Open Questions: Evolutionary history of animals

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Introduction

Cladistics

Ediacaran fauna

The Cambrian explosion

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books

Introduction

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 forms
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 four subdivisions, the questions of how they originated and evolved tend to be the most interesting.

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 name refers to the fact that

Possible triggers:

animals evolved visual sense -> predator arms race
climate change due to rapid continental drift
end of snowball earth period

Recommended references: Web sites

Site indexes

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 found at Netscape.

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 Phylogeny Wing.

Surveys, overviews, tutorials

Animal: Article from Wikipedia.
Cambrian explosion: Article from Wikipedia. See also Ediacaran.
Paleontology: Article from Wikipedia.
The Radiation of the First Animals: Good overview lecture by Jerry Lipps, at the BioForum site.
The Cambrian Explosion: Part of a PBS series on evolution. Contains background information and a short video.
The Ediacaran Assemblage: Good, encyclopedia-style article, with a bibliography and external refernces. "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 Cambrian Explosion.
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 explosion.)

Recommended references: Magazine/journal articles

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.
When Life Was Odd
Karen Wright
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 evolutionary puzzle.
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 process.
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
David Crews
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.

Recommended references: Books

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 themselves.
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.

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