Life evolution
Introduction
This chapter looks at human evolution. For the purposes of this discussion, Darwin's theory stating that all living forms originated from one common ancestor will be used. It will discuss the first life forms and the main steps of the evolution of life, the first human beings and their different forms.
First life forms
Early Earth had very little free oxygen and no ozone layer to absorb life-destroying ultra-violet (UV) radiation from the sun. These conditions were not suitable for most living organisms but they were suited to speeding up chemical reactions that otherwise would not have happened. That was the time when the first organic molecules were formed from non-organic elements. The formation of living forms from non-organic matter is called abiogenesis.
See animation 1.
These first, simple organic molecules accumulated in the ocean for millions of years, producing a so-called 'organic soup'. Fuelled by the still-extreme heat of early Earth, these molecules were interacted with each other, producing new, more complex organic compounds.
Some groups of organic molecules formed bubble-like structures called coacervates. Coacervates were made of organic molecules surrounded by a film of water molecules. Coacervates could selectively absorb different materials from the surrounding water and incorporate them into their structure. Some of these coacervates started to group together, grow and divide. Millions of years later some of them evolved into true biological cells. These cells evolved into the first unicellular organisms. It is believed they were Archaea-unicellular prokaryotic forms of life that could exist in the extreme conditions of early Earth. See image 1.
Biological evolution
Simple photosynthetic algae produced atmospheric oxygen. Unicellular creatures formed colonies that millions of years later evolved into multicellular organisms. The first algae also formed the first soil. Some types of algae evolved into land plants that further increased the level of atmospheric oxygen. Some multicelluar sea creatures adapted to the dry land conditions. As atmospheric oxygen levels increased and a protective ozone layer was established, life forms on Earth became more diverse. Biological evolution is a very slow process. The formation of new species takes millions of years. To say that 'organisms evolved into the new forms', does not mean that they changed into something new within their lifespan. It means that some of these organisms were slowly changing, generation after generation until completely new life forms were established. Also, when organisms evolve into their new forms, it does not mean that the whole population of these organisms changes. Some stay the same, while others choose a completely different evolutionary path.
Biological evolution is a continuous process that will only stop when all forms of life have ceased to exist. Evolution does not always mean improvement, it just means change. The pace of evolution depends on environmental conditions. The faster conditions change, the faster evolutionary changes will occur. 'Faster changes' still do not mean a few months instead of a few million years. Biological evolution is always a very lengthy process. That is why evolutionary changes of different species are sometimes difficult to trace or prove. That is also the reason why all evolutionary theories are just that, theories. All scientific theories have always been and always will be subject to constant change and reassessment. See image 2.
The first human fossils
We know about life forms that existed a long time ago from fossils. Mineralised remnants of plants and animals from previous eras are called fossils. The order of fossils relates to the order of appearance of life forms on Earth.
The radiometric method of fossil dating is the most accurate. There is a consistent pattern that occurs in the decay of radioactive elements that are the natural parts of all rocks. The original element or substance, called 'parent', gradually decays into 'daughter' substances. This process is irreversible. By measuring the amount of parent and daughter elements in a given rock (knowing the decay rate) it is possible to calculate the elapsed time passed since its formation. The age of the fossil equals the age of the rocks in which they are found.
According to Darwin's theory of evolution, all living organisms, including humans, originated from one ancestor. Humans belong to the order of primates. In modern zoology, primates are the highest order of mammalians (the class of warm-blooded vertebrates that nurse their young with milk). Primates include man, apes, monkeys and lemurs.
According to the fossil record, the first humans appeared in Africa. The oldest human skeleton was found in East Africa. One of the best preserved human remnants is a female skeleton found in Ethiopia. The young girl to whom this skeleton belonged was nicknamed Lucy by the anthropologists who found her. Lucy was dated between 3.6 million and 3 million years.
Thousands of ancient human fossils have been found but most of these fossils are just skeletal fragments or isolated teeth. Complete skulls and skeletons are very rare. See image 3.
