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| Pre-conditions for Evolution |
Pre-conditions for Evolution
The evolution of living things requires a number of pre-conditions. These preconditions are not causes of evolution, but set the stage and allow the actual mechanisms of evolution to operate.
Pre-condition #1: Reproduction of cells
Robert Hooke first detected cells in a thin slice of cork in 1665 using a microscope of his own construction. Investigations over the next two hundred years yielded more knowledge about the basic structure of all living things, and in 1838, Schleiden and Schwann published the cell theory . In modern terms, the cell theory states that:
1. All living things are composed of cells
2. New cells are formed by the division of pre-existing cells
3. All cells share fundamental chemical compounds and metabolic activities
4. The activity of an organism is the sum of the activities of its individual cells.
The mechanisms of cell division are of particular importance for understanding the theory of evolution. Every cell contains a structure called the nucleus. Within the nucleus, all the instructions to control the activity of the cell are contained in a few dozen pairs of chromosomes (23 pairs in humans). Under a microscope a chromosome appears as a short lumpy string of material. Biologists have determined that chromosomes contain thousands of substructures called genes made of deoxyribonucleic acid (DNA), and that the genes control inherited traits. In 1953, James Watson and Francis Crick determined that DNA molecules consist of two long chains held together in a double spiral structure, and that the chains are formed from just four simple organic compounds. The arrangements of these compounds along the double helix of DNA contain the code of life.
In cell division, each DNA structure unwinds into two long chains. Each chain then rebuilds the missing half from extra material in the cell, to form a new double helix of DNA. Then the two sets of DNA move to opposite ends of the cell, the middle of the cell contracts and the parent cell divides into two daughter cells. Each new daughter cell has a complete set of DNA. Any minor alteration in the replication of a cell’s DNA that is not fatal, is a mutation that is passed on to all future generations of that cell.
Pre-condition #2: Capacity for rapid population growth
Plants and animals have a tremendous capacity for reproduction.
Every tree has the reproductive potential to cover the entire Earth in just a few generations. For example, each year a single apple tree produces over a thousand apples. And each of these apples is capable of producing a whole new apple tree, if it happens to fall on fertile ground. It takes about fifteen years for an apple tree to reach maturity, so in fifteen years the first year’s apples could create a thousand more mature second-generation trees. Those trees could produce a million new trees a year; and so on. Just counting the newly matured trees at 15 year intervals, in 60 years a single apple tree could produce enough new trees to cover all the land on Earth.
In the animal kingdom, consider the racoon. A female racoon reaches maturity in two years, and after that can produce six kits a year. Suppose we started with five pairs of racoons and they produced six kits a year for ten years, then we would have over two thousand baby racoons. In thirty-eight years we would have over seven billion baby racoons. That’s more than enough for every person on Earth to have a pet racoon.
If all life has such a fantastic potential for growth why aren’t we up to our eyeballs in apple trees and racoons? Not all offspring survive. Disease, starvation, accidents, and battles over territory devastate the numbers in each new generation of every organism, from earthworms to humans. However, given a chance, any organism is ready to burst out from these restrictions and to populate the Earth with its descendants.
Pre-condition #3: Variations of characteristics in a family Within every one of us resides the potential to produce offspring with a wide range of characteristics.
Humans have 23 pairs of chromosomes in every cell in their bodies; one of each pair comes from the father and one from the mother. When an egg cell is created in the ovaries of a female it includes just element from each pair of chromosomes. Since the selection from each pair is random, there are over 8 million (2 to the power of 23) possible combinations for creating an egg cell. A similar process occurs in the production of sperm cells in the testicles of a man. When a sperm fertilizes an egg, the corresponding chromosomes reform as DNA pairs. There are an amazing 70 000 billion (2 to the power of 46) ways to achieve this. A single couple has the theoretical capability to produce billions of unique children.
