I have used some quotes to illustrate how science works. Quotes from anyone, whether they agree with the facts of evolution or disagree with the facts of evolution are not proof of any scientific theory. One of the first things that must be understood is that science is based on evidence. That evidence must be based on the scientific method of investigation. Quotes (authority), revelation (from a supernatural source), or tradition (voices from the past) play no part in science.
Be very suspicious of any source of material that does not give a bibliography or foot notes so that the information can be checked.
It is a pretty good idea to be very skeptical of anyone who says, "In my opinion...." or "It could have been..."
The quotes appear in bold type.
"Scientists do not invoke the supernatural to explain how the natural world works and they do not rely on divine revelation to understand it." Ernst Mayr - Prof of Zoology - Harvard
There have been many events on our planet that could be described as major. Two that stand out are the formation of the first ocean and the first land.
Imagine an ocean with pure water, with different degrees of light and temperature. Now think of the huge number of niches that would be there when life first evolved. A niche is the physical and biological range in which a species can live and reproduce.
Any form of life might fill a niche, but if competition became too strong, a slight change in that life form would allow it to move to another niche. Life forms would reproduce and expand to fill many niches, but eventually they were in competition with each other for space. The organisms that produced their own chemicals necessary for survival had a better chance of meeting competition. Other organisms would change to live off of these different life forms.
This same process happened on the first land. Imagine all of the land on earth with nothing living on it. The first organisms that ventured onto land had it all to themselves, but it did not last long. Others came and then there was competition for space and for food. Being able to survive and reproduce was challenging.
Evolution was progressing from the first life forms, to cells, and then to plants and animals. There is much that scientists want to learn about how life evolved on our planet. There is much that is known. There are many excellent books, beautiful and accurate video shows, and many magazines on the subject of evolution. Even with all of this, we still need to ask interesting and thoughtful questions.
In a series of short articles I would like to answer some of these questions - not as a scientist, but as a teacher. Most of the information in this booklet can be found in any good biology text book. My hope is that you will want to learn more, not by accepting what I have written, but by challenging it.
"The immensity of geologic time and the evolutionary origin of species are concepts that pervade modern geology and biology. These concepts must therefore be central themes of science in public schools; creationist's ideas (intelligent design) have no place in these courses because they are based on religion rather than science. Without knowledge of deep time and the evolution of life, students will not understand where they and their world came from, and they will lack valuable insight for making decisions about the future of their species and its environment." From, The Geological Society of America - Statement on Evolution, Report of the National Center for Science Education, Sept/Oct. 2002.
If plants and animals evolved at the same time, why are they so different?
This is a basic question which is often asked by students. The answer is not simple, and will take some understanding of what life is in order to answer.
The first thing to understand is that plants and animals are more alike than they are different.
Of course an apple tree is different from a dog, but the cells that make up an apple tree and the cells that make up a dog are very much alike. By comparing the cells in a plant and the cells in an animal, the similarities are easily seen.
An adult plant or animal has organs that carry on different functions. Cells have within them very small structures that also carry on different functions. These structures are called organelles. It is at the cellular level that most evolution takes place.
Cells need nourishment to carry out functions and they need to get rid of the waste that is produced from these processes.
It is the organelles in a cell, whether it be a plant cell, or animal cell that accomplish these tasks. Organelles are alive, but cannot live outside of the cell; just like a heart is alive, but cannot live outside of a body.
Organelles that are found in plant cells are almost exactly like those found in animal cells.
The typical plant or animal cell has a plasma membrane as the outermost covering. It has a mitochrondia which produces energy. It has a Golgi body which forms and packages proteins. These proteins are then secreted from the cell. These organelles and others are controlled by the DNA found in the nucleus of cells.
So plant cells and animal cells are very much alike. To answer the original question, we need to examine what makes them different.
Plant cells are very superior to animal cells because of a very important organelle, the chloroplast. This amazing structure enables the cell to carry on photosynthesis. This is a complicated chemical process, but it can be summed up as using sunlight to produce sugar and oxygen from carbon dioxide and water.
