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12.4: History of Earth’s Complex Life Forms

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

  • Describe how adaptations develop.
  • Describe the general development of Earth’s life forms over the last 540 million years.


  • adaptation
  • adaptive radiation
  • amniote egg
  • evolution
  • mutation
  • natural selection
  • paleontologist
  • tropical
  • variation


Organisms must adapt to their environment or they will die out. Most of the fossils are the remains of animals that are now extinct. The mechanism for change in a population of organisms is natural selection.

Adaptation and Evolution

The characteristics of an organism that help it to survive in a given environment are called adaptations. Adaptations are traits that an organism inherits from its parents. Within a population of organisms are genes coding for a certain number of traits; for example, a human population may have genes for eyes that are blue, green, hazel, or brown, but as far as we know, not purple or lime green.




An explanation of how adaptations develop.


This animation begins with the Big Bang, which will be discussed in the chapter about the solar system, and goes through the history of life on Earth: http://www.johnkyrk.com/evolution.html.

Ediacara Fauna

Although the explosion in the number and type of life forms did not come until the beginning of the Cambrian, life at the end of the Precambrian became more complex. Paleontologists find worldwide evidence of a group of extremely diverse multicellular organisms toward the end of the Precambrian (580-542 Ma). The Ediacara Fauna have a variety of forms of symmetry, range from soft to rigid, and they take the form of discs, bags, or even “quilted mattresses” (Figure below). The organisms seem to have appeared as Earth defrosted from its worldwide glaciation.

An example of an Ediacara organism.



  • Atmospheric oxygen was limited so complex organisms could not evolve.
  • The planet was too cold for complex life.

Scientists also do not know why the Ediacaran biota died out. Some possibilities include:

  • The evolution of predators with skeletons in the Cambrian.
  • Competition from more advanced Cambrian organisms.
  • Changes in environmental conditions caused by supercontinent breakups such as rising sea level, limited nutrients, or changing atmospheric and oceanic chemistry.

The existence of the Ediacaran biota does show that a diversity of life forms existed before the Cambrian.

Phanerozoic Eon

The Phanerozoic Eon is divided into three eras—the Paleozoic, the Mesozoic, and the Cenozoic--spanning from about 540 million years ago to the present (Table below). Life has undergone fantastic changes during the long span of the Phanerozoic Eon.

Notice that different types of organisms developed at different times.

Development of Life During the Phanerozoic Eon
Era Millions of Years Ago Major Forms of Life
Cenozoic 0.2 (200,000 years ago) First humans
35 First grasses; grasslands begin to dominate the land
Mesozoic 130 First plants with flowers
150 First birds on Earth
200 First mammals on Earth
251 Age of dinosaurs begins
Paleozoic 300 First reptiles on Earth
360 First amphibians on Earth
400 First insects on Earth
475 First plants and fungi begin growing on land
500 First fish on Earth

Extinction and Radiation

The eras of the Phanerozoic Eon are separated by mass extinctions. A mass extinction occurs when large numbers of species become extinct in a short amount of time. The causes of different mass extinctions are different: collisions with comets or asteroids, massive volcanic eruptions, or rapidly changing climate are all possible causes of some of these disasters (Figure below).

An extinct Tyrannosaurus rex. This fossil resembling a living organism.

After a mass extinction, many habitats are no longer inhabited by organisms because they have gone extinct. A change in the environment from one in which organisms live in all the available habitats to one in which many habitats are available gives an advantage to organisms that can adapt to new environments. Evolutionary processes act rapidly during these times and many new species evolve to fill those available habitats. The process in which many new species evolve in a short period of time to fill available niches is called adaptive radiation.

Paleozoic Life

The Cambrian began with a tremendous diversification of life forms. Shallow seas covered the lands so every major marine organism group evolved during this time. With the evolution of hard body parts, fossils are much more abundant and better preserved from this period than from the Precambrian.

The Burgess Shale formation in the Rocky Mountains of British Columbia, Canada, contains an amazing diversity of middle Cambrian life forms, from about 505 million years ago. One organism had a soft body like a worm, five eyes, and a long nose like a vacuum cleaner hose (Figure below). Paleontologists do not agree on whether the Burgess Shale fossils can all be classified into modern groups of organisms or whether many represent lines that have gone completely extinct.

This bizarre animal with five eyes lived during the Cambrian, which began about 540 million years ago.

Throughout the Paleozoic, seas transgressed and regressed. When continental areas were covered with shallow seas, the number and diversity of marine organisms increased. During regressions the number shrank. Large extinction events separate the periods of the Paleozoic. After extinctions, new life forms evolved (Figure below). For example, after the extinction at the end of the Ordovician, fish and the first tetrapod animals appeared. Tetrapods are four legged vertebrates, but the earliest ones did not leave shallow, brackish water.

Trilobites were shallow marine animals that flourished during the lower Paleozoic.



The largest mass extinction in Earth history occurred at the end of the Permian period, about 250 million years ago. In this catastrophe, it is estimated that more than 95% of marine species on Earth went extinct. Marine species with calcium carbonate shells and skeletons suffered worst. About 70% of terrestrial vertebrates (land organisms) also went extinct. This was also the only known mass extinction of insects.


Mesozoic Life

At the beginning of the Mesozoic, Pangaea began to break apart so more beaches and continental shelf areas were available for colonization. Climate during the entire era was warm and tropical. With many niches available after the mass extinction, a great diversity of organisms evolved.

Tiny marine plants called phytoplankton arose to become the base of the marine food web. On land, seed plants and trees diversified. Ferns were common at the time of the dinosaurs (Figure below). The earliest known fossil of a flowering plant is from the Cretaceous, 125 million years old.

