The Evolution Of Amphibians

THE EVOLUTION OF AMPHIBIANS Amphibians are a unique kingdom of animal which evolved from fish. However their evolution is very controversial amongst religious groups, in particular Creationists, or ‗Intelligent Design‘ theorists. In this essay the general features of amphibians and the main theory of amphibian evolution will be explained. Also, Creationists‘ attack on evolution and the Evolutionists‘ defence will be outlined and discussed.

Amphibians can be identified by certain physical features, most of which are mutations of features which they inherited from their fish ancestors. Most of the mutations that occurred were favourable as they enabled a new kingdom of animals to evolve with the characteristics that enabled them to search further for food and safety. However, inevitably, some of the mutations were not so favourable – for example, the amphibian lung. The features of amphibians are quite

 Figure A1 – Fish (left) and Amphibian (right) respiratory systems. (Essenfeld, 1994, p.580)

similar to that of fishes. The amphibian heart consists of two atriums (left and right) and one ventricle.

However,

oxygenated

and

deoxygenated blood mixes in the ventricle, making the amphibian respiratory system quite inefficient (see Figure A1). So, to compensate, they absorb oxygen through their skin and the walls of their mouths, as well as through their lungs. However, the way they breathe through their skin works in much the same way as gills,

 Figure A2 – Caecilian (Essenfeld, 1994, p. 575)

so they have to keep themselves moist (and thus don‘t venture too far from water). Some species of amphibian include frogs, caecilians, geckos, newts, salamanders and axolotls. There are more than 3,500 species of frogs, 360 species of salamanders and 170 species of caecilians (The

 Figure A3 – Albino axolotl (Essenfeld, 1994, p.585)

American Naturalist, 1992, p.105). Caecilians are worm-like amphibians that still have fish-like scales, much like their fish ancestors (see Figure 2), however, do not have limbs like the early amphibians. They have evolved to move along and under the ground in an earthworm-like fashion.

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Amphibians also have a unique breeding habit. The larvae hatch in water, and once they become adolescents, lose their gills and start breathing with their amphibian lungs. This allows them to become terrestrial to search for food and shelter. Once they have matured, amphibians only return to water to breed. They court in the water and the female lays eggs onto water plants‘ branches (the eggs are stuck fast by a sticky mucous). However, an exception to this rule is the axolotl, or Mexican Walking Fish (see Figure 3), which remains in its larvae stage its whole life. It lives in water and breathes through gills and surfaces occasionally to take a gulp of air into its lungs. Axolotls have four legs, but they are not strong enough to move the axolotls‘ body weight efficiently on land, so it generally resides in water.

Some fish have an air bladder, which helps to keep the fish upright in water, but is not used for absorbing oxygen into the blood. There is scientific evidence to suggest that by genetic mutation, many capillaries and cilia began to form in this air bladder, just like a primitive lung. This allowed these fish to stay for longer periods of time on land. Therefore, in times of drought, these lunged fish had a higher survival rate as they were able to relocate themselves to another nearby aquatic environment. This process is called natural selection.

 Figure B1 – Cladogram of the evolution of separate animal kingdoms from fish. (Essenfeld, 1994, p.555)

 Figure B2 - Cladogram of the gradual changes from fish to amphibians. (Tiktaalik rosae)

Some fish already had muscular, fleshy fins (lobe-finned fish), which when combined with the air bladder to lung mutation, enabled them to travel further on land. The fishes that used their weight-bearing fins to propel themselves on the land were able to survive and multiply and evolve their fins into limbs. As their limbs became more able to support their body weight on land, and their lungs were capable of absorbing efficient levels of oxygen while terrestrial, they were able to spend more time on land, and thus became the first amphibians (see figures B1, B2 and B3). In figure B2, the individual external physical changes in the transitional stages of evolution between fish as amphibians can be seen. Initially, some fish evolved lobe fins from spiny fins, evolved lungs from airbladders, and then changed the shape of their heads to suit surfacing for air. D:\My Documents\2007\2007sdon\Science\Biology\The Evolution of Amphibians.doc

