Charles Darwin (1809-1882) and Alfred Russell Wallace (1823-1913) proposed the theory at a meeting of the Linnaean Society in 1858. It has inspired many uncomfortable feelings amongst some people and many attempts have been mad to suppress it. What is it about this theory that makes it so controversial!

Some of the Oldest Questions Asked By Mankind

Darwin's theory of evolution proposes different answers for old questions about who we humans are and why we are here. For thousands of years, human beings have wondered about the meaning of life. The traditional answers were provided by religions and cultural myths. All traditional answers see humans as exceptional creatures. Humans are not just animals, humans have apirits or souls. Only humans have free will.

The theory of evolution challenges these traditional ideas. The American philosopher, Daniel Dennett (b. 1942), has described the theory of evolution as a kind of "universal acid". Like universal acid, the theory of evolution eats through just about every traditional explanation for origins. This is why Dennett calls it "Darwins Dangerous Idea".

Darwin's dangerous idea comes in two parts:

a. The Theory of Evolution

b. The Theory of Natural Selection

Let's look at each of these two theories in turn, and then put them together. It is only when the two theories are put together that they become really dangerous.

What is Evolution?

The theory of evolution states that species can change. One species can give rise to another. According to the theory of evolutin, humans are descended from non-human ancestors. Ultimately, every species on Earth is descended from a single comon ancestor, just as the braches on a tree all spring from a single trunk. The Tree of Life

This might seem like a big idea, but for thousands of years people in the West believed that species were fixed, unchanging entities. So it was a great shock to learn that this was not true.

The idea that species are fixed and unchanging dates back to Aristotle (384-322 BC), the Greek philosopher and scientist. He based his belief on the evidence of his own eyes. Cats give birth to cats and dogs to dogs. So, not unreasonably, Aristotle concluded that one species could never give rise to another. Monkeys would always be monkeys. And humans must always have been human.

For hundreds of years, scholars accepted Aristotle's theory of the fixity of species. They believed that a "Special Creation" had created each species independently at the beginning of time and that each species then remained exactly the same up to present.

Eventually, in the 18th century, some people began to suggest that species are not fixed. They realized that species might change gradually. An extremely critical component of this idea was time. Given enough time maybe it was possible for this gradual change, by a series of many small steps, to result in one species evolving into another species. However, the special creation story held that the Earth and life had been fairly recently produced. There simply was not enough time for species to evolve from one to another, let alone for all life to be descended from a single ancestor. The world was simply not that old, according to the scholars. The Age of the Earth

When Bishop Ussher (1581-1656) added up all the figures in the Bible he calculated that the world was created in 4004 BC. If the Bible was literally true, the earth could only be 6,000 years old. This was nowhere near enough time for evolution to take place.

But in the 19th century, geologists were already beginning to realize that the world was a lot older than this.

There are clear patterns of erosions where rivers have cut into the mountains - and yet it takes thousands of years for water to erode such rocks.

-Charles Lyell (1797-1875)

There is a mass of other scientific evidence has confirmed what those early geologists suspected. The worl is almost a million times older than Bishop Ussher thought. Current estimates put the edge of the earth at around 4.5 billion years. This improves the possibility that species may have evolved over time. However, it is not enough to know that evolution could have happened. Is there evidence that can show whether or not evolution really did happen?

The Bones Speak

In 1811, Mary Anning found the skeleton of an ichthyosaurus, a marine reptile that lived between 5 and 120 million years agu, in cliffs near Lyme Regis in Dorset, England. For thousands of years, people have come across old bones in the rocks. Some of these bones resemble those from animals that are seen around us, but others clearly come from animals unlike anything alive today.

Huge fossil teeth, for example, have been found that could not come from any currently existing species of animal. Where do they come from? Perhaps the fossils teeth originally belonged to dragons? Perhaps the world was once full of strabge creatures that no longer exist?

The existence of strange fossils does not provide conclusive evience against the fixity of species in itself. One can accept the idea that some species have become extinct without thinking that one species can change into another. However, when you compare the various fossils that have been found, it is immediately obvious that they form patterns.

Many fossils have been arranged in sequences, in which there is a more or less continuous series connecting an earlier fossil with a later one. When you compare the fossil at the beginning of the chain with the final one, they look very different. The supposed intermediate fossils show much less of a difference. A rational explanation for this pattern is that all these fossil species appear to be related to each other.