Plant cells need the sugar as a source of energy and to manufacture secretions (like sap and nectar). The oxygen is required for this process.
Cells that have chloroplasts can take carbon dioxide from the air, and by using the energy of sunlight, they can combine it with water to produce the two ingredients that support life.
With rare exceptions, all of the energy that animals get from eating comes from the sun. It has been stored in sugar by plants. Of course this sugar is often converted to starch or fat, but it is still basically sugar. It also means that the basic fuels that humans use (oil, coal, and natural- gas) are also basically sugar, because these fuels came from decomposing plant materials. Even when animals eat other animals, the energy provided is still proteins that were made from sugar.
To put it another way, when you had a hamburger for lunch in order to get materials for growth and energy, you were getting it from sugar.
Animals are just like plants in that they must combine sugar with oxygen to release the stored light energy. This oxygen is all produced by plants. Almost all of the free oxygen in our atmosphere is produced by plants and different forms of algae.
As I have stated, animals are just like plants in that they must combine sugar with oxygen to release the stored energy. This oxygen is all produced by plants and other living things that contain chlorophyll. Almost all of the oxygen in our atmosphere is produced by chlorophyll. Before the cells which carry on photosynthesis evolved, the earth's atmosphere was very different.
Oxygen is essential in this process, because it combines with the carbon molecules (sugar) to release the energy that it takes to hold these molecules together.
Photosynthesis is the process which takes two small molecules, water and carbon dioxide, and combines them to make larger molecules (sugar) and free oxygen. This takes energy supplied by the sun. When plant and animal cells use the products that are made from sugar, the process is reversed. Oxygen combines with sugar, and carbon dioxide and water are formed. The energy from the sun that held the larger molecules together is released in the form of heat energy. That is what enables warm blooded animals like humans to have a 98.6 body temperature. It is also why heat is released when wood, coal, or oil is burned. The burning of any carbon compound is just turning big molecules into small molecules. The same process happens in our bodies cells, it just happens much more slowly.
So, the short answer to the original question is: Plant cells evolved the ability to produce sugar and free oxygen for their survival. Animal cells evolved to survive off of the extra sugar and oxygen that plant cells produce. It is the overproduction by plants that enables animals to survive.
Of course there are some other basic differences between plant and animal cells.
One interesting difference between plants and animals is their method of handling cell waste. Excretion is a process that must take place or the cell will die. All animals have some method of removing waste. Some of the simple animals like sponges do not have a special organ to accomplish this. The more complex animals have kidneys that filter waste out of their blood. Each functioning cell deposits its waste in the blood which then gets pumped to the kidney where it is filtered. In the higher animals, this waste is then stored in the urinary bladder until it is emptied.
It is a different story for plants. Plants have evolved cell walls around some cells; they cannot get rid of waste. Instead it is stored in the cell in a bubble like structure called a vacuole. When the vacuole gets full, the cell dies. This is no problem for parts of plants that only live a few months, like flowers and leaves. It turns out to be a very good thing for some parts of the plant, like the stem.
These special cells that have evolved cell walls and a vacuole, are referred to as xylem (the stringy stuff in celery is xylem). It is better known to most of us as wood. It is the xylem that shapes trees and bushes Before wood evolved there were no tall plants, shrubs, or vines. If there were more of it in the celery, we would call it wood. It is good for us; it is the roughage of our diet.
Because animals can get rid of waste, they can rebuild and repair cells. Some of the cells in your body will live for years because they can be repaired and the waste removed. That is good for us and all animals.
Plants have evolved a different method and it is just as successful for them. They evolved so that almost all of their cells live less than a year.
These short chapters on evolution should be making you ask more questions than they answer. In science it is easier to answer the "how" questions than the "why" questions. We can answer how it is that certain plants have opposite leaves, but we cannot answer why it is that certain plants have opposite leaves.
"We humans pride ourselves on being the dominant life form on this planet, but these tiny beings-- unobtrusive, the perfect guests-- are in a sense running the show. Without them, almost all life on Earth would die." - Carl Sagan & Ann Druyan (talking about chloroplasts) Shadow of Forgotten Ancestors, 1992, page 101
It became obvious to early scientists that evolution was a theory that had much factual evidence to support it. That meant that they could begin to predict what they might find in the way of undiscovered living species (living fossils), and what might be uncovered in later excavations of preserved fossils.
