Chapter 8- MJ Tichich History of Life 3 of 3

Chapter 8- MJ Tichich History of Life 1 of 3

Sec. 8.1 Geologic Time

Scientists believe Earth to be about 4.6 Billion years old.

They believe life did not exist on Earth until about 3.4 billion years ago. The first organisms were believed to be simple cells without nuclei The atmosphere was made of gases poisonous to most living things today The land mass is thought to have been one large piece, that over time broke up into the continents we know today.

Earth’s Timeline A timeline scientists use to show the 4.6 Billion years is a scale of geologic time (ref poster), dividing earth’s geologic time into parts. The largest part is called and era. Eras are measured in millions of years.

There are four eras, but they are not equal in length, one era ends and another begins when important changes happened. Precambrian Era ended 3.5 billion years ago-much longer than the rest- few plant or animal fossils exist from this era During this time fungi and algae evolved, near the end of the era sponges, jellyfish and worms appeared

Paleozoic Era ended 590 million years ago- trilobite and other animals without backbones were common in oceans. Fish evolved, organisms moved onto land about halfway through this era. Reptiles and insects appeared. Paleozoic era ended with a large die off of oceanic life.

Mesozoic Era ended 245 million years ago- this is the time of the dinosaurs- mammals evolved early in the era with birds appearing about halfway through. It is during this era that Pangaea began to break up.

Cenozoic Era ended 66 million years ago- mammals were numerous and evolved into many different species. The horse and camel made an appearance about 37 million years ago. Large grazing animals and whales evolved. 4 times in the last 2 million years, glaciers covered much of the land. During this time first humans appeared.

History Preserved in Rock Scientists piece together Earth’s history by examining the sediments created by erosion from wind and water. Sediments build up layer-by-layer and over time are compressed into rocks called Sedimentary Rocks.

The youngest rocks are on the surface. Relative age of layers of sedimentary rocks can be determined by its position among the other layers. Pg 160 fig 8.2 shows the Grand Canyon whose rocks on the top are 200 million years old at the top and 2000 million years old at the bottom.

Radioactive Dating In radioactive dating, the amounts of two elements in the rock are measured. One is a radioactive element, which changes into another element at a constant rate. The other is the element the first one changed into. When the amounts of the tow elements are compared, a rocks age can be determined. In the oldest rocks, most of the radioactive element has been changed into the other element.

Science and Technology Particles of iron in certain rocks tend to line up with the magnetic field while the rocks are being formed. When the rocks cool, these particles are frozen in place, pointing north. If the rock moves, the iron particles will no longer be pointing north. Using this method, scientists can learn how continents have moved. Chapter 8- MJ Tichich History of Life 2 of 3

8.2 The Fossil Record Scientists use the records left by organisms that once lived to learn about past life on earth. Bones, teeth, shells, and prints from leaves, animal footprints, and outlines from body parts left in rocks are part of life’s record. These remains are called fossils.

Fossils provide evidence that earlier forms of life have dies out, or become extinct. Most individuals left no trace of their existence; it took an unusual condition for fossils to form.

Most fossils are found in rocks that formed in layers of sand, mud, or silt. They show the shape or outlines of the hard parts of the organism, like its shell or bones.

These types of fossils were formed after the organism had died and sediments covered them. The soft parts of the organism decayed and over a long period, the sediments became a layer of rock. Water seeped through the rock and dissolved the bones or shells, leaving an impression called a mold.

May fossils had one added step- minerals carried by the seeping water built up in the mold to form a cast.

Sometimes the soft part of an organism was preserved when it was buried where bacteria could not decay it. Leaves have been found that are millions of years old and still contain chlorophyll. Other organisms have been trapped in tree sap or frozen in ice.

Reading the Fossil Record The fossil record can be used to determine the order in which different forms of life have appeared. The oldest layers of rock where we find reptilian fossils always occur above fossils containing fishes. Based on this observation scientists conclude that fishes appeared on earth before reptiles.

Fossils can also provide clues to how different life forms are related to each other. Imagine fossils of fishes with leg like fins above fossils of fishes and above both of these are fossils of animals that have legs. These animals look like they have lived in the water part of the time. Above these animals, there is a layer of fossilized reptiles. These varied layers support the idea that reptiles evolved from fishes.

Evolutionary Tree From clues provided by fossils, scientists have developed a theory about the likely ancestry of earth’s life forms. (Pg 164) The base shows the earliest life forms with each branch leading in a different evolutionary “direction”.

Other Evidence By viewing the fossil record, scientists can see that life forms have changed over time. Similarities in DNA also show how closely organisms are related.

8.3 Human Evolution Humans belong to the group of mammals called primates. Primates appeared over 65 million years ago as animals adapted to living in trees. Their bodies had several adaptations needed for a life in the trees. The adaptations included hands to grasp branches, and arms with a large range of motion to allow for swinging from branch to branch. Eyes must be able to judge distances accurately for rapid movement. For this to happen the eye must be facing forward with overlapping fields of vision. Chapter 8- MJ Tichich History of Life 3 of 3

After they first appeared the primates evolved in different directions. Some continued to live in the trees (lemurs and monkeys), others adapted to living on the ground. These species evolved into the many species of modern apes, which include chimpanzees, gorillas, and orangutans.

Human Origins Between 4 and 8 million years ago, some primates began to adapt to living entirely on the ground. They evolved to have a skeleton for sanding and walking upright on two legs. These were the ancestors of modern humans. They were the first hominids or human-like primates. (Pg 168) shows picture of 3.6 million year old skeleton found in Africa in 1974.

From other hominid fossils, scientists have determined that sometime between 1.5 and 2 million years ago, a species of Australopithecus developed a larger brain and began using tools. This hominid was the first member of the genus Homo, which includes modern humans.

Neanderthals are the most widely known hominids that were classified as Homo sapiens. They lived between 230,000 and 30,000 years ago. They looked very different from the modern human we see today. They were very short, 1.5 to 1.7 meters. They had large bones and heavy brows. They used tools, fire and made clothes from animal hides. They lived in cooperative social groups and cared for the aged and the sick. They also used language, which allowed them to pass on knowledge from generation to generation. They disappeared about 30,000 years ago.

Cro-Magnons were the first group of modern humans. They appeared about 100,000 years ago, possibly in Africa, later migrating to other areas. They looked similar to modern humans, with smaller teeth, longer faces, and straighter heads. They too used language, creating music and art. More than 200 caves in Europe contain their artwork.

The Piltdown Hoax On December 12, 1912 headlines announce the discovery of the “missing link” between man and ape. The find was named Piltdown Man and was just what scientists were looking for- a primitive human halfway between modern humans and apes. Many accepted the Piltdown Man as real. In 1953, the Piltdown Man was exposed as a forgery, a composition of a modern human skull with an orangutan jaw. The discovery of the forgery shows how science works. It is self-correcting and scientists adjusted their theories accordingly. The current theory states humans and modern apes share a common ancestor.