Ozone Protection

The Earth has a layer of ozone in the upper part of the oxygen-filled lower atmosphere. Ozone is made by a reaction that takes place when ultraviolet (UV) radiation from the Sun strikes oxygen molecules. When sunlight strikes oxygen molecules, the molecules are split apart; they recombine with other elements in the atmosphere to form the band of ozone that surrounds the planet. That ozone is

Earth's Changing Environment

Millions of Years Ago (mya)

Paleozoic Mesozoic Cenozoic

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Atmospheric oxygen levels have gradually increased as photosynthetic organisms have flourished.

2 The global climate has alternated between icehouse and hothouse conditions. Such changes probably stem from shifts in the Earth's orbit around the Sun. Major meteorite impacts may have played a role, too.

* Major glaciations

3 Sea levels have changed markedly, partly in response to changing climate but also as a result of tectonic changes such as continental drift. Alterations in ocean currents have, in turn, had widespread effects on climate.

A-G Major marine extinction events. These usually coincide with falls in sea level.

Environmental conditions on Earth change at time scales ranging from decades to eons. Some changes (1) are unidirectional, but most physical and chemical changes oscillate in an irregular fashion (2-3) in response to variables such as levels of volcanism, the drift of the continents, changes in Earth's orbit, and meteorite impacts.

unstable and soon breaks apart, but it is continually replenished at the same time as it is breaking up.

Ozone is a natural filter; it prevents most of the deadly ultraviolet radiation from the Sun from striking the surface of the Earth. Without the ozone layer to shield plants and animals from UV radiation, Earth would be uninhabitable. Atmospheric oxygen levels of the early Paleozoic reached concentrations great enough to form a respectable ozone layer, thus supporting life in the seas and also on the land, where plants and animals were greatly more exposed to direct sunlight.

requirements for living on land

With the greening of the Middle Paleozoic world, the stage was set for the slow movement of some lines of sea organisms onto the land. For life to flourish on land, however, the land-invading organisms had to evolve certain adaptations to make life outside the water practical. Water is in many ways an ideal medium in which to live. The fluid nature of water, particularly seawater, relieves an organism from having to support its full weight. Swimming requires less energy than locomotion on land. Water carries life-giving nutrients, gases, and food particles. The ocean is also less prone to dramatic temperature changes and so provides a protective shield against temporary climate extremes in the outside air. Significant, too, is the role of water in the sexual reproduction of marine organisms. Water enables sexual reproduction to occur reliably by serving as a medium for the dispersal of reproductive molecules.

Compared with life in the water, life on land would appear to be enormously challenging. First, there is gravity to contend with. On land, an organism requires a stronger and more supportive anatomy to maintain its body weight and allow motion. Because they are not immersed in water, terrestrial organisms must develop ways to retain moisture and protect their bodies from drying out. Other formidable challenges to life on land include the need to develop alternative means of respiration without gills, the need to find new ways to feed, and the need to reproduce successfully outside the nurturing environment of water.


This chapter explored the changing geology and climate of the Paleozoic Earth that created environments suitable for life on land.

1. Life on land was not feasible until the end of the Ordovician Period, about 444 million years ago.

2. Some of the key components that make up a successful terrestrial ecosystem are a stable ground cover of soil and plants, an abundance of water, breathable air, and shelter from ultraviolet solar radiation.

3. A craton is the large, tectonically stable interior of a continent that remains fairly intact over geologic time.

4. During the latter part of the Paleozoic Era, the topography of continental cratons became increasingly varied in terms of elevation, climate, and the availability of varied habitats.

5. An important early stage in the development of a habitable terrestrial environment was the formation of a viable ground cover of soil. The first soils were formed during the Ordovi-cian Period and only sparsely covered the mostly barren rock surfaces of dry land.

6. Water is essential for life on land; water works in concert with ground cover to maintain a stable, self-sustaining foundation for the terrestrial ecosystem.

7. Levels of atmospheric oxygen rose steadily during the Paleozoic, creating a more inviting atmosphere for terrestrial life. During the Late Carboniferous Period, atmospheric oxygen levels peaked at an astounding 35 percent, which is 67 percent greater than the level found today.

8. Atmospheric oxygen levels of the early Paleozoic reached concentrations great enough to form a respectable ozone layer; this layer protects life in the sea and especially on land from lethal ultraviolet radiation from the Sun.

9. To live successfully on land, organisms had to evolve adaptations to cope with the greater effects of gravity and to find new ways to breathe, to move about, to take in nourishment, to protect against changing temperatures, and to reproduce.

The First Land Plants

Early Cambrian Earth encompassed two sharply different worlds.

Warm, shallow oceans teemed with the many colors of life, from single-celled algae and bottom-dwelling plants to the first complex metazoans—multicelled organisms—that diversified to occupy every depth and niche of the thriving, sunlit sea. Outside of the nourishing waters of the sea, however, one-third of the planet existed as a colorless, lifeless, rocky domain. The only signs of life on dry land during the Early Cambrian Period were the dark stains left by bacteria and other single-celled organisms washed ashore by the tides and left behind by the receding waters to dry up on the oceans' barren, rocky coasts. The world beyond the coasts was devoid of green. The landscape consisted of one craggy horizon after another, the view never broken by a scrub of ground-covering plants or a patch of trees. No plants existed outside of the water. During the earliest Paleozoic, dry land was a world of unsheltered grayness.

Yet life insisted on migrating to land even though the odds were stacked strikingly against it. While the ocean contained a complete and robust ecosystem, the land had no such habitat in which to sustain life. The biological modifications needed to adapt a waterborne physiology to that required to thrive on dry land were enormous. This chapter traces the development of the first terrestrial plants and animals prior to the move of vertebrates from sea to land. It was a time when the world began to become green and when arthropods were the dominant creatures of the continental coasts and were expanding inland, to the interiors of the continents.

the evolution of early land plants

The first organisms to conquer the land were the green algal ancestors of plants. After being water bound for 500 million years, photosynthetic algae slowly crept onto near-shore rocky surfaces, where they began to adapt anatomical strategies for improving their chances of living outside the ocean. The result was myriad evolutionary changes that led to the domination of plants in all known land environments. Today, there are about 300,000 species of land plants found in terrestrial habitats.

The early evolution of plants is one of the most important in the history of life. From humble, single-celled, water-bound beginnings, plants came ashore and transformed dry land in ways that would dramatically influence the evolution of all other organisms and the ecology of the planet.

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