Pre-condition #4: Variations of characteristics in a population
In a large population composed of many families, any measurable characteristic is found to vary over a wide range of values. For example, suppose you measured the weight of a million adult European males and constructed a frequency graph of the results. You might find that weights ranged from less than 45 kg (100 lb) to more than 180 kg (400 lb), with most individuals in the 70 kg (154 lb) to 110 kg (242 lb) range.
You could conduct similar studies of any characteristic, such as the strength of index fingers or the ability to hear soft sounds, and you would obtain similarly shaped distribution graphs.
Pre-condition #5: Time and the age of the Earth
Evolution requires extended periods of time, corresponding to thousands of generations.
The order of significant events in the geological history of the Earth is recorded in layers of rock. You can see these layers in any cliff or eroded bluff. More impressive layers can be seen across larger features such as the Alps in Switzerland or along the walls of the Grand Canyon in Arizona. Layers are visible because they have different textures and contain different types of minerals.
Geologists have developed a classification system to describe the relative ages in Earth’s history based on the structure of these layers in the Earth’s crust. This system describes the Earth’s history in eons, eras, periods, and epochs. Eons are the largest chunks of time; periods are the shortest. Geologists and prospectors have used their knowledge of rock layers to discover and trace the seams and veins of valuable mineral deposits.
Some layers of rock contain fossils. A fossil is the mineralized remains of an ancient plant or animal. Specific types of fossils tend to occur in specific layers, and the layers can cover extensive regions. For example, the Burgess Shale in Yoho National Park in British Columbia has been declared a World Heritage Site because it contains a wide range of fossil invertebrate animals from the Middle Cambrian age. Layers of sedimentary rock from the Jurassic and Cretaceous periods are famous for the fossilized dinosaur bones they contain.
How can the actual age of these layers and the Earth be estimated? Several approaches have been tried. In 1650 Archbishop James Ussher of Armagh published the results of his analysis of the Bible, correlated with Babylonian history, in which he determined that the world was created in 4004 BCE. The work of Ussher was a valid interpretation of the available historical data. His notion, that a rational process could be used to determine the age of the Earth, inspired many future investigations of the same question.
In 1779 the French naturalist Comte du Buffon estimated the age of the Earth to be 75 000 years. This age was based on experiments exploring the cooling of the Earth from an original molten state. In 1862, the physicist William Thomson refined this method and found an age for the Earth between 20 and 400 million years. In 1899, John Joly estimated the age of the Earth to be 90 million years based on the rate at which salt accumulated in the oceans.
Radioactivity was discovered in 1896. When an atom decays by emitting radiation, it is transformed into another type of atom, and the rate at which this happens can be measured in the laboratory. Knowledge of the rates of decay for radioactive material can be used to determine how much time has elapsed since a rock was in the molten state. For radiometric dating of rocks the most useful reactions involve the transformation of uranium into lead. The age of a rock sample can be determined by comparing the relative amounts of lead and uranium that the rock contains. The oldest rocks on Earth found with this method are about 4.55 billion years old.
Using models of stellar evolution, astrophysicists estimate the age of the Sun at 5 billion years. And based on additional astronomical observations, cosmologists estimate the age of the whole universe to be about 13 billion years. So an age of 4.55 billion years for the Earth is consistent with basic astronomical data.
This age for the Earth can then be used to date the geological periods based on rock layers, and the fossils in the corresponding layers can also be dated. The oldest fossils of bacteria are thus estimated to be about 3.5 billion years old. The oldest creatures such as worms and trilobites are about 550 million years old. The dinosaurs roamed Earth during the period from 250 to 65 million years ago. The oldest hominoid fossils have been found in Early Miocene deposits (22 million years old) in Kenya.
If an average human generation spans 22 years, then there have been about a million generations for human evolution to take place.
Pre-Condition #6: Energy
It takes energy to power life. Plants gather their energy from sunlight, herbivores harvest the sun-energy stored in plants, and carnivores harvest the sun-plant-energy collected by herbivores. Living forms are more successful when they are more effective in gathering and converting the available energy to grow and reproduce.
With an understanding of the six pre-conditions described above, we are ready to delve into the actual mechanisms of evolution.
