Diversity And Unity Of Life

The diversity, or variety, of life, is amazing. For example, there are single-celled organisms that thrive inside thick Antarctic ice that never thaws. There are whales that contain about 1,000 trillion cells that can easily cruise the Pacific and migrate each year from Alaska to Mexico. There are even plants that can capture and eat insects. Biologists have identified more than 1.5 million species on Earth. And there may be many more species that remain to be identified.

Unity in the Diversity of Life

Life is so diverse, yet life is also characterized by unity, or features that all living things have in common. One feature is the genetic code, the rules that govern how cells use the hereditary information in DNA. Another unifying feature is the presence of organelles that carry out all cellular activities.

The "tree of life" shown in Figure 1-5 is a model of the relationships by ancestry among organisms. All living things share certain genes, yet no two types of organisms have the same full sets of genes. One way biologists build a "tree of life" is to place organisms that have more similar sets of genes on closer branches, or lineages, of the "tree." They place the more distantly related organisms on more distant branches. The placement of all kinds of organisms produces a "tree" that relates and unites life's diversity.

Fungi

Plants

Animals j

So, how does the "tree of life" represent the unity in the diversity of life? Scientists think that all living things have descended with modification from a single common ancestor. Thus, all of life is connected. Yet, there are many different lineages representing different species. This diversity stems from the fact that genetic changes accumulate over the years. Also, organisms change as they become suited to their own special environments.

Three Domains of Life

Notice in Figure 1-5 that the "tree" has three main branches. Biologists call these major subdivisions of all organisms domains. The three domains are Bacteria, Archaea, and Eukarya. Bacteria and Archaea have less complex cells than those of Eukarya. Later chapters describe these domains in more detail. Notice that the largest and most familiar organisms, the animals, plants, and fungi, occupy parts of the Eukarya branch of the "tree."

Another system of grouping organisms divides all life into six major categories called kingdoms. The six kingdoms consist of four kingdoms within the domain Eukarya (the Kingdoms Animalia, Plantae, Fungi, and Protista), one kingdom in the domain Archaea (Kingdom Archaea) and one kingdom in the domain Bacteria (Kingdom Bacteria). Many biologists recognize these six kingdoms and three domains, but some biologists use other systems of grouping.

Word Roots and Origins ecosystem from the Greek word, eco, meaning "house," and system, meaning "ordered parts in a whole"

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