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Unit 9: Biodiversity Decline // Section 3: Counting Species

As discussed in Unit 1, "Many Planets, One Earth," life first appeared on Earth as early as 3.8 billion years ago. The earliest life forms were single-celled bacteria and archaea that harvested energy through chemical reactions before free oxygen began to accumulate in Earth's atmosphere. Early photosynthetic bacteria appeared about 3.5 billion years ago, but several billion years passed before multicellular organisms developed. This step took place around 600 million years ago, when Earth's atmosphere and oceans were accumulating increasing amounts of oxygen.

Early life forms were limited to the oceans until plants and animals evolved to live on land about 400 million years ago. Life on Earth became increasingly diverse as organisms adapted to environments on land, despite several waves of mass extinctions (Fig. 3).

Time table of the evolution of complex life forms on earth

Figure 3. Time table of the evolution of complex life forms on earth
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Source: Dennis O'Neil/

How do scientists estimate past and current numbers of species? Fossil records are key sources. By looking carefully at fossil records, scientists can use plant and animal fossils to trace species' evolution over time, estimate rates of speciation (the formation of new biological species), and assess how various organisms responded to known environmental changes in the past (footnote 4). It is important to note that fossil records are imperfect: not all organisms leave recognizable, well-preserved skeletons, so some species are easier to count than others. As a result, it is very difficult to make precise estimates of the number of species on earth at specific points in time, but the records do indicate trends in biodiversity levels.

Based on analyses of fossils, scientists estimate that marine biodiversity today is about twice the average level that existed over the past 600 million years, and that biodiversity among terrestrial organisms is about twice the average since life adapted to land about 440 million years ago. Fossil records also indicate that on average species exist for about 5 to 10 million years, which corresponds to an extinction rate of 0.1 to 1 species per million species-years (footnote 5).

Molecular phylogenetics is a newer tool for studying biodiversity. By measuring the degree of similarity between DNA, RNA, and proteins in the cells of closely related organisms, scientists can reconstruct these organisms' evolutionary histories and see how species are formed. For example, although it was long believed that fungi were closely related to plants, genetic analyses led by Mitchell Sogin of the Marine Biological Laboratory in Woods Hole, Massachusetts, have shown that fungi are more directly related to animals (footnote 6).

Our current understanding of biodiversity is uneven: for example, we know more about animals than we do about protists. As discussed in Unit 1, "Many Planets, One Earth," biologists classify life on Earth into three broad domains: Eukarya, Bacteria, and Archaea. The first group includes all animals and plants, as well as fungi and protists, while the latter two groups comprise different types of microbes (Fig. 4). None of these groups are descended from the others, and all have unique features.

Three domains of life

Figure 4. Three domains of life
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Source: National Aeronautics and Space Administration.

The biodiversity of Bacteria and Archaea is poorly understood for several reasons. Scientists do not agree on how to define a species of bacterium and do not have accurate estimates of the total number of bacterial individuals. From a practical standpoint, counting species of bacteria and other microbes is harder than counting bird species because birds have more defining features, including color, songs, and shapes. Visual and behavioral differences are clearer to the human observer. And many microbes are found in extremely challenging habitats, such as vents in the ocean floor.

Finally, humans tend to see animal and plant species as more interesting and important than other, smaller life forms. A discovery of a new monkey species is apt to become major science news, but new species of fungi are routinely reported in specialized journals without attracting serious popular interest. And the scientific literature focuses heavily on mammals and birds at the expense of other groups such as invertebrates and insects (footnote 7).

Based on fossil records and the opinions of experts who study various groups of living organisms, biodiversity on Earth appears to be at a historical peak today. Some 1.5 million species have been identified and described, but estimates suggest that at least another 5 to 15 million species remain to be catalogued (footnote 8). Insects and microorganisms are thought to account for large shares of these uncounted species. In July 2006, scientists involved in the Census of Marine Life (a ten-year project to measure ocean biodiversity) stated that there could be as many as five to ten million different types of bacteria in the oceans, some 10 to 100 times more than previously estimated. Many of these species may exist in relatively low numbers, but could have important ecological functions (footnote 9).

"Just as scientists have discovered through ever more powerful telescopes that stars number in the billions, we are learning through DNA technologies that the number of marine organisms invisible to the eye exceeds all expectations and their diversity is much greater than we could have imagined."

Mitchell L. Sogin, Marine Biological Laboratory (Woods Hole, MA)

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