Then as they moved out of the water, their eyes moved to the top of their heads and their lobe fins metamorphosed into weight-baring limbs (see figure B3). Scientists occasionally discover great deposits of transitional species‘ fossils concentrated in one particular area. This suggests that certain species in a certain era and location had to evolve to suit their changing environment, and so they evolved through the transitional stages and became new species. Technically, all species are ‗transitional‘ because they are all constantly changing. Some animals change quickly enough to adapt to changing environments, while others can‘t evolve fast enough, so they can‘t survive, and thus die. Scientists can even study how amphibians‘ environments changed thus causing them to evolve, by looking at physical changes in their jaw and teeth structure and arrangement for how and what they ate. They can study their leg structure and skeleton for how they walked. The fossil in Figure B4 shows the hind limb of an Ichthyostega – one of the earliest amphibians. In 2004, three American palaeontologists, Neil Shubin, Edward Daeschler and Farish Jenkins, came across a group of Tikataalik roseae fossils. Tikataalik roseae is a ‗transition species‘ between primitive fish and the earliest amphibians, which lived in the Devonian period. Jenny Clack (another palaeontologist who specialises in fish evolution) said, ―the fossil combines features of fish and tetrapods such that it fits perfectly between the two…this is another gap closed that a deity no longer needs to fill.‖ (Perlman, 2006).  Figure B4 Fossil of a hind limb of an Ichthyostega (Ichthyostega and the Origins of Land Vertebrates).

There are always some people who can‘t, or won‘t, believe that evolution is a fact, despite the extensive amount of scientific research and evidence that provides logical explanations. Creationists support the biblical theory that an intelligent being designed and created every species of plant and animal that ever lived on Earth. They attack Darwin‘s theories and claim that scientists have never found ‗transitional species‘. Scientists have gained much evidence since Darwin made his claims, and can now say that his theories are ‗old‘ and flawed. Also, scientists have found many fossils of ‗transitional species‘. However, the most likely reason why there are not as many as the creationists seem to expect, is that if a mutation occurred that didn‘t help or even handicapped a species, it simply wouldn‘t have survived, and been either bred out or further

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 Figure B3 – Strong appendages enabled early amphibians to support their own weight on land. (Essenfeld, 1994, p.573)

evolved into something more useful to the species. Therefore there wouldn‘t be very many ‗transitional species‘‘ fossils to find. Creation theory states that, the Earth and everything on it (including plants, animals and humans) was created by God, some 6,000 to 10,000 years ago. It also states that there is no such thing as evolution. Therefore, they believe that God made the fishes and amphibians discretely, and that amphibians could not have evolved from fish. It is a wonder how such a statement can be made and supported by so many people,  Figure D1 2001 Gallup Poll conducted in America (Shermer, 2002, p.25)

when there is so much evidence to the

2001 Gallup Poll Results - Creation vs. Evolution

contrary. A 2001 Gallup poll, showed

Evolution

Belief

that 45 percent of Americans who Blended (God helped evolution)

responded believe that ―God created humans pretty much in their present

Creation

form at one time within the last 10,000 0

10

20

30

40

Percentage of people with correspoding belief

50

years or so‖; 37 percent believe that ―human

beings

developed

over

millions of years from lass advanced forms of life, but God guided this process‖; and a mere 12 percent stand by the scientific theory of evolution, that ―human beings have developed of millions of years from less advanced forms of life, but God had no part in this process.‖ Only 34 percent of people who responded to the poll said that they considered themselves to be ―very informed‖ about evolution (Shermer, 2002, p.25). Scientific American (a scientific journal) published an article entitled 15 Answers to Creationist Nonsense in July, 2002, that lists some common questions asked by creationists, to push their theory of creationism and discredit evolution science. Often these questions can be asked of creationism also, and one could expect a just as nonsense answer. One particular question outlined in the article says, ―if humans descended from monkeys, why are there still monkeys?‖. John Rennie, the author of the article, compares this Creationist argument with the question, ―if children descended from adults, why are there still adults?‖. (Rennie, 2002, p.62-69) Another argument made by creationists is that nobody has ever seen a new species evolve (Rennie, 2002, p.66). This is tantamount to saying that nobody has seen God. However, there are species of animals today that have evolved from their ancestors that were physically similar, that scientists have collected fossils of. When they are carbon dated, and it can be noticed that the different specimens with similar characteristics have been evolving for millions of years. The carbon dating results show that these specimens are evenly spaced along the timeline of evolution, so they did evolve according to changes in their environments, and weren‘t simply ‗created‘ by ‗intelligent design‘ and placed on the Earth as discrete species at one point in time and have not evolved since.

