In the summer that 1909, one American scientist called Charles Doolittle Walcott (Figure 12.15) was in the Rocky mountains of brothers Columbia, Canada. He was a paleontologist, which is a scientist that studies past life top top Earth. That was trying to find fossils. Riding top top horseback, he to be making his means down a mountain trail as soon as he noticed something on the ground. He quit to choose it up. It to be a fossil! He began to dig roughly the area and found even an ext fossils. The fossils the Walcott discovered were of several of the most bizarre organisms anyone had ever seen. Among the organisms preserved in the fossils had a soft body like a worm, 5 eyes, and also a lengthy nose choose a vacuum cleaner hose (Figure 12.16). Many of the fossils were the continues to be of animals that carry out not live today. They are currently extinct, which method that nothing of their sort lives and also that they are gone forever.

You are watching: The _______ era featured the longest periods with no life forms on earth.


Figure 12.16: This bizarre pet with five eyes lived throughout the Cambrian. Fossils of the were uncovered by Charles Walcott.
The organisms in Walcott’s fossils lived throughout a time of geologic history known together the Cambrian. The Cambrian period began about 540 million years ago. It marked the beginning of the Phanerozoic Eon. The also significant the beginning of many brand-new and complicated life forms showing up on Earth. In fact, the hatchet Phanerozoic means “time of well-displayed life”. Us still live now in the Phanerozoic Eon. However, life on earth is an extremely different this particular day than it to be 540 million year ago. This lesson covers several of the history of life top top Earth. It will display you exactly how living points have arisen and changed over the last 540 million year of the Phanerozoic Eon. You will certainly learn around how species adapt and also evolve end time.

Lesson Objectives

Describe just how adaptations develop.Explain exactly how the fossil document shows united state that species evolve over time.Describe the general breakthrough of Earth’s life creates over the critical 540 million years.

Evaluating front Knowledge

Be sure that you can answer the adhering to questions prior to you start this lesson.

What is a fossil?How is geologic time divided?How perform organisms rely on their setting to live?

Earth’s Diversity


There space over 1 million types of plants and also animals well-known to be currently alive on earth (Figure 12.17). Scientists think there room millions more that have not been discovered yet. Look approximately you and also you notification that the organisms on this planet have incredible variation. One of the most remarkable functions of Earth’s organisms is their capability to make it through in their particular environments. Because that example, polar bears have thick hair coats that help them stay warmth in the icy waters that they hunt in (Figure 12.18). Reindeer have sponge-like hoofs that assist them to walk on snowy ground there is no slipping and falling. Plants that live in dried desert environments have special stems and leaves that help them conserve water.

Figure 12.18: many animals, like this polar bear, have actually special body functions that assist them live in a specific environment.
Other organisms have special functions that help them hunt because that food or stop being the food of one more organism. For example, as soon as zebras in a herd operation away from lions, the zebras’ dark stripes confused the lions and also make it hard for them to emphasis on just one zebra during the chase. Hummingbirds have long slim beaks that aid them drink nectar indigenous flowers. Some plants have poisonous or foul-tasting building materials in them the keep animals from eating them.

Adaptations and Evolution

The qualities of an biology that help it endure in a given environment are called adaptations. Adaptations build when particular variations in a population help some members survive better than rather (Figure 12.19). Often the variation originates from a mutation, or a random readjust in one organism’s genes. The persons that survive pass favorable traits on to their offspring.


To help you understand adaptation, think about a populace of oak trees. Imagine that most of the trees are quickly killed by a specific fungus however that every now and also then, over there is one tree that has a natural capacity to make it through the fungus. The one tree screens a variation that gives it a far better chance of surviving its environment. It likewise has a better chance of living to develop seeds and have offspring. It will reproduce and also carry ~ above the species, while the various other trees will certainly die off. The tree with the natural ability to resist the fungus will certainly pass that trait on to its offspring. The various other trees will certainly not live and also have offspring. Ultimately the populace will readjust so that most of the individual trees have actually the characteristics to make it through the fungus. This is one adaptation. Adaptations room inherited traits the an organism it s okay from that is parents. In time traits that aid an organism endure become more common. Traits that hinder survival eventually disappear.

Changes and adaptations in a types accumulate end time. Ultimately the descendants are really different from their ancestors and may become a whole brand-new species. Alters in a species over time are dubbed evolution. Us learn about evolution indigenous the fossil record. It mirrors us that many of the life creates that live today occurred from earlier, different life forms. Because that example, equine fossils display us that about 60 million years back horses were much smaller than they are today (Figure 12.20). Fossils also show united state that horses’ teeth and hooves have adjusted several time as horses have adapted to changes in the environment.


Figure 12.20: The steed has progressed over the critical 60 million years. Equines today room much larger than earlier horses.

Studying the Fossil Record

Like the organisms that were represented in Walcott’s fossils, plenty of of the biology that once lived on earth are currently extinct. Earth’s in its entirety environmental conditions have adjusted many times because the Cambrian, and also many biology did not have actually the characteristics to endure the changes. Those that did make it through the alters passed properties on to your offspring. They offered rise come the varieties that live today.

We examine fossils to learn about how varieties responded to readjust over the Earth’s lengthy history. Fossils show us that basic organisms overcame life on earth for its an initial 3 exchange rate years. Then, between 1 and also 2 billion years ago, the very first multi-cellular organisms appeared on Earth. Life creates gradually evolved and also became much more complex. During the Cambrian period, animals became more diverse and complex. We occasionally refer to this component of the Phanerozoic Eon together the Cambrian Explosion—meaning a time once the earth “exploded” with incredible numbers of new complex life forms.