The evolution of living things requires a number of pre-conditions. These preconditions are not causes of evolution, but set the stage and allow the actual mechanisms of evolution to operate.
Pre-condition #1: Reproduction of cells
Robert Hooke first detected cells in a thin slice of cork in 1665 using a microscope of his own construction. Investigations over the next two hundred years yielded more knowledge about the basic structure of all living things, and in 1838, Schleiden and Schwann published the cell theory . In modern terms, the cell theory states that:
1. All living things are composed of cells
2. New cells are formed by the division of pre-existing cells
3. All cells share fundamental chemical compounds and metabolic activities
4. The activity of an organism is the sum of the activities of its individual cells.
The mechanisms of cell division are of particular importance for understanding the theory of evolution. Every cell contains a structure called the nucleus. Within the nucleus, all the instructions to control the activity of the cell are contained in a few dozen pairs of chromosomes (23 pairs in humans). Under a microscope a chromosome appears as a short lumpy string of material. Biologists have determined that chromosomes contain thousands of substructures called genes made of deoxyribonucleic acid (DNA), and that the genes control inherited traits. In 1953, James Watson and Francis Crick determined that DNA molecules consist of two long chains held together in a double spiral structure, and that the chains are formed from just four simple organic compounds. The arrangements of these compounds along the double helix of DNA contain the code of life.
In cell division, each DNA structure unwinds into two long chains. Each chain then rebuilds the missing half from extra material in the cell, to form a new double helix of DNA. Then the two sets of DNA move to opposite ends of the cell, the middle of the cell contracts and the parent cell divides into two daughter cells. Each new daughter cell has a complete set of DNA. Any minor alteration in the replication of a cell’s DNA that is not fatal, is a mutation that is passed on to all future generations of that cell.
Pre-condition #2: Capacity for rapid population growth
Plants and animals have a tremendous capacity for reproduction.
Every tree has the reproductive potential to cover the entire Earth in just a few generations. For example, each year a single apple tree produces over a thousand apples. And each of these apples is capable of producing a whole new apple tree, if it happens to fall on fertile ground. It takes about fifteen years for an apple tree to reach maturity, so in fifteen years the first year’s apples could create a thousand more mature second-generation trees. Those trees could produce a million new trees a year; and so on. Just counting the newly matured trees at 15 year intervals, in 60 years a single apple tree could produce enough new trees to cover all the land on Earth.
In the animal kingdom, consider the racoon. A female racoon reaches maturity in two years, and after that can produce six kits a year. Suppose we started with five pairs of racoons and they produced six kits a year for ten years, then we would have over two thousand baby racoons. In thirty-eight years we would have over seven billion baby racoons. That’s more than enough for every person on Earth to have a pet racoon.
If all life has such a fantastic potential for growth why aren’t we up to our eyeballs in apple trees and racoons? Not all offspring survive. Disease, starvation, accidents, and battles over territory devastate the numbers in each new generation of every organism, from earthworms to humans. However, given a chance, any organism is ready to burst out from these restrictions and to populate the Earth with its descendants.
Pre-condition #3: Variations of characteristics in a family Within every one of us resides the potential to produce offspring with a wide range of characteristics.
Humans have 23 pairs of chromosomes in every cell in their bodies; one of each pair comes from the father and one from the mother. When an egg cell is created in the ovaries of a female it includes just element from each pair of chromosomes. Since the selection from each pair is random, there are over 8 million (2 to the power of 23) possible combinations for creating an egg cell. A similar process occurs in the production of sperm cells in the testicles of a man. When a sperm fertilizes an egg, the corresponding chromosomes reform as DNA pairs. There are an amazing 70 000 billion (2 to the power of 46) ways to achieve this. A single couple has the theoretical capability to produce billions of unique children.