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Evolution can be seen all around us. It is slow, of course, but each species is constantly changing. Evolution can even be controlled. For example, if axolotls were bred in a small tank with many other axolotls, and only a minimal amount of food was supplied, the smallest and fastest axolotls would survive to mate. These small and fast traits would then be passed down to their offspring and so on. Eventually generally the population of axolotls in the tank would be characteristically small and fast and would continue to evolve into a smaller and faster species. Eventually this evolution would plateau because there would no longer be a need to be even smaller and faster. If one denies natural selection, they are denying genetics. Creationists even push for ―intelligent design‖ to be taught in schools as well as, or in place of evolution. So far they have achieved this in several states in America, but each attempt was short lived – the courts soon overturned the decision to teach school children theory over fact. Everybody has the right to learn facts that have substantial supporting evidence, rather than be forced to believe and follow claims made by a book written thousands of years ago – the bible – with no evidence to support its claims. It is convenient that the bible states that one should not question the doings of the almighty God (Exodus 17:1-3). Another point of creationism outlined in 15 Answers to Creationist Nonsense is, ―Evolution is unscientific, because it is not testable of falsifiable. It makes claims about events that were not observed nor can be re-created.‖ Can this same statement not be applied to Creationism? Evolution can not be re-created immediately in a laboratory, but there are many fossils that provide the ―missing links‖ in the timeline of evolution. Nobody has contacted God and asked him to demonstrate ―intelligent design‖ in the modern day. Yet another question often posed by creationists is, ―Evolution cannot explain how life first appeared on Earth.‖ (Rennie, 2003, pp.62-69) This can be challenged by asking the question, ―if God made the universe, who made God?‖ which outlines yet another flaw of Creationism belief. What is God? What is God made of? How can one say that God made the universe if such a concept is incomprehensible? At least evolution ‗theory‘ can explain where the first forms of life came from. In the moment following the ‗big bang‘, sub-atomic particles became attracted to each other to form atoms, to form molecules and compounds, to form cells and chemicals such as chloroform which the earliest prokaryotic cells used to absorb light from the sun to create the energy needed to multiply and multiply until the unicellular organisms became multicellular and so on. Referring to the earlier mentioned discovery of the Tiktaalik roseae fossils, Chairman of aquatic biology at California Academy of Sciences, John McCosker says, ―these fish fill in the gaps in the fossil record that marks the transition of life from water to land, and the discoveries provide additional evidence that disputes the often heard and unfounded criticism made by creationists that adequate evidence doesn‘t exist to support these proposed ancestries…the only thing better than this would be to catch a live one‖ (Perlman, 2006)

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―Creation science‖ is an oxymoron. Science encompasses hypothesising, testing and questioning. Creationism involves taking the bible‘s word for what it says about creation theory, and not questioning at all. Creationists sometimes use the assertion that evolution scientists often argue and question their findings, so even scientists don‘t agree with their own theory of evolution. Scientists do not question evolution, they question within evolution – they question how evolution has been happening, in what ways, why it happens, how fast it happens, and what it affects. It is curious to observe that only non-creationists question creationism, however. They accept what is stated in the bible – the only source they have – and close their minds to the possibility of change and evolution over time that began before the bible even says time began. Just the fact that creationists have to twist the words of scientists and manipulate evidence to support their creation theory, suggests that they are having a great deal of trouble finding their own supporting evidence. Evolution science, however, had its own evidence, so scientists don‘t need to attack creation theory to prove evolution‘s validity.

Creationists believe that God made the fishes and the amphibians as completely separate and individual species which He ‗intelligently designed‘. Of course, there is an extensive amount of research and evidence to support the contrary, evolution ‗theory‘. It is one thing to be close minded to the evidence that today‘s scientists and technology can provide, but it is another to deny school children of opportunity to learn about fact (evolution) as well as theory (creationism) so that they can make up their own mind eventually. Unfortunately not enough people are educated about evolution and simply go with what they‘ve grown up with – the stories of Adam and Eve and the six days in which God created the Earth and the universe. Churches are able to use scare tactics to control what people ‗choose‘ to believe in, but scientists can only hope that there are enough free thinkers out there who look at the evidence and make a logical conclusion for themselves and realise that evolution is not a theory, but a fact.

