Evolution and Classification
Evolution can be defined as the change in hereditary characteristics of biological populations over several successive generations. These characteristics are the expression of genes that are passed from the parent generation to the offspring generation through reproduction. It takes place through processes such as natural selection. The characteristics are different from another due to genetic recombination, mutation and various other factors.
What evolution did is, it created huge biodiversity between organisms and hence they were all classified based on their characteristics such as similarities and dissimilarities. These characteristics can be appearance, form, behaviour, function etc.
The hierarchy of classification begins with the cell as it is the structural and fundamental unit of all organisms. Then come the body design and the level of organisation followed by the development of organs. As we move towards the top of the hierarchy, the number of the organism which has common characteristics become less to which we can conclude that the number of common characteristics that are shared between organisms the more chance they have that they share a common ancestor. In this topic, we will discuss fossil biology and how fossils help us with tracing evolutionary relationships.
Evolutionary Relationships Meaning
Evolutionary relationships can be determined by key characteristics between two different organisms which share similar characteristics which may lead to the idea that they might share a common ancestor. An evolutionary tree or a phylogeny is important to determine key characteristics to establish evolutionary relationships to detect patterns between organisms. Characteristics which help in determining evolutionary relationships are:
Structural similarities
Breeding behaviour
Geographical distribution
Biochemistry
Geographical distribution
Cladistics
Fossil Biology
A fossil can be described as the mineralised complete or partial form of an organism or of an organism’s activity which has been preserved in a mould, impression or a cast. It gives tangible and physical evidence of ancient life and provides the basis of the theory of evolution in the absence of preserved soft tissues. Fossils can be categorised into four classes, these are
Mould Fossils: This is a fossilised impression made in a substrate which gives a negative image of the organism
Cast Fossils: A cast fossil forms when a mould is filled in.
Trace Fossils: These are also called ichnofossils. Some examples are, fossilised nests, burrows, footprints, gastroliths etc
True Form Fossils: Fossils of an actual part of the animal or the complete body of the animal.
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Tracing Evolutionary Relationships
How Do Fossils Provide Evidence for the Theory of Evolution?
Biologists use several types of information to trace and reconstruct evolutionary relationships between organisms. We will discuss some of these types of information below:
Anatomy and Embryology - Common anatomical features shared between organisms which would include ones that are visible only during embryonic development can indicate a shared evolutionary ancestry.
Molecular Biology - Similarities and differences between the same gene in different organisms, that is, a pair of homologous genes can be utilised to determine how the organisms are related.
Biogeography - The geographical distribution of species, meaning the habitat of the organism can help biologists reconstruct their evolutionary histories.
Fossils - Although the fossil record is not a complete record of evolutionary history, it confirms the existence of now-extinct species and in a few cases, captures potential in-between forms on the path to modern species.
How Are Fossils Helpful in Developing Evolutionary Relationships?
Over the years, palaeontologists have recovered and studied fossil remains of several thousands of organisms that lived in the past. This fossil record shows that several extinct organisms were different in form from any their present counterparts. The record also shows successions of organisms through time, and through that, it can be determined their transition from one form to another.
When an organism dies, it is generally decomposed by other forms of life and by the weathering processes. However, on certain occasions, some body parts of the deceased organism, specifically hard ones such as shells, teeth, or bones are preserved as they are buried in mud or protected in some other way from decomposers and the environment. Eventually, they are petrified and preserved indefinitely with the rocks in which they are embedded.
Methods such as radiometric dating indicate that the earth was formed almost 4.5 billion years ago and the earliest fossils resemble microorganisms such as bacteria and cyanobacteria. Fossils of these microorganisms appear in rocks and are more than 3.5 billion years old The oldest known animal fossils over 700 million years old and come from the Edicara fauna which are small wormlike creatures with soft bodies.
