Some of these questions include the following: How long have animals existed on Earth? The periods that followed the Cambrian during the Paleozoic Era are marked by further animal evolution and the emergence of many new orders, families, and species. The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. Some scientists question the validity of this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well. They use fossils to observe and explain how life evolved on Earth and how species interacted with each other and with the environment. Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. However, a small number of animal species representing each phylum were usually able to survive each extinction event, allowing the phylum to continue to evolve rather than become altogether extinct. On land, the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles to emerge, the dinosaurs. Additionally, work experience in a museum or in a paleontology lab is useful. It is believed that most of the animal phyla in existence today had their origins during this time, often referred to as the Cambrian explosion (Figure 27.4.3). Examples of these changes include rising atmospheric oxygen levels ((Figure)) and large increases in oceanic calcium concentrations that preceded the Cambrian period. Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist. One theory states that environmental factors led to the Cambrian explosion. However, recent fossil discoveries have revealed that additional, larger, and more complex animals existed during the Ediacaran period, and even possibly earlier, during the Cryogenian period. While animal diversity increased during the Cambrian period of the Paleozoic era, 530 million years ago, modern fossil evidence suggests that primitive animal species existed much earlier. The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. The history of the Earth is divided into geological eras characterized by the formation of minerals and the appearance of animal and plant species, and humans. Until recent discoveries suggested otherwise, animals existing before the Cambrian period were believed to be: Plant life first appeared on land during which of the following periods? The most rapid diversification and evolution of animal species in all of history occurred during the Cambrian period of the Paleozoic Era, a phenomenon known as the Cambrian explosion. For example, the rise in atmospheric oxygen and oceanic calcium levels helped to provide the right environmental conditions to allow such a rapid evolution of new animal phyla. Until recently, scientists believed that there were only very few tiny and simplistic animal species in existence before this period. One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision ((Figure)a,b,c,d). Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period (Figure \(\PageIndex{5}\)). Yet other hypotheses claim genetic and developmental reasons for the Cambrian explosion. The cause of the Cambrian explosion is still debated, and in fact, it may be that a number of interacting causes ushered in this incredible explosion of animal diversity. How is it that most, if not all, of the extant animal phyla today evolved during the Cambrian period if so many massive extinction events have taken place since then? Was there really an “explosion” of life at this particular time? On land, the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles to emerge, the dinosaurs. Some of the dominant phyla in the world’s oceans, such as the trilobites, disappeared completely. An undergraduate degree in earth science or biology is a good place to start toward the career path of becoming a paleontologist. The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. The appearance and dominance of flowering plants in the Cenozoic Era created new niches for pollinating insects, as well as for birds and mammals. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes that obliterate life, can result in devastating losses of diversity. Changes in animal species diversity during the late Cretaceous and early Cenozoic were also promoted by a dramatic shift in Earth’s geography, as continental plates slid over the crust into their current positions, leaving some animal groups isolated on islands and continents, or separated by mountain ranges or inland seas from other competitors. Paleontologists are scientists who study prehistoric life. The rapid diversification of animals that appeared during this period, including most of the animal phyla in existence today, is often referred to as the Cambrian explosion ((Figure)). Figure 3. Another mass extinction event occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an end. Briefly describe at least two theories that attempt to explain the cause of the Cambrian explosion. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Was there really an “explosion” of life at this particular time? Most often, a graduate degree is necessary. For this reason, there are a number of hypotheses that attempt to answer this question. (b) Stages on the geological time scale are represented as a spiral. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Original content by OpenStax (CC BY 4.0; Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87). Many scientists now believe that animals may in fact have evolved during the Cryogenian period. The appearance and dominance of flowering plants in the Cenozoic Era created new niches for insects, as well as for birds and mammals. By the end of this section, you will be able to do the following: Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched and debated, as new fossil and molecular evidence change prevailing theories. The warm and stable climatic conditions of the ensuing Mesozoic Era promoted an explosive diversification of dinosaurs into every conceivable niche in land, air, and water. There is also support for hypotheses that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. There are no living representatives of these species, which have left impressions that look like those of feathers or coins ((Figure)). While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period ([link]). The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure \(\PageIndex{1}\)). Yet other theories claim genetic and developmental reasons for the Cambrian explosion. There is evidence that both supports and refutes each of the theories described above, and the answer may very well be a combination of these and other theories. Natural history museums contain the fossil casts of extinct animals and information about how these animals evolved, lived, and died. The most rapid diversification and evolution of animal species in all of history occurred during the Cambrian period of the Paleozoic Era, a phenomenon known as the Cambrian explosion. What were the earliest members of the animal kingdom, and what organism was their common ancestor? We want to hear from you. Another theory states that ecological factors such as competitive pressures and predator-prey relationships reached a threshold that supported the rapid animal evolution that took place during the Cambrian period. The rapid diversification of animals that appeared during this period, including most of the animal phyla in existence today, is often referred to as the Cambrian explosion ( (Figure) ). As animal phyla continued to diversify, new species adapted to new ecological niches. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. However, a small number of animal species representing each phylum were usually able to survive each extinction event, allowing the phylum to continue to evolve rather than become altogether extinct. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Paleontogists are scientists who study prehistoric life. As animal phyla continued to diversify, new species adapted to new ecological niches. It is true that multiple mass extinction events have taken place since the Cambrian period, when most currently existing animal phyla appeared, and the majority of animal species were commonly wiped out during these events. Figure 6. OpenStax College, Biology. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Additionally, work experience in a museum or in a paleontology lab is useful. origin and evolution of species [1] Since its formation some 4.6 billion years ago, the Earth has witnessed the genesis of continents and oceans and the appearance of animals and vegetation. Environmental changes may have created a more suitable environment for animal life. They use fossils to observe and explain how life evolved on Earth and how species interacted with each other and with the environment.

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