Fossils of the ginkgo tree have been found in North America that date back seventy-five million years. They were thought to be extinct. Imagine the delight of western scientists when they discovered that they had been cultivated around Buddhist temples in China for thousands of years. There may be some wild ginkgoes in remote areas of China, but it appears that there is only one surviving species of a group of plants that were very numerous during the age of the dinosaurs. This has made the ginkgo a popular tree to cultivate, and it can safely be said that there are ginkgoes on almost every college campus in the United States. It is a classic example of a living fossil.
There are, of course, a number of living fossils. Plants and animals that were thought to be extinct only because no one was looking for them. Some were discovered by accident, like the coelacanth fish, and others were discovered through diligent scientific work, like the Peripatus (an animal referred to as "walking worms" that has characteristics of an earthworm and an arthropod).
An interesting example of a living fossil discovered through scientific research is the alligator. For years it has been classified as a reptile. After all, it looks like a reptile - but it does not behave like a reptile.
Alligators build a nest; reptiles do not. They communicate with their young before and after they hatch from eggs; reptiles do not. Both alligator parents guard the nest and young, and bring them food; reptiles do not. Alligators stay in family groups for years; reptiles do not.
There are physical differences between alligators and reptiles. Alligators have a four-chambered heart like birds and mammals, reptiles have a three-chambered heart. Alligators have a breathing muscle, the diaphragm; reptiles do not. They also have a highly developed brain part called a cerebral cortex; reptiles do not.
Their behavior and their special anatomy are more bird-like than reptile like. Now that scientists are able to sequence the DNA of living things, it is possible to determine close relationships by doing comparisons. It was not surprising to find that the DNA of alligators is more closely linked to birds than it is to reptiles.
Biology text books still classify the alligator as a reptile, but it is apparent that alligators are somewhere between birds and reptiles in the evolutionary scheme of things.
If evolution is a fact, why are there some scientists who do not believe it?
It is true, there are some so-called scientists who do not accept evolution as a fact, but we must be careful to know who are scientists and who are not.
There are people and organizations that do not accept evolution. It is important to know that there is no evidence to support any other cause for the diversity of life on earth. There is no other theory to explain life on earth because a theory must be supported by evidence. There are scientists who do not accept evolution and yet offer no theory to take its place. That is not the way science works. Proving a theory to be wrong does not make another theory right. As an example: when it was discovered that the flu is not caused by bacteria, we still did not know what did cause it. It took more research to discover that it was caused by a virus.
If the theory of evolution is wrong, then there must be evidence to support another theory that would explain the diversity of life on the planet. No one has discovered such evidence.
Every college and university in the world that does scientific research accepts evolution. Every major scientific publication and organization accepts evolution.
Long before Darwin wrote, On The Origin of Species, evidence was being gathered to support the theory of evolution. It should be apparent, even to the most uninformed, that advances have been made in gathering evidence to support the theory. Charles Darwin did not have the benefit of computers, ultra-sound, the electron microscope, radioactive dating, X-ray, plate tectonics, DNA sequencing, or any other scientific advancement of the last one hundred and fifty years.
The scientific instruments that are used in the study of medicine, immunology, endocrinology, and other health related fields are the same instruments that are used to study evolution. The instruments that are used to search for natural resources, develop agriculture, advance animal husbandry, investigate crimes, and further environmental studies are the same instruments that are used to study evolution. It would not be good science to accept all of the knowledge gained in these areas and ignore the knowledge acquired about evolution.
It is these advanced scientific instruments and the scientific method that supports all scientific research.
The scientific method is based on observation and experimentation. These must be repeatable, and others must get the same results. Every scientific theory must have evidence to support it. The word "theory" is often not used scientifically. We must be careful to use it correctly.
The evidence that supports evolution should be able to stand the test of reliability. That is what science is all about and why evolution is based on science.
We should be able to evaluate and challenge any textbook or authority, whether it supports evolution or not.
