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Introduction

There are millions of different types of living organisms on Earth. Even though all organisms have different characteristics, all their DNA molecules and proteins are made of the same structural units. All cellular metabolic and DNA replication mechanisms are also very similar. This chapter will look at evidence of biological evolution.

How do we know?

The evidence for evolution comes from different sources.

1. Fossil record

Mineralised remnants of plants and animals from previous eras are called fossils. The Earth had changed systematically over long periods of time. According to the geological law of superposition, deeper sediments were formed earlier than shallower sediments, which mean they are older. Fossils found in deeper sediments belong to more primitive organisms. Fossils found in sediments that are closer to the surface belong to more sophisticated living forms. The order of fossils relates to the order of appearance of life forms on Earth.

See animation 1.
 

2. Chemical similarities of living organisms

There are 118 chemical elements in the periodic table, but all living organisms use only 7 to build their bodies. They are: oxygen, hydrogen, carbon, iron, silicon, calcium and nitrogen. These elements form 20 different amino acids that in turn form thousands of different types of proteins from which all living things are made.

Before true living forms evolved, proteins would have been synthesised first. In order to find out how the first organic molecules were formed, the scientists Oparin, Miller and Urey recreated the early Earth's environment in a laboratory. When gases containing inorganic elements like hydrogen, oxygen, nitrogen and carbon were heated with water and energised by electrical discharge or by UV radiation, they formed small organic molecules that led to the formation of more complex organic molecules like proteinoids. Proteinoids are protein-like molecules inorganically formed from amino acids.

The formation of living forms from non-organic matter is called abiogenesis. Apparently, sediments like clay played an important role in the formation of the first protein and RNA molecules. Due to its structure, clay attracts small organic molecules that combine, forming more complex organic polymers. It also contains metal catalysts like zinc and iron. Clay also collects energy from radioactive decay and releases it when the temperature or humidity changes. These first proteins formed coacervates. Coacervates consist of macromolecules surrounded by a film of water molecules. Coacervates selectively absorbed different materials from surrounding water and incorporated them into their structure. Millions of years later, coacervates evolved into true biological cells.

Some scientists think that RNA molecules were formed first, which later led to protein synthesis. See image 1.

3. Anatomical similarities of living organisms

All living organisms start their existence as a single cell. They also develop and reproduce by similar cell division processes. All living things have a limited life span at the end of which they die.

All of these major chemical and anatomical similarities between living things can be explained by assuming that they all originated from the same ancestor and/or came into existence as a result of similar natural processes.

4. Sources of energy

All living things need energy for growth, development and reproduction. They get this energy either from the sun, chemical reactions or by consuming organisms that can utilise the sun's energy.

5. Geographic distribution of related species

Plant and animal species on isolated land areas evolved differently from the evolution of continental species. Some species are endemic because they have been evolving in isolation from the rest of the world for millions of years. The environment plays a very important role in the process of biological evolution.

6. Genetic similarities

Similarities in DNA structure prove that all living things are related. DNA code is the same for all living things. Some species share similar body structures because they inherited them from a common ancestor. All vertebrates, for example, have limbs with the same types of bones (humerus, radius and ulna) because they shared a common, ancient vertebrate ancestor.

7. Ontogenesis and phylogenesis

Phylogenesis means the sequence of the main events of evolution. The development of a living organism from a fertilised egg to mature form is called ontogenesis. There is a theory stating that ontogenesis is a short story of phylogenesis. That means that the entire evolutionary history of an organism could be traced by examining different stages of its embryo development. The table below shows there are definite similarities between embryos of different organisms. See image 2. The relationship between embryonic development and evolution is probably much more complex, but it is a very interesting theory that deserves further research.

8. Transitional forms

Some living organisms, or their fossils, are referred to as transitional forms. A large set of individual fossils can often show gradual changes between species, called species-to-species transition. Transitional forms are organisms that are considered to be 'links' between different groups of organisms. Examples of transitional forms include the ancient, feathery, flying reptile Archaeopteryx, the transitional form between reptiles and birds and the air-breathing 'walking' ancient fish Latimeria, the transitional form between fish and amphibians.

Mitochondria's DNA - witness of life evolution

DNA of the cell's mitochondria is often used to construct evolutionary trees.Mitochondria are a eukaryotic cell's organelles responsible for turning nutrients into energy. Mitochondria have their own genome and are thought to be an evolutionary result of the symbiosis between prokaryotes and eukaryotes. As all eukaryotic cells have plenty of mitochondria, fewer cells are required for research.Mitochondria's DNA mutates more easily than nuclear DNA, making it easier to resolve differences between closely-related organisms. Also, because mitochondria are inherited only from the mother, it is possible to trace a direct genetic line. See image 3.


Chapters: Evidence of evolution Moving forces of evolution

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1. Mineralised remnants of plants and animals from previous eras are called

Gems

Minerals

Fossil fuel

Fossils

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