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By the end of this section, you will be able to:
  • Describe the features that characterized the earliest animals and when they appeared on earth
  • Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity that took place during that time
  • Describe some of the unresolved questions surrounding the Cambrian explosion
  • Discuss the implications of mass animal extinctions that have occurred in evolutionary history

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. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom, and what organism was their common ancestor? 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.

Pre-cambrian animal life

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 ( [link] ). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protest species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

Table A describes eras in earth’s history. The earth’s history is divided into four eons, the Pre-Archean, Archaea, Proteozoic, Phanerozoic. The oldest eon, the Pre-Archean, spans the beginning of earth’s history to about 3.8 billion years ago. The Archean eon spans 2.5 to 3.8 billion years ago, and the Proterozoic spans 570 million to 2.5 billion years ago. The Pharenozoic eon, from 570 million years ago to present time, is sub-divided into the Paleozoic, Mesozoic and Cenozoic eras. The Paleozoic era, from 240 to 570 million years ago, is further divided into seven periods: the Cambrian from 500 to 570 million years ago, the Ordovician from 435 to 500 million years ago, the Silurian from 410 to 435 million years ago, the Devonian from 360 to 410 million years ago, the Missisippian from 330 to 360 million years ago, the Pennsylvanian from 290 to 330 million years ago, and the Permian from 240 to 290 million years ago. The Mesozoic era, from 66 to 240 million years ago, is divided into three periods, the Triassic from 205 to 240 million years ago, the Jurassic from 138 to 205 million years ago, and the Cretaceous, from 66 to 138 million years ago. The Cenozoic era, from 66 million years ago to modern times, is divided into two eras, the Tertiary and the Quaternary. The tertiary period spans 66 to 1.6 million years ago. The quaternary period spans 1.6 million years ago to modern times. Illustration B shows geological periods in a spiral starting with the beginning of earth’s history  at the bottom and ending with modern times at the top. The diversity and complexity of life increases toward the top of the spiral.
(a) Earth’s history is divided into eons, eras, and periods. Note that the Ediacaran period starts in the Proterozoic eon and ends in the Cambrian period 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 only tiny, sessile, soft-bodied sea 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.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. 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). Other fossils from the Ediacaran period are shown in [link] ab .

Part a shows a fossil that resembles a wheel, with spokes radiating out from the center, imprinted on a rock. Part b shows a fossil that resembles a teardrop shaped leaf, with grooves radiating out from a central rib.
Fossils of (a) Cyclomedusa and (b) Dickinsonia date to 650 million years ago, during the Ediacaran period. (credit: modification of work by “Smith609”/Wikimedia Commons)

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. 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. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.

Questions & Answers

the diagram of the digestive system
Assiatu Reply
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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