Pre-condition #4: Variations of characteristics in a population
In a large population composed of many families, any measurable characteristic is found to vary over a wide range of values. For example, suppose you measured the weight of a million adult European males and constructed a frequency graph of the results. You might find that weights ranged from less than 45 kg (100 lb) to more than 180 kg (400 lb), with most individuals in the 70 kg (154 lb) to 110 kg (242 lb) range.
You could conduct similar studies of any characteristic, such as the strength of index fingers or the ability to hear soft sounds, and you would obtain similarly shaped distribution graphs.
Pre-condition #5: Time and the age of the Earth
Evolution requires extended periods of time, corresponding to thousands of generations.
The order of significant events in the geological history of the Earth is recorded in layers of rock. You can see these layers in any cliff or eroded bluff. More impressive layers can be seen across larger features such as the Alps in Switzerland or along the walls of the Grand Canyon in Arizona. Layers are visible because they have different textures and contain different types of minerals.
Geologists have developed a classification system to describe the relative ages in Earth’s history based on the structure of these layers in the Earth’s crust. This system describes the Earth’s history in eons, eras, periods, and epochs. Eons are the largest chunks of time; periods are the shortest. Geologists and prospectors have used their knowledge of rock layers to discover and trace the seams and veins of valuable mineral deposits.
Some layers of rock contain fossils. A fossil is the mineralized remains of an ancient plant or animal. Specific types of fossils tend to occur in specific layers, and the layers can cover extensive regions. For example, the Burgess Shale in Yoho National Park in British Columbia has been declared a World Heritage Site because it contains a wide range of fossil invertebrate animals from the Middle Cambrian age. Layers of sedimentary rock from the Jurassic and Cretaceous periods are famous for the fossilized dinosaur bones they contain.
How can the actual age of these layers and the Earth be estimated? Several approaches have been tried. In 1650 Archbishop James Ussher of Armagh published the results of his analysis of the Bible, correlated with Babylonian history, in which he determined that the world was created in 4004 BCE. The work of Ussher was a valid interpretation of the available historical data. His notion, that a rational process could be used to determine the age of the Earth, inspired many future investigations of the same question.
In 1779 the French naturalist Comte du Buffon estimated the age of the Earth to be 75 000 years. This age was based on experiments exploring the cooling of the Earth from an original molten state. In 1862, the physicist William Thomson refined this method and found an age for the Earth between 20 and 400 million years. In 1899, John Joly estimated the age of the Earth to be 90 million years based on the rate at which salt accumulated in the oceans.
Radioactivity was discovered in 1896. When an atom decays by emitting radiation, it is transformed into another type of atom, and the rate at which this happens can be measured in the laboratory. Knowledge of the rates of decay for radioactive material can be used to determine how much time has elapsed since a rock was in the molten state. For radiometric dating of rocks the most useful reactions involve the transformation of uranium into lead. The age of a rock sample can be determined by comparing the relative amounts of lead and uranium that the rock contains. The oldest rocks on Earth found with this method are about 4.55 billion years old.
Using models of stellar evolution, astrophysicists estimate the age of the Sun at 5 billion years. And based on additional astronomical observations, cosmologists estimate the age of the whole universe to be about 13 billion years. So an age of 4.55 billion years for the Earth is consistent with basic astronomical data.
This age for the Earth can then be used to date the geological periods based on rock layers, and the fossils in the corresponding layers can also be dated. The oldest fossils of bacteria are thus estimated to be about 3.5 billion years old. The oldest creatures such as worms and trilobites are about 550 million years old. The dinosaurs roamed Earth during the period from 250 to 65 million years ago. The oldest hominoid fossils have been found in Early Miocene deposits (22 million years old) in Kenya.
If an average human generation spans 22 years, then there have been about a million generations for human evolution to take place.
Pre-Condition #6: Energy
It takes energy to power life. Plants gather their energy from sunlight, herbivores harvest the sun-energy stored in plants, and carnivores harvest the sun-plant-energy collected by herbivores. Living forms are more successful when they are more effective in gathering and converting the available energy to grow and reproduce.
With an understanding of the six pre-conditions described above, we are ready to delve into the actual mechanisms of evolution.
