Open Questions: Evolutionary history of animals
See also: Developmental biology --
Evolutionary milestones --
Molecular evolution --
Within the domain of Eukarya (life forms based on cells having a nucleus),
there are several subdivisions. Taxonomy (the classification of life
forms) is in a state of ferment at the present time, but the principal
categories of Eukarya are:
- Protists -- a variety of single-celled creatures such as amoeba,
some kinds of algae, diatoms and protozoans
- Fungi -- yeast, mushrooms, molds, and related single and multi-celled
- Green plants -- single and multi-celled organisms that use
chlorophyll for photosynthesis of food
- Animals -- multi-celled organisms dependent on other organisms to
provide energy through their metabolism
One can consider the evolutionary history of each of
these forms of life, and there are interesting open questions in each area.
However, since instances of animals are generally the most complex of the
the questions of how they originated and evolved tend to be the most
Then too, humans are animals, so it's quite natural for them to have
some interest in the origins and history of their own ancestors.
The big question here is, "What caused the Cambrian explosion?" Or perhaps
better, "Why did the Cambrian explosion happen?"
But first, what is the "Cambrian explosion"?
The name refers to the fact that
- animals evolved visual sense -> predator arms race
- climate change due to rapid continental drift
- end of snowball earth period
Open Directory Project: Paleontology
- Categorized and annotated links. A version of this
list is at
Google, with entries sorted in "page rank" order. May also be
Sites with general resources
Museum of Paleontology
- At the University of California, Berkeley. Excellent site which
really covers much more of biology than just paleontology.
Features include a variety of exhibits, such as the
History of Life. Also includes external links, an extensive
glossary, and a directory of biological taxa in the
Surveys, overviews, tutorials
- Article from
- Article from
- Article from
Sponge larvae: Your unlikely ancestors
- May 2009 New Scientist article.
"We know very little about the origins of complex animal
life, but there are hints that it all began with sponges
that refused to grow up."
The Radiation of the First Animals
- Good overview lecture by Jerry Lipps, at the
The Cambrian Explosion
- Part of a PBS series on
Contains background information and a short video.
The Ediacaran Assemblage
- Good, encyclopedia-style article, with a bibliography and
"This page describes the geological and chronological settings of
the Ediacaran forms, the range of morphologies of the fossils
themselves, and concludes with some remarks about their
relationships with other organisms."
There are additional pages on Ediacaran paleontology and the
The origin of sex chromosomes
- October 1999 news article about a study that reconstructs
the evolutionary history of mammalian sex chromosomes.
Life Goes for a Spin
- August 1997 Scientific American In Depth article,
subtitled "A topsy-turvy earth may have triggered an evolutionary
big bang." (Suggesting that a period of rapid continental drift
caused drastic climate change, leading to the Cambrian
Science News, January 31, 2009
- Transitional fossils are the hardest to find, but
sometimes tell the best stories.
Science News, June 17, 2006
- Vertebrates' transition to dry land took some fancy footwork.
Changes in the Air
Science News, December 17, 2005
- Variations in atmospheric oxygen have affected evolution
in big ways.
Getting a Leg Up on Land
Jennifer A. Ckack
Scientific American, December 2005
- The Origin of Birds and Their Flight
Kevin Padian; Luis M. Chaippe
Scientific American, February 1998, pp. 38-47
- The evolutionary orgins of birds has long been a subject of
debate and controversy. But the evidence now appears to be overwhelming
that birds evolved from small theropod dinosaurs.
Out of the Swamps
Science News, May 22, 1999
- How early vertebrates established a foothold - with all
10 toes - on land.
- When Life Was Odd
Discover, March 1997, pp. 52-61
- The biota known as Ediacarans were the earliest large scale
life forms. Developing just before the Cambrian explosion they
are unlike any subsequent forms of life and present a major
- The Origin of Animal Body Plans
Douglas Erwin, James Valentine, David Jablonski
American Scientist, March-April 1997, pp. 126-137
- Larger multicellular animals first appeared about 565 million
years ago, just prior to the Cambrian explosion. The body plans
of all currently existing animals originated around that time,
and molecular biology provides some significant clues to the
- The Ordovician Radiation
Mary L. Droser; Richard A. Fortey; Xing Li
American Scientist, March-April 1996, pp. 122-131
- The Ordovician Radiation, which occurred about 500 million
years ago, was a time of rapid diversification of animal species,
genera, and families. There are good indications of how extrinsic
events, such as favorable climatic and geologic changes,
contributed to this diversification, just as at other times
such events have contributed to mass extinctions.
- Animal Sexuality
Scientific American, January 1994, pp. 108-114
- Sexuality in many animal species is determined by factors
other than chromosome type, such as termperature or even social
environment. Understanding such factors may help clarify the
origin and function of sexuality.
- Richard Fortey -- Trilobite: Eyewitness to Evolution
Vintage Books, 2001
- Fortey, a paleontologist at London's Natural History Museum,
in 1999 published an enjoyable history of the whole history of
life on Earth, entitled Life. This sequel focuses on his
favorite class of animals, the trilobites. Although now extinct
for about 250 million years, the trilobites persisted for an
even longer span of time, having their origins in the Cambrian
period. Fortey's history is as interesting for its lively
portrayal of what else was going on in the time of the trilobites
as it is for what it tells about these distinctive animals
- Simon Conway Morris -- The Crucible of Creation: The Burgess
Shale and the Rise of Animals
Oxford University Press, 1998
- The Burgess Shale is a geologic formation in Canada which
contains the best fossil record of living creatures which
appeared during the remarkable "Cambrian explosion". In this book the
story is told of those animals and how they may be related
to life forms which came both earlier and later. There are
innumerable unresolved questions about these creatures, as
well as important issues in theoretical biology that they pose.
- Mark A. S. McMenamin; Dianna L. Schulte McMenamin -- The
Emergence of Animals: The Cambrian Breakthrough
Columbia University Press, 1990
- The Cambrian "explosion" was easily the most dramatic
period in the development of animal life, but perhaps even
more interesting -- and mysterious -- were the evolutionary
events which preceded it. About all we know of that time is
represented in the scant fossil history of the Ediacaran
fauna. The key issue is whether the ancestors of Cambrian
animals can be found among the Ediacara -- or whether the
latter left any descendents at all. This is one of the few
books to explore the controversial topic.
Copyright © 2002 by Charles Daney, All Rights Reserved