The important thing to remember is that the scientists in all of the major research universities are gathering more evidence every day in support of the theory of evolution. This information is available to everyone who wants to read the books and journals.
One of the best places to go to find out about the theory of evolution is a natural science museum. There are major museums like the Smithsonian or the Natural History Museum in New York City. Every state has smaller excellent natural history museums. These are usually found on the campus of larger universities, and always at state universities. Most of these museums have information on the Internet, but a trip to observe fossil evidence, preserved specimens, and displays that show and explain evolution is something that every educated person should take the time and energy to do.
There will always be someone who does not accept this research. What science demands is that they show their evidence for the challenge and the evidence for their replacement theory.
There are hundreds of natural history museums around the world that have excellent fossil collections and displays on evolution for the general public. The London Natural History Museum has an "Origin of Species" wing which explains the concept of evolution by using Charles Darwin's approach.
It became evident to scientists around the world that much of the evidence was not on display in most museums. They understood that there should be a place where scientists could send specimens, borrow specimens, and study all evolutionary collections.
That place is the new Darwin Centre in London. The goal is to systematically classify all plants and animals living and extinct that have ever lived on earth. This is a tremendous challenge. At present the first phase has been completed. There are 22 million specimens, 450,000 jars, and over 15 miles of shelving. Many of the specimens were collected by Charles Darwin.
There will be video links from the labs, and from researchers in the field, to the museum and the Web.
Phase two, which will be completed in 2007, will add 28 million insects and 6 million plants.
Imagine what this will mean for the continuing battle with those who support intelligent design. It is not that we will contact the museum often, but we will have a resource with world wide status supported by scientists and universities everywhere.
Everything available at the Darwin Centre will be available to anyone with a computer. On completion, there will be over 60 million specimens and an unbelievable amount of scientific information to support the theory of evolution. if you are on line just type in www.DarwinCentre.com to access and put yourself in the world of evolutionary science.
We humans seem to get great joy from comparing things. We do it in sports, music, intelligence, and even with living things.
Of course we put ourselves at the top and then compare the lower forms with each other. Hopefully, by now you will have learned that there is usually another way to look at most questions involving living things.
The amoeba is a very interesting lower form. It must carry on most of the functions that most living things do, and yet it is a single cell. This single cell must find food, digest it, get rid of waste, use oxygen, repair worn out parts, and reproduce - not bad for a lower form.
Let us compare this single cell with any single cell in your body. How about a white blood cell? They look very much like an amoeba and they carry on the same functions. What about a muscle cell or a liver cell? Well, they don�?t look much like an amoeba, but they must carry on the same functions to stay alive. In the case of the muscle cell, it must contract, and in the case of the liver cell, it must secrete chemicals useful to other cells.
Now you can see what I am getting at. Our bodies are composed of lots of cells that we considered lower forms. The main reason that the lower forms in our body differ from the lower forms found in a pond is that our cells have become so specialized that they need help from other cells in order to stay alive and do their jobs. They have become so specialized that they cannot make it on their own like an amoeba can.
Would you say that the specialized cells that make up our body are a higher form of life than the amoeba which can survive on its own? I do not want an answer to that question, I just want you to think about what your answer would be and why.
This leads to an even more interesting question. What is the reason for the specialized cells gathering together to form organs? We all know that the heart pumps blood, the lungs take in oxygen, the kidneys filter out waste and the muscles make us move, but why?
Most of us want to say that our brain must be kept alive. After all, that is what makes us who we are. True, but let us use that same logic on an earthworm or a pine tree. What is the function for the organs in an earthworm? They do the same for the earthworm as they do for us. For some reason the earthworm brain does not seem that important to us. And what about the organs in a pine tree? Of course they have organs too. What is the real function of the leaves, roots, bark, and stem?
Let us go back to the lower forms of life. What do they do that all the higher forms do in order to insure that their species survives? They pass on their DNA to future generations. All of the organelles in one-celled creatures, and all of the organs in higher plants and animals must keep the individual alive long enough in order pass on DNA.
