Which Of The Following Periods Is The Earliest During Which Animals May Have Appeared?

Introduction to Animate being Diverseness

The Evolutionary History of the Animal Kingdom

OpenStaxCollege

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Learning Objectives

By the end of this section, y'all will be able to:

• Describe the features that characterized the earliest animals and when they appeared on globe
• Explain the significance of the Cambrian menstruation for beast evolution and the changes in animate being diversity that took place during that time
• Depict some of the unresolved questions surrounding the Cambrian explosion
• Hash out the implications of mass beast extinctions that have occurred in evolutionary history

Many questions regarding the origins and evolutionary history of the animal kingdom go along to be researched and debated, as new fossil and molecular prove change prevailing theories. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the brute kingdom, and what organism was their common ancestor? While animal diversity increased during the Cambrian period of the Paleozoic era, 530 million years ago, modernistic fossil bear witness suggests that primitive creature species existed much before.

Pre-Cambrian Creature Life

The fourth dimension earlier the Cambrian period is known every bit the Ediacaran menstruation (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era ([link]). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protist species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses accept revealed similar sequence homologies in their DNA.

(a) Globe's history is divided into eons, eras, and periods. Annotation that the Ediacaran period starts in the Proterozoic eon and ends in the Cambrian catamenia of the Phanerozoic eon. (b) Stages on the geological time scale are represented as a spiral. (credit: modification of work by USGS)

The earliest life comprising Ediacaran biota was long believed to include just tiny, sessile, soft-bodied bounding main creatures. Withal, recently there has been increasing scientific evidence suggesting that more varied and complex beast species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in Due south Australia. These sponge-like fossils, named Coronacollina acula, date back as far equally 560 meg years, and are believed to bear witness the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about five cm long). Other fossils from the Ediacaran period are shown in [link]ab.

Fossils of (a) Cyclomedusa and (b) Dickinsonia date to 650 1000000 years ago, during the Ediacaran menses. (credit: modification of piece of work by "Smith609"/Wikimedia Commons)

Another recent fossil discovery may represent the earliest animal species e'er found. While the validity of this merits is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from S Australia date back 650 million years, actually placing the putative animal before the slap-up water ice historic period extinction event that marked the transition between the Cryogenian menses and the Ediacaran flow. Until this discovery, most scientists believed that in that location was no animal life prior to the Ediacaran catamenia. Many scientists at present believe that animals may in fact have evolved during the Cryogenian flow.

The Cambrian Explosion of Brute Life

The Cambrian period, occurring betwixt approximately 542–488 million years ago, marks the virtually rapid evolution of new fauna phyla and animal diverseness in Earth's history. Information technology is believed that nearly of the animate being phyla in existence today had their origins during this fourth dimension, often referred to as the Cambrian explosion ([link]). Echinoderms, mollusks, worms, arthropods, and chordates arose during this catamenia. One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was amid the starting time animals to showroom a sense of vision ([link]abcd).

An artist'southward rendition depicts some organisms from the Cambrian period.

These fossils (a–d) belong to trilobites, extinct arthropods that appeared in the early Cambrian period, 525 million years ago, and disappeared from the fossil record during a mass extinction at the end of the Permian period, about 250 million years ago.

The cause of the Cambrian explosion is even so debated. There are many theories that endeavour to answer this question. Ecology changes may have created a more suitable environment for animal life. Examples of these changes include ascent atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period ([link]). 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-be. There is also back up for theories that contend that ecological relationships between species, such every bit changes in the food web, competition for nutrient and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other theories merits genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complication of animal development afforded past the development of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the fourth dimension of the Cambrian menses. Theories that effort to explain why the Cambrian explosion happened must exist able to provide valid reasons for the massive animal diversification, as well as explicate why information technology happened when information technology did. 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.

The oxygen concentration in Globe'south temper rose sharply effectually 300 one thousand thousand years ago.

However, unresolved questions nearly the animal diversification that took identify during the Cambrian period remain. For instance, we do not understand how the evolution of and so many species occurred in such a short catamenia of time. Was in that location really an "explosion" of life at this item time? Some scientists question the validity of the this idea, because in that location is increasing evidence to propose that more animal life existed prior to the Cambrian period and that other similar species' so-chosen explosions (or radiations) occurred afterwards in history as well. Furthermore, the vast diversification of fauna species that appears to have begun during the Cambrian catamenia connected well into the post-obit Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian catamenia marked a time of impressively rapid animal development and diversification that is unmatched elsewhere during history.

Link to Learning

View an animation of what ocean life may have been like during the Cambrian explosion.

Post-Cambrian Development and Mass Extinctions

The periods that followed the Cambrian during the Paleozoic Era are marked by further brute evolution and the emergence of many new orders, families, and species. Every bit animal phyla continued to diversify, new species adapted to new ecological niches. During the Ordovician period, which followed the Cambrian period, plant life first appeared on land. This change allowed formerly aquatic fauna species to invade land, feeding directly on plants or decaying vegetation. Continual changes in temperature and moisture throughout the residual of the Paleozoic Era due to continental plate movements encouraged the development of new adaptations to terrestrial beingness in animals, such as limbed appendages in amphibians and epidermal scales in reptiles.

Changes in the environs often create new niches (living spaces) that contribute to rapid speciation and increased diversity. On the other paw, cataclysmic events, such as volcanic eruptions and shooting star strikes that obliterate life, tin can upshot in devastating losses of diverseness. Such periods of mass extinction ([link]) have occurred repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve into the empty niches left behind. The terminate of the Permian period (and the Paleozoic Era) was marked by the largest mass extinction event in World's history, a loss of roughly 95 pct of the extant species at that time. Some of the dominant phyla in the world's oceans, such as the trilobites, disappeared completely. On state, the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles to emerge, the dinosaurs. 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. Plants, too, radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became very large on the abundant food bachelor.

