27.4 The Evolutionary History of the Animal Kingdom

Annelids and arthropods were thought to be related due to their similarities.

The ecdysozoan clade and annelids are more closely related to mollusks, brachiopods, and other arthropods than previously thought.
The two clades now make up the Protostomes.

The emergence of a new worm called Acoelomorpha is one of the reasons for a change to former groupings.

The acoel flatworms were thought to be related to the phylum Platyhelminthes.
This was originally thought to be a false relationship due to the fact that acoels represented some of the earliest bilaterians.
The hypothesis that the acoels are related to deuterostomes has been called into question by more recent research.
Scientists agree that with enough data, their true phylogeny will be determined.

The Ctenophora is the main clade of the animal kingdom.

Ctenophora and the sponges were once considered to be sister groups of the Cnidaria.
This view of the relationships among simple animal forms was strengthened by the presence of nerve and muscle cells in the Ctenophores and the Cnidaria.

Many of the genes that support neural development in other animals are absent from the Ctenophore genome.

The muscles are derived from cells in the mesoglea.
The Ctenophores does not have many genes found in other animal mitochondrial genomes.
The absence of Hox genes from the Ctenophores has been used to argue that the group should be considered a sister group of the Porifera, and that the evolution of specialized nerve and muscle tissue may have occurred more than once in the history of the group.
Ctenophores have been shown to be more closely related to other animals than other animals have been shown to be.

Changes to the tree can be difficult to understand and can be evidence of the process of science.

Data and analytical methods are used in the development of phylogenies.
We can't dismiss a former tree as inaccurate because of the incomplete analysis.
A group of evolutionary biologists reanalyzed the evidence and found that comb jellies are not the oldest metazoan group.
The study, which used more sophisticated methods of analyzing the original genetic data, supports the view that the sponges were the first species to differ from the metazoans.
The ongoing discussion about the location of sponges and comb jellies on the animal's family tree is an example of what drives science forward.

Many questions about the evolutionary history of the animal kingdom are still being researched and debated.

Fossil evidence shows that primitive animal species existed much earlier than 530 million years ago.

The first fossils were found in the hills of Southern Australia.

There are no living representatives of these species who have left impressions that look like feathers or coins.
Early animal life is thought to have evolved from protists.

The Ediacaran period begins in the Proterozoic eon and ends in the Cambrian period of the Phanerozoic eon.

Scientific evidence suggests that more varied and complex animal species lived before the Ediacaran period.

The fossils are believed to be the oldest animals with hard body parts.

These spongelike 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 thimble-shaped body.

A recent fossil discovery may be the earliest animal species ever found.

The primitive fossils appear to be small, one-centimeter long, sponge-like creatures with internal tubes or canals.
Most scientists believed that there was no animal life prior to the Ediacaran period.
Many scientists think that animals may have evolved during the Cryogenian period.

If the fossils of the Ediacaran and Cryogenian periods are enigmatic, those of the following Cambrian period are much less so, and include body forms similar to those living today.

The most rapid evolution of new animal species in Earth's history took place during the Cambrian period.
Animals resembling echinoderms, mollusks, worms, arthropods, and chordates arose during this period.
An arthropod-like creature named Anomalocaris may have been the top predator of this period.

An artist has created a depiction of organisms from the Cambrian period.

Anomalocaris is in the upper left corner of the picture.

The trilobite was one of the first animals to have a sense of vision.

Modern horseshoe crabs are similar to trilobites.
Not a single species from the Cambrian period is still alive today.

The trilobites fossils are from the early Cambrian period, 525 million years ago, and disappeared from the fossil record during a mass extinction about 250 million years ago.

The cause of the Cambrian explosion is still debated, and it may be that a number of interacting causes ushered in this incredible explosion of animal diversity.

There are many hypotheses that attempt to answer this question.

Changes in the environment may have created a better environment for animals.

The changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations.
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 coexist.
There is support for hypotheses that argue that changes in the food web, competition for food and space, and predatorprey relationships were primed to promote a sudden massive coevolution of species.
Other hypotheses claim that there are genetic and developmental reasons for the explosion.
The evolution of Hox control genes may have given rise to increased animal morphologies at the time of the Cambrian period.
Hypotheses that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification.

The answer may be a combination of these and other theories, as there is evidence that both supports and refutes each of the hypotheses.