Now comes the lesson. Of vital importance is the fact that sometimes DNA has changes in its chemical structure. Those changes may lead to individuals that are more able to survive. This is the wonder of that process called evolution. What we refer to as the lower forms of life have evolved into earthworms, pine trees, and you and me. Feel better about the lower forms of life now?
Charles Darwin said, "It is not the stronger of the species that survive, nor the most intelligent, but the one most responsive to change."
Philip Johnson, Darwin on Trial: In reference to Intelligent Design, declared, "This isn't really, and never has been, a debate about science. It's about religion and philosophy." From Natural History 4/02
Just what is a lower form of life? It depends on the definition used. If the definition is one of structure and physiology, then the lowest form would probably be bacteria. (Virus is not considered living; it is classified as an infectious chemical.)
If the definition includes the functions of all organisms in the biosphere, it is a different story. Bacteria are not lowly. They are one of the main sources of decomposition, and life on this earth would not be possible without them. The same can be said for the fungi.
Just what would an eco-biologist classify as lower forms of life?
The best way to answer that is to ask, "What would an eco-biologist classify as the highest form of life?"
Hands down it is a maple tree (you may substitute any other tree that is wind pollinated, but be careful). Maple trees overproduce leaves which fall to the ground, and with the help of bacteria, become soil. They carry on photosynthesis and overproduce sugar which is the building block for all forms of life (without sugar there would be no higher forms of life; you are basically sugar).
Through this same process they overproduce oxygen.
Some is used, but the rest is snapped up by the environment and makes a suitable environment for animals. This process also takes ground water and releases it into the atmosphere, helping to make rain clouds. The maple tree also overproduces seeds - billions of them. The extra serve as a great source of nourishment for a multitude of critters. While this is going on, the tree is an ecosystem for hundreds of living things.
They give off enough warm moisture, through transpiration, to cause air currents. The maple tree produces very tiny male plants called pollen. This pollen travels with these air currents to other trees where they meet a female flower which has an ovary. The little pollen grain then grows a tube to unite with the ovary. It then produces a sperm which swims through this tube and fertilizes the egg produced by the ovary. It even make its own delivery system for sex.
I tell you this story to let you know that the maple tree is almost completely self-sufficient.
Let's add this up. Maple trees make their own soil, which others use. They make their own food, which others use. They make their own oxygen, which others use. They make rain clouds, which others use. They produce seeds, which others use. They furnish habitat for others. They do not count on other living things for reproduction. So if some biologists think the maple tree is the highest form of life, what is the lowest?
To a biologist, the highest form of life is the gene, and the lowest is the adult that any gene helps produce. A harsh statement? No, think of it this way: the gene passes from generation to generation for thousands, millions, and even hundreds of millions of years by the process of replication, sometimes with little or no change. Adults on the other hand, last a few years or decades at most. Almost every human and maple tree alive today will be dead in seventy-five years. They will be replaced, and none will be exactly like those alive today. But, that future gene pool will most likely be almost exactly like the one in existence today.
What does any adult of any living organism actually do? It preserves and passes on genes. All of the organs in any living thing, plant or animal, have cells that are there for only one purpose - to preserve and pass on genes. The genes in any organ, heart, liver, leaf , etc. have no effect on the gene pool of future generations. Even the cells that make up the ovaries in plants and animals, and the cells that make up testicles and pollen are of little consequence. It is the genes in eggs and sperm that determine the next generation. It is a mutation or change in the DNA of a gene that accounts for the process of evolution. Genes evolved cells, organs, and organisms to preserve their existence.
But most of the genes in that species will be found in similar species that survive. The basic principle of evolution is the survival of genes and other genetic material.
In 1976, The Selfish Gene, by Richard Dawkins was published. It created quite a controversy, especially in religious circles. Imagine saying genes produce adult humans. Not the way we usually think, humans producing humans (the biblical approach) by passing on genes. The concept of the gene being much more important than the adult is very different and sometimes it is compared to the "which came first, the chicken or the egg," riddle. Of course by understanding evolution, we know that the egg came first. Obviously, the parents of the chicken may have been very different from her, and her offspring may not be like her or the rooster who mated with her. It is the egg and sperm that cause a difference, and given enough time and the right environment, may even lead to a new species. The chicken, or for that matter, you and I, are just the factories where the genes do their work.
