Evolution of Animal Kingdom

Study Notes

Animal Evolution

Animal evolution began in the ocean over 600 million years ago (MYA) with tiny creatures that do not resemble any living organisms today.
This led to a highly diverse kingdom with over one million extant species of animals identified and still being discovered.
It is estimated that there are between 3 and 30 million extant species of animals.

Mass Extinctions

99% of all animals that ever existed are now extinct.
Most species linger for about 5 million years and then become extinct.
There have been five major mass extinctions:
- Ordovician Period (440 MYA): 85% of species lost due to continental drift and climate change.
- Late Devonian (375 MYA): 75% of species lost due to lack of oxygen in the oceans, quick cooling of air temperatures, and possibly volcanic eruptions and/or meteor strikes.
- End Permian (251 MYA): 96% of species lost due to possibly asteroid strikes, volcanic activity, climate change, and microbes.
- End Triassic (200 MYA): 80% of species lost due to major volcanic activity with basalt flooding, global climate change, and changing pH and sea levels of the oceans.
- End Cretaceous (66 MYA): 76% of all species lost due to extreme asteroid or meteor impact.

Sixth Mass Extinction

Biologists suspect we are living through the sixth major mass extinction, known as the Holocene extinction or Anthropocene extinction.
This extinction is mainly caused by human activity, including:
- Habitat destruction
- Overhunting
- Toxic pollution
- Invasion by alien species
- Climate change
The ultimate cause of these factors is human overpopulation and continued population growth, and overconsumption.

Definition of an Animal

An animal refers to any organism that belongs to the kingdom Animalia.
The word "animal" comes from the Latin word "animalis," meaning "having breath."
The definition of an animal encompasses a diverse range of creatures, from sponges (simplest animals) to humans (most complex animals).

Five Kingdom Classification

Organisms are divided into five major kingdoms: Plants, Animals, Fungi, Monera, and Protists.
The five kingdom classification was proposed by R.H. Whittaker in 1969.
The classification is based on:
- Complexity of cell structure: Prokaryotic and Eukaryotic
- Complexity of organisms: Uni- and Multicellular
- Mode of nutrition: Plantae (Autotrophs), Fungi (Heterotrophs and saprobic absorption), Animalia (Heterotrophs and ingestion)
- Life style: Producers (Plantae), Consumers (Animalia), Decomposers (Fungi)
- Phylogenetic relationships: Prokaryotes to Eukaryotes, Unicellular to Multicellular organisms

Evolution of Animals on Land

Animals evolved in the sea and remained there for at least 600 million years due to the absence of a protective ozone layer, which made the land uninhabitable due to lethal levels of UV radiation.
Once photosynthesis had raised atmospheric oxygen levels high enough, an ozone layer formed, making it possible for living things to venture onto land.
The first metazoa, Ediacaran fauna, appeared between 620-550 MYA and consisted of relatively large, complex, soft-bodied multicellular animals.
They left no skeletal material behind and disappeared from the fossil record at the end of the Vendian period (544 MYA).

Description

Explore the history of animal evolution, from its origins in the ocean to the diverse range of species that exist today. Learn about the rates of extinction and the estimated number of extant species.