The Radioisotope Dating and Relative of Fossils

Dating a fossil in terms of approximately how many years old it is can be possible using radioisotope-dating of igneous rocks found near the fossil. Unstable radioactive isotopes of elements, such as Uranium-235, decay at constant, known rates over time (its half-life, which is over 700 million years). An accurate estimate of the rock's age can be determined by examining the ratios of the remaining radioactive element and its daughters. For example, when lava cools, it has no lead content but it does contain some radioactive Uranium (U-235). Over time, the unstable radioactive Uranium decays into its daughter, Lead-207, at a constant, known rate (its half-life). By comparing the relative proportion of Uranium-235 and Lead-207, the age of the igneous rock can be determined. Potassium-40 (which decays to argon-40) is also used to date fossils.

The half-life of carbon-14 is 5,568 years. That means that half of the C-14 decays (into nitrogen-14) in 5,568 years. Half of the remaining C-14 decays in the next 5,568 years, etc. This is too short a half-life to date dinosaurs; C-14 dating is useful for dating items up to about 50,000 - 60,000 years ago (useful for dating organiams like Neanderthal man and ice age animals).

Radioisotope dating cannot be used directly on fossils since they don't contain the unstable radioactive isotopes used in the dating process. To determine a fossil's age, igneous layers (volcanic rock) beneath the fossil (predating the fossil) and above it (representing a time after the dinosaur's existence) are dated, resulting in a time-range for the dinosaur's life. Thus, dinosaurs are dated with respect to volcanic eruptions.

Finally, by Relative Dating, we mean that we determine the age of a fossil by looking at its relative position in the stratigraphic (layers of rock) record. This technique is often called stratigraphic dating. To use relative dating, we look at the sediments above and below a fossil; this allows us to say that an animal lived after one species (found in the rock below it), and before another species (found in the rock above it).

The Significance of the Dating Procedures

It is obvious that these dating procedures have supported the theorized age of the earth and its inhabitants. There is, however, another factor that this dating considers. The sequences of fossils that we mentioned. These sequences date out as being older, or younger, than their fossil relatives. This time sequence helps support the notion that these fossils represent the remains of evolving organisms. The fossil at the beginning of the chain is older than the next one, which is older than the next one, and so on. If all species had been created independently, we should not expect them to appear in the fossil record in any order at all, let alone the order of their physical similarity. The pattern of fossils thus appears to demonstrate that evolution is a reality.

The Tree of Life

From fossil and other evidence, biologists have been able to reconstruct the broad history of life on earth. Life appears to have begun here on Earth somewhere around 4 to 3.5 billion years ago. The logic is that the first form, or forms, of life must have been very simple.

3.5 billion years ago - Prokaryotic Cells

1.8 billion years ago - Eukaryotic Cells

600 million years ago - First multicelled life forms appeared-jellyfish and worms

500 million years ago - Fish and Plants

400 million years ago - Bony fish

370 million years ago - Animals begin to colonize the land Vertebrates and invertebrates

35 million years ago - Primates (lemurs)

5 million year ago - Australopithecines

100,000 years ago - Modern humans first appeared in Africa 

Living Fossils

The general outline of the Tree of Life is relatively well known, but discovering the exact shape of he branches is quite hard. Fossils provide only part of the evidence. The other evidence lies in our genes.

-98 percent of human genes are also found in the chimpanzee.

-There are also human genes in bananas, but not so many.

-Ths suggests that humans are more related to chimpanzees than to bananas.

In other words, the common ancestor of humans and chimpanzees was more recent than the common ancestor of humans and bananas. All living things carry a record of the history of their descent in their genes. Genes are like living fossils.

1. All organisms tend to increase at a geometric rate.

2. In each generation, however, the number of individuals in a species remains nearly constant.

3. Therefore, there must be a struggle to survive.

4. Variations (which may be inherited) are found among individuals in each species.

5. Some variations are favorable to an organism in a particular environment and help it to survive so it may reproduce. Surviving organisms pass on their variations to their offspring.

MICROEVOLUTION

Evolution is defined as a change in the genetic makeup of a population.

Variations in specific genotypes can change the overall appearance, or phenotype, of a population in time.

Natural Selection is a process that leaves organisms behind that survive environmental

conditions. As the process continues the population will change.

The Theory Of Natural Selection

In time, great differences arise, until finally a new species has evolved from an old species.

MACROEVOLUTION

The Classification System reflects the sequence of evolutionary events as we perceive them.

Dating Fossils 1

Dating Fossils

Relative Time

A Radiometric Dating Resource List

The Geology Wing

The Phylogeny Wing

The Evolution Wing