Phanerozoic Eon

The Phanerozoic Eon is divided into 3 chunks that time dubbed eras—the Paleozoic, the Mesozoic, and also the Cenozoic (Table (12.1). They expectancy from about 540 million years back to the present. Us live now in the Cenozoic Era. The table listed below shows exactly how life has changed during the long span of the Phanerozoic Eon. Notification that different types of organisms occurred at various times. However, every organisms evolved from a typical ancestor. Life progressively became much more diverse and new species branched out from that common ancestor. Most modern-day organisms progressed from species that are currently extinct. To acquire an idea of how an biology can change from a solitary common ancestor to plenty of different types, think about all the different types of dogs. Every dogs progressed from a common wolf ancestor. Today there are hundreds of varieties the dogs the all look very different.

Table 12.1: breakthrough of Life throughout the Phanerozoic EonEraMillions of year AgoMajor creates of Life
Cenozoic0.2 (200,000 years ago)First humans
35First grasses; grasslands begin to overcome the land
Mesozoic130First plants through flowers
150First bird on Earth
200First mammals ~ above Earth
251Age the dinosaurs begins
Paleozoic300First reptiles on Earth
360First amphibians top top Earth
400First insects on Earth
475First plants and also fungi start growing ~ above land
500First fish ~ above Earth

The ages of the Phanerozoic Eon room separated by events referred to as mass extinctions. A massive extinction occurs when huge numbers of organisms end up being extinct in a short amount that time. In between the Paleozoic and also the Mesozoic, almost 95% of all varieties on Earth passed away off. The reason or causes of this extinction room still gift debated.

Between the Mesozoic and the Cenozoic, around 50% of every animal types on Earth passed away off. This massive extinction, 65 million year ago, is the one in which the dinosaurs came to be extinct. Although there are other hypotheses, most scientists think the this mass extinction took location when a giant meteorite struck earth with the power of the most powerful nuclear weapon. The influence kicked increase a substantial dust cloud. When the particles rained earlier onto the surface they boil the environment until it ended up being as hot as a kitchen oven, roasting animals. Dust that remained in the atmosphere blocked sunlight for a year or more, leading to a deep freeze and ending photosynthesis. Sulfur indigenous the impact mixed through water in the setting to kind acid rain, which liquified the shells the the tiny marine plankton that form the basic of the food chain. With small food being developed by land plants and also plankton, animals starved. Carbon dioxide was likewise released indigenous the affect and ultimately caused global warming. Life forms can not make it through the dramatic temperature swings.

Earth’s climate changed numerous times during the Phanerozoic Eon. Just prior to the start of the Phanerozoic, much of the earth was cold and also covered with glaciers (Figure 12.21). As the Phanerozoic began, however, the climate was transforming to a heat and tropical one (Figure 12.22). The glaciers were changed with tropic seas. This permitted the Cambrian explode of many brand-new life forms on Earth. Throughout the Phanerozoic, Earth’s climate has gone v at least 4 significant cycles between times that cold glaciers and also times of warm tropical seas. Part organisms survived environmental transforms in the climate; others ended up being extinct when the climate adjusted beyond their capacity to cope with it.

Figure 12.21: Just prior to the Phanerozoic, plenty of parts of planet were extended with glaciers. After ~ the Earth started to warm and also many of the glaciers melted, there was an to explode of new life on Earth. The glaciers in this snapshot are native the present. However, glaciers are much less usual on planet today than at various other times in Earth’s history.
Figure 12.22: The Phanerozoic Eon to be often characterized by time of warm tropical climates. The age of the dinosaurs was particularly mild for many of the Earth. This enabled plants and also animals to spread over large areas that land. This snapshot shows plants in a contemporary rainforest. Tree of the Phanerozoic may have actually looked similar.

Lesson Summary

Adaptations are favorable traits the organisms inherit. Adaptations develop from variations in ~ a population and help organisms to survive in their given environment.Changes in populaces accumulate over time; this is called evolution.The fossil record shows us that present day life forms developed from earlier different life forms. It reflects us the the an initial organisms on planet were straightforward bacteria that conquered the earth for numerous billion years.Beginning around 540 million years ago more complicated organisms developed on Earth. Throughout the Phanerozoic Eon every one of the plant and animal varieties we understand today have evolved.Many types of organisms that as soon as lived are now extinct. Eart’s all at once environment, particularly the climate, has readjusted many times, and also organisms readjust too end time.

See more: Dramatic Irony In Romeo And Juliet Act 3 Scene 1, The Drama Of Shakespeare: Act 3, Scene 1

Review Questions

Describe what is meant by adaptation.The an initial animals on earth had soft bodies. Gradually many animal species evolved that had hard outer parts referred to as exoskeletons extending their bodies. How might an exoskeleton be a favorable adaptation?Explain why unfavorable traits execute not usually gain passed come offspring.List the bespeak in which the major types of pets appeared top top Earth.How can climate have influenced the capacity of tree to prosper over big areas during a provided time?One cause of fixed extinctions is meteorite or comet impacts. What could be some extr causes of fixed extinctions?


adaptationA trait the an organism inherits that helps it endure in its herbal environment.evolutionThe readjust in an organism’s traits over time such that a new species is frequently the result.glaciersLarge sheets of flowing ice.paleontologistA scientist who research studies Earth’s previous life forms.tropicalA climate the is warm and humid.variationHaving numerous differences.