating a way of life that has not been replicated. Most researchers agree that the Mesozoic Era ended at least in part due to the impact of an asteroid.
The Cenozoic Era:
Age of Mammals
The Cenozoic Era is the last and most recent of the geologic periods. Its name means “new life” coming from the Greek root kainos, meaning “new,” and zoic, “life.” While this new life came to refer to mammals-thus coined The Age of Mammals- this new life could have just as easily been the angiosperm or flowering plants, the insects, the newest fish (teleostei) or modern birds. All of these groups, including the mammals, continued to evolve during this era.
The Rise of The Mammals
During the Cenozoic, mammals evolved from their somewhat insignificant stature during the Mesozoic to include giant species that have gone extinct in modern times. While none of the mammals ever reached the size of the dinosaurs, there were some species that dwarfed their modern-day relatives. Everyone knows about mammoths, but during the Cenozoic Era some birds stood 7-feet tall. There were beavers 7 feet long! These creatures were typical of the growth achieved by the “new life” in the early Cenozoic. During the Paleogene Period rodents, primates, pigs, cats, dogs, bears, and whales appear
Flowering plants or angiosperms were widespread in the Cenozoic Era. This was beneficial to insects, many of which evolved symbiotic relationships with flowering plants.
The Continents Move
During this time, the continents continued the separation that had begun at the end of the Mesozoic Era during the Cretaceous Period. The Atlantic Rift was widening and forcing more continental separation, in particular Greenland from Europe. Other ocean spreading rifts caused the separation of Australia from Antarctica and Africa from India. The supercontinents of Gondwanaland and Laurasia that had been the result of tectonic movement during the Mesozoic, were now transforming into the continents of modern day.
Volcanic Activity Builds Mountains
The rifts that occurred around the globe resulted in volcanic activity that formed mountain ranges. The Cascade Range that extends along the coast of North America from British Columbia to California is one example of this rifting/volcanic activity. Volcanic activity in Europe, Asia and Africa resulted in the formation of the Himalayan and Alpine mountain systems about 50 million years ago.
The Cenozoic Era: Time Marches On
And so the Cenozoic era continues. During the Neogene period, the first hominids, or early humans, appear. This is the era we live in, though we could hardly say this is the era of humans. We have been present as a species only about 1.5 million years of the 65 million of the current era. That represents about 7 seconds on the clock of eras!