Invertebrate

V

Evolution

As the simplest animals, invertebrates date back to the time when animal life first began in ancient shallow seas. Zoologists are uncertain when this was, because the first invertebrates were small and soft-bodied and left no direct fossil remains. However, some scientists believe that strange patterns preserved in sedimentary rocks dating back to 1 billion years ago may be the fossilized tracks and burrows of ancient invertebrates. Other scientists, studying genetic material in living animals, believe that the earliest invertebrates may have appeared even earlier and may already have begun to separate into different phyla before 1 billion years ago.

The oldest recognized fossils of invertebrates date back to the close of the Precambrian period, about 550 million years ago. The best known of these fossil finds, from the Ediacaran Hills in southern Australia, include animals that look like jellyfish and annelid worms. Zoologists disagree about their status. Some think that they might well be ancestors of animals alive today, but others believe they belong to a group of invertebrates that eventually became extinct (see Ediacaran Fauna).

With the start of the Cambrian period 542 million years ago, invertebrate life evolved with almost explosive speed. Due to the appearance of the first invertebrates with exoskeletons, the fossil record provides a rich record of invertebrate life in the Cambrian period. By the time the Cambrian period ended 488 million years ago, all the invertebrate phyla alive today were established.

Between that time and the present, invertebrates spread through the seas and also invaded land. Scientists believe that the first land dwellers were almost certainly arthropods, including the forerunners of wingless insects. During the Carboniferous period, which began 359 million years ago, flying insects appeared, including giant dragonflies with a wingspan of up to 75 cm (30 in). But on land the great expansion of invertebrate life occurred during the Cretaceous period, which started 145 million years ago. Flowering plants first evolved in this period, enabling insects to exploit a whole new source of food and triggering a huge growth in insect life that has continued to this day.

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While many invertebrates flourished, some of the most successful groups of invertebrates in the fossil record nonetheless became extinct. Giant sea scorpions and trilobites were types of arthropods that thrived for much of the Paleozoic era, about 270 million years ago, but were unable to survive the great mass extinction at the end of the Permian period 251 million years ago. Ammonites (mollusks related to today’s octopuses and squids) fared better. They first appeared during the Silurian period about 440 million years ago and lived into the Mesozoic era, only to vanish at the same time as the dinosaurs, about 65 million years ago. Their intricate massive spiral shells were often superbly preserved as fossils, some measuring almost 2 m (7 ft) across.

VI

Importance of Invertebrates

The continued prominence of invertebrates, measured by their great diversity and abundance, indicates that these animals have adapted to their ecosystems over millions of years. In so doing, invertebrates have become necessary to the health of Earth’s ecology. For instance, all ecosystems support one or more food chains that form food webs. Each chain begins with plants, known as primary producers, which convert light energy into food. Primary producers are eaten by primary consumers, and secondary consumers eat the plant-eating primary consumers. Decomposers derive their energy from the dead remains of plants and animals. Invertebrates occupy several niches in this food web, acting as primary consumers, secondary consumers, and decomposers.

Many invertebrates have a direct and invaluable impact on their environment. For example, the common earthworm burrows deep below the surface, consuming soil along the way. Coiled soil masses known as casts are excreted from the worm’s digestive system, making the soil more fertile. The earthworm’s burrowing action continually moves mineral-rich soil to the surface, which improves plant growth. The burrowing action also aerates soil, enhancing drainage. In another example, as honey bees, butterflies, and moths flit from flower to flower collecting nectar, they inadvertently transport pollen from the male reproductive structure of one flower to the female reproductive structure of another flower. Known as pollination, this leads to the fertilization of the plant’s seeds—an essential stage in the process of reproduction.

Other invertebrates form mutually beneficial partnerships with other animals. For example, some crabs form alliances with sea anemones, which they fasten to their backs. In this alliance, the crab is protected from predators by the anemone’s stinging tentacles. The anemone, in turn, receives food particles as the crab tears up meat from the animals it consumes. As the crab grows, it periodically sheds its body case. Before doing so, it removes the anemone, and then afterwards puts it back in place.

Humans sometimes share a troubled relationship with invertebrates. A number of invertebrate organisms cause many parasitic diseases in humans and farm animals. These parasites survive by feeding and reproducing inside a host, often causing internal destruction. Some of the most damaging parasites include the flatworm Schistosoma, which causes schistosomiasis; the roundworms that cause hookworm infection; and the roundworm larvae of Trichinella spiralis that cause trichinosis. Other invertebrates are agricultural pests, destroying plant crops. Insects such as leaf beetles, flea beetles, and young caterpillars feed on the leaves, stems, roots, and flowers of plants. Sucking insects, including aphids, leafhoppers, and scales, remove plant sap, weakening the plants. Sucking insects can also spread disease-causing viruses and bacteria to plants. The larvae and adult stages of some roundworms are parasites of plants. Using specialized structures called stylets, these roundworms pierce plants at the roots to extract cell content, killing the plant.

Although invertebrates can cause problems for humans, they are more often beneficial. In many cultures, invertebrates such as squid, octopuses, cuttlefish, clams, mussels, crabs, and lobsters are considered popular food items. Scientists use invertebrates for a variety of experiments that have profound benefits for human health. Scientists have performed delicate surgery on the glandular systems of caterpillars and roaches to uncover clues to the function of glands in humans. In other experiments, scientists have given spiders different types of drugs and observed the animals as they created spider webs. The different pattern of spider webs offered a way to distinguish and measure the effects of various drugs.

The vinegar fly Drosophila melanogaster, also known as the fruit fly, has long been the standard test subject in the field of genetics. In the 1910s and 1920s American geneticist Thomas Hunt Morgan used the vinegar fly to demonstrate that genes lie in a linear fashion on chromosomes, establishing the chromosomal basis of inheritance. In early 2000 studies of vinegar flies continued to advance the field of modern genetics when researchers sequenced the fly’s entire genetic makeup, or genome. The techniques used to reveal the vinegar fly genome were then applied to the efforts to decode the human genome.

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