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Bibliography: (1999) The American Naturalist, http://www.journals.uchicago.edu/AN/journal/available.html (21/5/07) Since its inception in 1867, The American Naturalist has maintained its position as one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. While addressing topics in community and ecosystem dynamics, evolution of mating systems, organismal adaptation, and genetic aspects of evolution, American Naturalist emphasizes sophisticated methodologies and innovative theoretical syntheses—all in an effort to advance the knowledge of organic evolution and other broad biological principles. Eusthenopteron, http://en.wikipedia.org/wiki/Eusthenopteron (23/5/07) Wikipedia is written collaboratively by volunteers from all around the world. With rare exceptions, its articles can be edited by anyone with access to the Internet. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference Web sites on the Internet. Because Wikipedia is an ongoing work to which in principle anybody can contribute, it differs from a paper-based reference source in some very important ways. In particular, older articles tend to be more comprehensive and balanced, while newer articles may still contain significant misinformation, unencyclopedic content, or vandalism. Users need to be aware of this in order to obtain valid information and avoid misinformation which has been recently added and not yet removed. However, unlike a paper reference source, Wikipedia can be constantly updated, with articles on topical events being created or updated within minutes or hours, rather than months or years for printed encyclopedias. Tiktaalik rosae, http://lancelet.blogspot.com/2006/04/tiktaalik-rosae.html (23/5/07) Musings on natural history by a partially-qualified graduate student. Perlman, D. (2006) Oldest evidence yet of fish moving to land, http://www.sfgate.com/cgibin/article.cgi?file=/c/a/2006/04/06/MNGCGI4CAD1.DTL (23/5/07) Northen California‘s largest newspaper posts several articles on its website. (2006) Ichthyostega and the Origins of Land Vertebrates, http://afarensis.blogsome.com/category/vertebrates/amphibians/ (23/5/07) An archaeologist‘s blog on which he/she posts extensively researched facts and archaeological concepts. Several pictures of fossils and cladograms of the evolution of amphibians from fish are also placed throughout his/her entries. The reliability is questionable, however the information illustrated in his/her cladograms is frequently supported by information with supporting evidence in several other sources used in this essay. Essenfeld, B.E.; Gontang, C.R.: Moore, R. (1994) Biology, Addiaon-Weasley Publishing Company, Inc., United States of America. Biology is a very comprehensive 950-page text book for high school biology students. It covers topics ranging from different animal kingdoms and anatomy to cell biology and genetics, containing excellent pictures, diagrams and illustrations. It is a genuine and reliable source of scientific facts up to date of 1994 and was moderated before publishing by several academic authorities. (2007) ―How Fins Became Fingers‖, Scientific American, May 2007. Clack, J.A. (2005) ―Getting A Leg Up On Land‖, Scientific American, issue. December 2005, pp.100-107. Doyle, R. (2002) ―Down with Evolution!‖, Scientific American, issue. March 2002, pp.20. Dunham, W (2007) ―Primitive Fish had Genetic Wiring for Limbs‖, Scientific American, May 2007. Rennie, J. (2002) ―15 Answers to Creationist Nonsense‖, Scientific American, issue. July 2002, pp.62-69. Rennie, J. (2006) ―Just Another Lousy Week for Creationism‖, Scientific American, April 2007. Shermer, M. (2002) ―The Gradual Illumination of the Mind‖, Scientific American, issue. February 2002, pp. 25. Dr. Michael Shermer writes for several magazines and scientific journals, including Skeptic and Scientific American. He‘s given lectures at numerous university across the United States of America and has written many books of his own. Shermer writes controversial satire articles that correspond

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with religious material he comes across in the media. His work is very much opinionated and makes one question their personal point of view pertaining to numerous different topics, relating back to science, psychology and philosophy. A credible source of data and opinions from the scientific community in general in relation to evolution versus creationism. (2003) Evolution, http://www.creationism.org/topbar/evolution.htm, (16/5/07) (2003) Carbon-14, http://www.creationism.org/topbar/carbon14.htm, (16/5/07)

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