Fern fossil.

Mammals appeared near the end of the Triassic but the Mesozoic is known as the age of the reptiles. In a great advance over amphibians, which must live near water, reptiles developed adaptations for living away from water. Their thick skin keeps them from drying out and the evolution of the amniotic egg allowed them to lay their eggs on dry land. The amniote egg has a shell and contains all the nutrients and water required for the developing embryo.

Of course the most famous Mesozoic reptiles were the dinosaurs. Dinosaurs reigned for 160 million years and had tremendous numbers and diversity. Species of dinosaurs filled all the niches that are currently filled by mammals. Dinosaurs were plant eaters, meat eaters, bipedal, quadrupedal, endothermic (warm-blooded), exothermic (cold-blooded), enormous, small, and some could swim or fly. Although nearly all species of dinosaurs went extinct, modern birds evolved from theropod dinosaurs (Figure below).

Archeopteryx, the earliest known bird, lived during the late Jurassic.

Between the Mesozoic and the Cenozoic, 65 million years ago, about 50% of all animal species, including the dinosaurs, became extinct. Although there are other hypotheses, most scientists think that this mass extinction took place when a giant meteorite struck Earth with the energy of the most powerful nuclear weapon (Figure below).

An artist’s painting of the impact that caused the Cretaceous extinctions.

The impact kicked up a massive dust cloud and when the particles rained back onto the surface they heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust that remained in the atmosphere blocked sunlight for a year or more, causing a deep freeze and temporarily ending photosynthesis. Sulfur from the impact mixed with water in the atmosphere to form acid rain, which dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced by land plants and plankton, animals starved. Carbon dioxide was also released from the impact and eventually caused global warming. Life forms could not survive the dramatic temperature swings.

Asteroid impacts have profoundly affected earth history from the very beginning, by bringing in water and amino acids for the oceans, atmosphere, and life, and by forming the Moon. Mass extinctions that have occurred throughout Earth history may also have been caused by asteroid impacts. The best known is the impact that brought about the extinction of the dinosaurs (1f):

http://www.youtube.com/watch?v=z2CnH_0V5_I (2:01), http://www.youtube.com/watch?v=uEFYkOh3YYA (1:23).

Cenozoic Life

The extinction of so many species again left many niches available to be filled. Although we call the Cenozoic the age of mammals, birds are more common and more diverse.

The adaptations allow mammals to spread to even more environments than reptiles because mammals are endothermic and have fur, hair, or blubber for warmth. Mammals can swim, fly, and live in nearly all terrestrial environments. Mammals initially filled the forests that covered many early Cenozoic lands. Over time, the forests gave way to grasslands, which created more niches for mammals to fill.

As climate cooled during the ice ages, large mammals were able to stand the cold weather and so many interesting megafauna developed (Figure below).


The Evolution of Life in 60 Seconds scales all 4.6 billion years of Earth history into one minute. Don’t blink at the end (1i - I&E Stand.): http://www.youtube.com/watch?v=YXSEyttblMI (1:03).

The saber-tooth cat lived during the Pleistocene.

Humans also evolved during the later Cenozoic. Bipedal primates first appeared about 6 million years ago when grasslands were common. Standing on two feet allows an organism to see and also to use its hands and arms for hunting (Figure below). The brain size of this bipedal primate grew rapidly.


The genus Homo appeared about 2 million years ago. Humans developed tools and cultures (Figure below). Homo sapiens, our species, originated about 200,000 years ago in Africa.


The ice ages allowed humans to migrate. During the ice ages water was frozen in glaciers and so land bridges such as the Bering Strait allowed humans to walk from the old world to the new world.

Modern Biodiversity

There are more than 1 million species of plants and animals known to be currently alive on Earth (Figure below) and many millions more that have not been discovered yet. The tremendous variety of creatures is due to the tremendous numbers of environments (Figure below).

There is an amazing diversity of organisms on Earth. How do the organisms in this picture each make their living?

Polar bears have thick fur coats to stay warm as they hunt in icy waters.



Other adaptations help an organism move or gather food. Reindeer have sponge-like hoofs that help them walk on snowy ground without slipping and falling. Hummingbirds have long thin beaks that help them drink nectar from flowers. Organisms have special features that help them avoid being eaten. When a herd of zebras run away from lions, the zebras’ dark stripes confuse the predators so that they have difficulty focusing on just one zebra during the chase. Some plants have poisonous or foul-tasting substances in them that keep animals from eating them. Their brightly colored flowers serve as a warning.

Lesson Summary

  • The fossil record shows us that present day life forms evolved from earlier life forms.
  • Many types of organisms that once lived are now extinct. Earth’s overall environment, especially the climate, has changed many times, and organisms change over time, too.

Review Questions

1. Describe how an adaptation comes about.

2. What is evolution? How is natural selection the mechanism for evolution?

3. How might a hard external skeleton, called an exoskeleton, be a favorable adaptation for the soft bodied organisms that had lived before?

4. Explain why unfavorable traits usually do not get passed to offspring.

5. Why did it take 4 billion years of Earth history for multicellular organisms to evolve and diversify to the point of the Ediacara biota?

6. List the order in which the major types of animals appeared on Earth.


8. One cause of mass extinctions is a meteorite or comet impact. What might be some additional causes of mass extinctions?

9. What happens immediately after a mass extinction to the diversity of organisms? What happens thousands or millions of years later?

10. Describe the big advance reptiles had over amphibians.

11. Why are there so many different species on Earth today?

Points to Consider

  • How did climate affect the evolution of life?
  • Evolution is well documented in the fossil record. Why is it so controversial?

For Table above, from top to bottom:

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