If you have any interest in evolution, get a copy of The Selfish Gene. It is easy reading and guaranteed to make you think. There were biologists who disagreed with what Dawkins had to say, but he quieted most with a second book, The Extended Phenotype (1982). This book is not as easy to read, but it is well worth the effort.
Secretions are the useful chemicals produced by a cell. The cell is living, but secretions are nonliving substances. They are not to be confused with excretions, the waste materials given off by a cell. All secretions are valuable because they aid in the survival of the organism. Some of them are obvious and well known to us, like claws, teeth, and bone. Others are not as obvious, but just as important.
About three billion years ago, there were no multi cellular living things. All bacteria and cells were unicellular. That is, each cell carried on all the functions to maintain life. Then one of the most important events in the history of evolution occurred. Mutations in the DNA caused a secretion that allowed cells to remain together. This was extremely important because it meant that organisms could be multi-cellular. Because of this development, some cells in the organism could develop special functions and, therefore, be more efficient. Some cells could produce a secretion for digestion, some could produce a secretion for movement, some could produce a secretion for protection. The possibilities were enormous. With all of these different cells working for the survival of the organism, it helped to ensure that the cells that carried on reproduction in the organism had a better chance for survival.
The process of cells forming different specialized tissue is known as differentiation and it is what led to the beginning of plants and animals.
In our own bodies we can see how important it is for muscle, skin, and nerve tissue to hold together. We can also see that some cells do not produce the secretion or else blood would not be a liquid and sperm would not be able to swim.
Many other secretions that are responsible for growth and development in both plants and animals are the hormones. Each hormone is the result of a specific bit of DNA (a gene) producing the proper protein.
The beak of a bird, the cartilage of an ear, the venom of a snake, the lens of an eye, milk, honey, and saliva are all secretions.
In plants, the same secretion that forms cell walls also forms the veins of a leaf and the wood of a tree. Plant cells also produce other secretions, such as peanut oil, maple sugar, sap, and poison.
Almost everything we think of as living is made of cells and the secretions made from those cells.
Whenever a biological process takes place and energy is stored or released, or chemicals are changed in form, some waste material is a by-product. Therefore, when the energy stored in sugar is released or a secretion is produced, a waste product is produced which must be removed (excreted).
Because this is a waste, it can be dangerous to the cell or organism. If the waste is inside of a plant cell it cannot escape because of the non-living cell wall. In this case, the waste is stored in a vacuole inside the cell until the cell dies, usually in less than a year. This is the reason that some types of wood have a different color.
If the waste is produced by a one celled organism living in water, it simply protects itself by discharging it and moving away from the waste. If the waste is produced by a cell in a multi-cellular organism like yourself, the waste must be carried away. When any cell in your body functions, it will produce some waste. That waste will be picked up by the blood stream and carried to the kidneys. The kidneys are made up of special cells which form nephrons. They have the ability to remove the waste and transport it to the bladder in the form of urine. It can be stored safely there until urination. All vertebrate animals function this way.
This process is important, because if the kidneys stopped working you would be very ill in a few hours, and dead in a few days. This rarely happens, and usually enough nephrons function to keep us healthy, or at least alive, when we have kidney infections until some other biological mechanism (the immune system) saves us.
As you might imagine, the evolution of these specialized cells was very important in the evolution of all forms of animal life. So much so that the nephrons found in frogs, fish, and you are almost identical. They are also very much like the nephridia, which perform the same function in worms and insects. It was the evolution of this specialized cell that enabled multi-cellular animals to develop. Without nephrons and nephridia, no animal life could exist.
Of course, different cells produce different wastes. Yeasts produce alcohol, and bacteria produces a variety of acids.