Another mass extinction event occurred at the end of the Cretaceous flow, bringing the Mesozoic Era to an finish. Skies darkened and temperatures fell as a large shooting star impact and tons of volcanic ash blocked incoming sunlight. Plants died, herbivores and carnivores starved, and the more often than not cold-blooded dinosaurs ceded their potency of the landscape to more than warm-blooded mammals. In the following Cenozoic Era, mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs, and birds, the warm-blooded offshoots of one line of the ruling reptiles, became aerial specialists. The appearance and dominance of flowering plants in the Cenozoic Era created new niches for insects, as well equally for birds and mammals. Changes in animal species diversity during the late Cretaceous and early Cenozoic were likewise promoted past a dramatic shift in World's geography, as continental plates slid over the chaff into their current positions, leaving some creature groups isolated on islands and continents, or separated by mountain ranges or inland seas from other competitors. Early in the Cenozoic, new ecosystems appeared, with the evolution of grasses and coral reefs. Belatedly in the Cenozoic, further extinctions followed by speciation occurred during ice ages that covered high latitudes with ice then retreated, leaving new open spaces for colonization.

Link to Learning

Scout the post-obit video to acquire more than nigh the mass extinctions.

Mass extinctions accept occurred repeatedly over geological fourth dimension.

Career Connection

PaleontologistNatural history museums contain the fossil casts of extinct animals and information nigh how these animals evolved, lived, and died. Paleontogists are scientists who report prehistoric life. They utilise fossils to detect and explain how life evolved on Earth and how species interacted with each other and with the environment. A paleontologist needs to be knowledgeable in biology, ecology, chemistry, geology, and many other scientific disciplines. A paleontologist'south work may involve field studies: searching for and studying fossils. In add-on to excavation for and finding fossils, paleontologists also prepare fossils for further study and analysis. Although dinosaurs are probably the first animals that come to mind when thinking about paleontology, paleontologists study everything from found life, fungi, and fish to bounding main animals and birds.

An undergraduate caste in world scientific discipline or biology is a good place to first toward the career path of becoming a paleontologist. Almost oftentimes, a graduate degree is necessary. Additionally, piece of work experience in a museum or in a paleontology lab is useful.

Section Summary

The near 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. Until recently, scientists believed that there were only very few tiny and simplistic animal species in existence before this period. Yet, contempo fossil discoveries take revealed that boosted, larger, and more circuitous animals existed during the Ediacaran period, and even possibly before, during the Cryogenian flow. Still, the Cambrian period undoubtedly witnessed the emergence of the bulk of animal phyla that we know today, although many questions remain unresolved about this historical miracle.

The remainder of the Paleozoic Era is marked past the growing advent of new classes, families, and species, and the early colonization of land by certain marine animals. The evolutionary history of animals is also marked by numerous major extinction events, each of which wiped out a bulk of extant species. Some species of most animal phyla survived these extinctions, allowing the phyla to persist and continue to evolve into species that nosotros see today.

Review Questions

Which of the post-obit periods is the earliest during which animals may take appeared?

1. Ordovician period
2. Cambrian period
3. Ediacaran period
4. Cryogenian period

D

What type of data is primarily used to determine the existence and appearance of early animate being species?

1. molecular data
2. fossil data
3. morphological data
4. embryological development data

B

The fourth dimension between 542–488 million years ago marks which period?

1. Cambrian period
2. Silurian period
3. Ediacaran period
4. Devonian flow

A

Until contempo discoveries suggested otherwise, animals existing before the Cambrian period were believed to exist:

1. modest and body of water-dwelling
2. small and non-motile
3. small and soft-bodied
4. minor and radially symmetrical or asymmetrical

C

Institute life kickoff appeared on land during which of the post-obit periods?

1. Cambrian menstruation
2. Ordovician period
3. Silurian catamenia
4. Devonian catamenia

B

Approximately how many mass extinction events occurred throughout the evolutionary history of animals?

1. 3
2. 4
3. 5
4. more than v

D

Free Response

Briefly describe at least ii theories that try to explicate the cause of the Cambrian explosion.

One theory states that environmental factors led to the Cambrian explosion. For example, the rising in atmospheric oxygen and oceanic calcium levels helped to provide the right environmental conditions to allow such a rapid evolution of new animal phyla. Some other theory states that ecological factors such every bit competitive pressures and predator-casualty relationships reached a threshold that supported the rapid animal evolution that took place during the Cambrian period.

How is it that nearly, if non all, of the extant animal phyla today evolved during the Cambrian period if and then many massive extinction events take taken place since and so?

Information technology is true that multiple mass extinction events take taken identify since the Cambrian period, when almost currently existing fauna phyla appeared, and the majority of animal species were normally wiped out during these events. However, a minor number of animal species representing each phylum were unremarkably able to survive each extinction event, allowing the phylum to keep to evolve rather than become birthday extinct.

Glossary

Cambrian explosion

time during the Cambrian period (542–488 million years agone) when most of the animal phyla in existence today evolved

Cryogenian period

geologic menstruum (850–630 million years ago) characterized past a very cold global climate

Ediacaran catamenia

geological flow (630–542 million years ago) when the oldest definite multicellular organisms with tissues evolved

mass extinction

event that wipes out the majority of species within a relatively brusque geological time period

Source: http://pressbooks-dev.oer.hawaii.edu/biology/chapter/the-evolutionary-history-of-the-animal-kingdom/#:~:text=The%20Cambrian%20Explosion%20of%20Animal,animal%20diversity%20in%20Earth's%20history.

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