Skip to main content

Geologic Time

Geologic Time
The Earth is about 4.5 billion years old. Fossil evidence suggests that the first living things were simple cells that appeared about 3.5 billion years ago. The time line in Table above shows that the period of time from the beginning of Earth to 700 million years ago, the largest part of the Earth’s past, is known as the Precambrian era. 

The Paleozoic era began about 570 million years ago and lasted until 280 million years ago. Fish, insects, amphibians, and reptiles were some of the major groups of animals that developed in this period. Both terrestrial and aquatic plants also formed in this time span. The Mesozoic era extended from 250 million years ago until 135 million years ago. A period dominated by reptiles, the Mesozoic is known as the age of the dinosaur. Late in the era, mammals and birds developed. The most recent period, the Cenozoic era, began 65 million years ago and extends to the present. During this time, birds and mammals flourished. Humans made their appearance late in the era, about 3 million years ago.

The geologic time scale shows significant events in the development of life on Earth.
The geologic time scale shows
significant events in the development of life on Earth.
To visualize the amount of time that has passed since the first coral reef appeared on Earth 2 billion years ago, one can compare time to a human’s walking stride. For example, a person’s stride, a distance of about 3 feet (0.9 m), could represent a period of 50 years. In such an analogy, walking two steps back would take one back a century in time. The distance of 40 steps would represent the time that has passed since the birth of Jesus (the beginning of the Christian era [C.E.]), and 200 strides would bring one to human’s prehistoric period. Yet, to reach the time when reefs were first formed on Earth, one must walk a distance equal to the Earth’s circumference at the equator (24,902 miles [40,076 km], or 43,827,520 strides)!

Popular posts from this blog

Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the animal species are equipped with a hard, outer covering called an exoskeleton. The functions of exoskeletons are similar to those of other types of skeletal systems. Like the internal skeletons (endoskeletons) of amphibians, reptiles, birds, and mammals, exoskeletons support the tissues and give shape to the bodies of invertebrates. Exoskeletons offer some other advantages. Serving as a suit of armor, they are excellent protection against predators. Also, because they completely cover an animal’s tissues, exoskeletons prevent them from drying out. In addition, exoskeletons serve as points of attachment for muscles, providing animals with more leverage and mechanical advantage than an endoskeleton can offer. That is why a tiny shrimp can cut a fish in half with its claw or lift an object 50 times heavier than its own body.
Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them …

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea.

Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows)…