As you might imagine, because evolution is a never ending process, some organisms have evolved to use the waste of others. So the alcohol excreted by yeasts in making beer, wine, and other alcoholic beverages is consumed to keep bacteria alive, and they in turn excrete different acids. A good example is sweet cider, containing sugar, which the yeast use for food. The yeast then give off alcohol that the bacteria use, and the bacteria then excrete acid: sugar to alcohol to acid. In other words, sweet cider to hard cider to vinegar. The alcohol in hard cider or wine is "yeast pee" and the acid in vinegar is "bacteria pee."
The reason that milk is pasteurized is to kill the bacteria which would produce acid and make the milk curdle (sour). On the other hand, we can introduce certain bacteria to cause the milk to curd and form cheese.
Wine and beer can also be pasteurized to stop bacterial growth.
"The rock record is a treasure trove of fossils, and by 1841, eighteen years before Charles Darwin published On the Origin of Species, geologists had not only assembled much of the geologic time scale from physical relationships among bodies of rock, but they had also recognized that fossils document profound changes in life through earth's history. Darwin showed that biological evolution provides an explanation for these changes. Since the time of Darwin, geologists have continued to uncover details of life's history, and biologists have continued to elucidate the process of evolution. Thus, our understanding of life's evolution has expanded through diverse kinds of research, much of it in fields unknown to Darwin, such as genetics, biochemistry, and micropale- ontology. In short, the concept of organic evolution has not only withstood the test of time - the ultimate test of any scientific construct - but has been greatly enriched." From: The Geological Society of America- Statement on Evolution, Report of the National Center for Science Education, Sept/Oct. 2002.
Digestion is the chemical process of getting food ready for cell use. Almost all digestion takes place outside of the cell and usually involves turning large chemicals, like proteins, into smaller chemicals, such as amino acids, so they can be used by the cell.
Plants must digest the sugars and starches that they store before they can be used by the cells. Animals evolved a different way when it comes to digestion. As the food enters an animals mouth, it may bring with it many useless and dangerous chemicals for cell survival.
As animals evolved and became more complicated, with many types of tissues and organs, a gut evolved to carry on digestion. At the same time, the gut protects the cells that make up the animal. Animals have a protective outer layer of cells (skin) to protect the living cells from the external environment and they also have a layer of cells lining the gut to protect cells from the substances in the gut.
Although it may seem like the food we eat is inside of us, it is not. It is in a tube that starts with our mouth and ends with our anus. In order for food to get into our body it must enter the blood. Almost everything we put into our mouths would be poison to our cells if it went directly into the blood. Meat, and eggs are just a few things that must be digested before they enter the blood stream or they would kill us.
Because evolution is an ever changing process, some animals evolved digestive juices to digest only certain foods. Many insects live off of only one type of plant. Some animals, like owls, evolved a method of regurgitating indigestible materials. Snakes have very strong digestive juices and can break down almost anything they eat.
This information is important in the study of evolution because some bacteria and worms have evolved that not only can survive in a gut, but in many cases must survive in a gut. Almost every animal from insects to humans have these parasites.
An understanding of how our digestive system works and the parasites that invade it has been, and will continue to be, significant in medicine and healthful living. It is possible to follow the branches of the evolutionary tree back to the first digestive systems. These first systems have been very successful, because many of the animals that have them have been on earth long before animals evolved to live on land. There are many similarities between your digestive system and that of a shark.
There are many interesting side issues in the digestive process that are very meaningful to people around the world. Animal "waste" (feces) is still one of the main sources of fertilizer. In many countries, intestinal "waste" is dried and used as fuel. And, of course, there are thousands of insects, worms, fungi, and bacteria that survive because of what we term as "waste."
"Life on Earth began about 3.5 billion years ago. At first, living organisms were simple, like present-day bacteria, in both their metabolism and structure. However, over thousands of millions of years cells gradually increased in complexity through evolution by natural selection. The process is easy to understand: As organisms reproduce, their offspring differ slightly from each other in their features - they are not identical. Offspring with features that make them poorly adapted to the habitat probably do not grow well and reproduce poorly if they live long enough to become mature. Offspring with features that cause them to be well-adapted grow well and reproduce abundantly, passing on the beneficial features to their own offspring." From, Botany - An Introduction to Plant Biology, James D. Mauseth, 1998, Page 9.