Around 300 million years ago, the oxygen concentration in Earth's atmosphere rose sharply.

There are still unresolved questions about the animal diversification that took place during the Cambrian period.

We don't know how the evolution of so many species occurred in such a short period of time.
Some scientists are questioning 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.
The vast diversity of animal species that appears to have begun during the Cambrian period continued into the following Ordovician period.
Despite some of the arguments, most scientists agree that the Cambrian period marked a time of impressively rapid animal evolution and diversity of body forms.

The animation shows what ocean life might have looked like during the Cambrian explosion.

The Paleozoic Era was marked by the emergence of many new orders, families, and species.

New species were adapted to new ecological niches.

Plant life first appeared on land during the Ordovician period.

Previously aquatic animal species were able to invade land and feed on plants.

Changes in the environment can create new niches that invite rapid speciation and increased diversity.

Disasters, such as volcanic eruptions and meteor strikes that destroy life, can result in devastating losses of diversity to some clades, yet provide new opportunities for others to fill in the gaps.
The end of the Permian period marked the largest mass extinction event in Earth's history, a loss of 95 percent of the species.
The trilobites, one of the most dominant species in the world's oceans, disappeared completely.
It was possible for a new line of dinosaurs to emerge because of the disappearance of some of the dominant species from land.

The warm and stable climatic conditions of the ensuing Mesozoic Era promoted an explosion of dinosaurs into every conceivable niche in land, air, and water.

Plants created complex communities of producers and consumers, some of which became very large on the abundant food available.

The end of the Cretaceous period brought about another mass extinction event.

After a large meteorite impact and tons of volcanic ash ejected into the atmosphere, skies darkened and temperatures fell.
The dinosaurs gave up their dominance of the landscape to the more warm-blooded mammals.
The warm-blooded descendants of one line of the ruling reptiles became aerial specialists in the Cenozoic Era.
The emergence and dominance of flowering plants in the Cenozoic Era created new niches for pollinating insects, as well as for birds and mammals.
Changes in animal species diversity were 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.
Grasses and coral reefs appeared early in the Cenozoic.
Ice ages that covered high latitudes with ice and then retreated left new open spaces for colonization.

The mass extinctions are discussed in the following video.

Over geological time, mass extinctions have occurred.

Scientists who study prehistoric life are called paleontologists.

Fossils are used to explain how life evolved on Earth and how species interacted with each other.
A paleontologist needs to be knowledgeable in many scientific disciplines.
Field studies may be a part of a paleontologist's work.
Paleontologists prepare fossils for further study and analysis after digging for and finding fossils.
Dinosaurs are probably the first animal that comes to mind when you think of ancient life, but paleontologists study a wide range of life forms, from plants, fungi and invertebrates to the fishes, amphibians, reptiles, birds and mammals.

If you want to become a paleontologist, an undergrad degree in biology is a good place to start.

A graduate degree is required most of the time.

Work experience in a paleontology lab is useful.

There is an incredibly diverse kingdom of animals.

Animals range in complexity from genes called Hox genes, which directs the organization of the major simple sea sponges to human beings, and most members of the animal body plans.
There are animals in the kingdom.

Classify was used with a fixed body plan.

New data and Placozoa are changing the details of the Eumetazoa and more.

The Eumetazoa is divided into those with radial versus bilateral modifications of the understanding of symmetry.

Animals with bilateral animal species in all of history are usually characterized by the development of the mesoderm, a third embryologic germ layer, during the Paleozoic Era.

Until recently, scientists believed that there were only a few tiny and simplistic animals in existence before this period.
The coelom is a recent fossil.
The presence of a coelom has led to the discovery of additional, larger, and more many advantages for animals with a coelom, as well as true coelomates or pseudocoelomates.

The emergence of one of two groups called protostomes the majority of animal phyla that we know today, although and deuterostomes, based on a number of developmental questions remain unresolved.

Scientists are interested in the history of semiaquatic arthropods.

There are three main sources of data that scientists use to major extinction events, each of which wiped out a majority of the species.

Fossil record data, which survived the extinctions, continue to evolve into species that we see today.

Parazoans don't display bilateral symmetry.

As with the emergence of the Acoelomorpha phylum, it develop into specific groups of tissues or organs during a time when it is common for data to misplace animals in close stage called _____.

An animal's development is marked by a D.

There are at least two theories that attempt to explain the cause of the Cambrian explosion.