Fossils - Tracing Evolutionary Relationship

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.