Fossils of the first vertebrates show that they appeared about 400 million years ago and the first mammals appeared around less than 200 million years ago. However, the fossil record is incomplete. Only a tiny section of the fossils available on earth have been recovered and studied by palaeontologists and in that only in some cases has the succession of forms been reconstructed in detail. One example is the evolution of the horse.
The horse can be traced to an animal which has the size of a dog with several toes on each foot and teeth appropriate for browsing. The animal is called the dawn horse (genus Hyracotherium) and is supposed to have lived more than 50 million years ago. The most recent form, the modern horse (Equus), is much larger, has only one toe and teeth appropriate for grazing. The transitional forms of this animal are well preserved as fossils, as are many other kinds of extinct horses that evolved in different directions and left no living descendants.
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Recovered fossils also help palaeontologists reconstructed examples of radical evolutionary transitions in form and functions of different animals. For example, the lower jaw of reptiles contains many bones, but that of mammals only has one. Similarities between the other bones in the reptile jaw and the bones in the mammalian ear have been found and it has been established that they have been evolved from the former.
Such a transition might seem unlikely as it is hard to imagine what function such bones could have had during their intermediate stages. Yet, palaeontologists have discovered two transitional forms of mammal-like reptiles which they called therapsids, having a double jaw joint. One joint consists of the bones that persist in the mammalian jaw whilst the other joint is composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear.
FAQs on Fossils - Tracing Evolutionary Relationship
1. What exactly are fossils and how are they formed?
Fossils are the preserved remains or traces of ancient plants, animals, and other organisms. They are not the actual organism, but a rock-like copy. They typically form when a dead organism is quickly buried by sediment, like sand or mud. Over millions of years, minerals in the surrounding sediment seep into the remains, replacing the organic matter and turning it into stone.
2. How do fossils help us trace and understand evolutionary relationships?
Fossils act like a timeline of life on Earth, providing direct evidence for evolution. They help us in several ways:
- Showing Past Life: They prove that life forms in the past were different from those today.
- Revealing Gradual Changes: By arranging fossils in order of their age, scientists can see how species have changed gradually over millions of years.
- Finding Common Ancestors: Fossils can show features of two different groups of animals, suggesting a common ancestor. For example, some fossils have features of both reptiles and birds.
3. Why is it important to figure out the age of a fossil?
Determining a fossil's age is crucial for understanding its place in the evolutionary timeline. There are two main methods:
- Relative Dating: This method compares the position of a fossil to others. Fossils found in deeper layers of rock are generally older than those found in upper layers.
- Absolute Dating: This method, often using radiometric dating, measures the decay of radioactive elements in the fossil or surrounding rock to calculate a more precise age in years.
4. Can you give an example of a fossil that shows an evolutionary connection?
A famous example is Archaeopteryx. This fossil is considered a transitional form between dinosaurs and birds. It had features of both groups: like a dinosaur, it had teeth, a bony tail, and claws on its wings. But like a modern bird, it had feathers and wings. This fossil provides strong evidence that birds evolved from reptile-like ancestors.
5. How do fossils show that complex organisms evolved from simpler ones?
The fossil record clearly demonstrates that evolution happened in stages. When we examine rock layers, the fossils in the oldest, deepest layers belong to very simple organisms. As we move up to younger layers, the fossils become progressively more complex. This pattern shows that life did not appear all at once but evolved from simple forms to more complex forms over vast periods of time.
6. Is the fossil record a complete story of evolution?
No, the fossil record is incomplete. Fossilisation is a very rare event that requires specific conditions, like quick burial in sediment. Most organisms simply decompose after they die, leaving no trace. Therefore, the fossil record has many gaps and only provides a snapshot of the history of life, not the complete picture.
7. How is the evidence from fossils different from the evidence from homologous organs?
Both provide evidence for evolution, but in different ways. Fossils provide a direct, historical record of past life forms and show how they have changed over time. Homologous organs, like the forelimb of a human, the wing of a bat, and the flipper of a whale, provide indirect evidence. They show that different species share a similar underlying structure from a common ancestor, even if the organs are used for different functions today.
