The Theory of Evolution
By Geoffrey Dobson
Evolution is the theory that describes how complex organisms like us can have developed from simple organisms over thousands of years. This occurs as characteristics pass down through generations (Mendelism), while the species as a whole adapts to changes in the environment (Darwinism).
The idea that all species developed from simple organisms was first mooted by the Ancient Greeks, but was suppressed by religious leaders until being revived by philosophers in the late 18th century. There have always been religious objections to the idea of evolution, as opposed to supernatural creation. This dispute continues today, particularly in the USA, where an alternative theory of “intelligent design” has been proposed.
In 1859 Charles Darwin proposed that species could evolve by a process of adapting to their environment. The central idea is that variations occur at random during reproduction. Changes that are improvements to the species slowly become dominant and weaker variants die out (i.e. “Natural Selection” leading to “Survival of the Fittest”). Some changes lead to new species (“branching”). Darwin used fossil evidence to construct a diagram showing the progression from simple to a variety of complex organisms. He based this diagram on the “Tree of Life”, an ancient religious symbol. Similar but much more complex versions of the Tree of Life are still used today to illustrate the progress of evolution, including dozens of animal species, as well as plants, fungi, and single-celled life forms like bacteria.
While Darwin showed the relationships between differing species, he could not explain the mechanism of how characteristics pass between generations. In 1865 Gregor Mendel developed the theory of genetics showing how generations develop in a predictable manner. For a time the two theories were in conflict until in the 1920s Haldane combined them into a single theory of evolution. In the 1940s the theory was confirmed by the discovery of DNA as the chemical molecule that controlled inherited traits, and was finally completed in 1953 when Crick and Watson identified the structure of DNA by X-ray analysis.
In summary, modern genetic theory assumes that the genes of a population of sexually interbreeding animals or plants constitute a gene pool. New genes originate as a result of errors in copying, and spread through the gene pool by sexual mixing. The strongest gene changes alter the species while weak ones die out. Today the structure of individual “genomes” of many species (including humans) have been identified and genetic theory has become the basis of all modern biology.
When the earth was formed about 5000 million years ago, it had an atmosphere containing simple gases like water, carbon dioxide, nitrogen, oxygen and methane. Violent electrical storms initiated chemical reactions in this mixture gradually forming a soup of more complex chemicals like amines and amino acids. This has all been simulated in laboratory experiments but nobody knows how the next step happened – the creation of life.
About 4000 million years ago some of these complex chemicals developed the ability to self-replicate. Once self-replicating molecules had formed they combined in a random manner forming many different types of molecule. These new molecules themselves replicated and, by natural selection, the most robust varieties became the dominant species. Gradually the changes continued until complex self- reproducing molecules similar to DNA were formed. It was then a short step to the formation of proteins and hence simple living cells.
By about 3000 million years ago, simple bacteria existed on Earth. The rest of evolution continued by replicator molecules (genes), building for themselves multi-cellular bodies to enable their own preservation and reproduction. Richard Dawkins called this the progress of “The Selfish Gene”. By about 500 million years ago simple vertebrate fishes had developed. However, the progress of evolution did not run smoothly. There have been five periods of mass extinction during Earth’s history, probably caused by massive volcanic activity. The first two (450 and 360 million years ago) wiped out most of the new life in the oceans, but the species that survived continued to evolve.
About 300 million years ago amphibians began to colonise the land areas. Then about 250 million years ago the « Permian Mass Extinction » again wiped out 96 percent of all life. Fungi, insects and bacteria survived but among the animals, only a few amphibian creatures were left. These evolved over time into mammals, birds and, importantly, giant reptiles. The rule of the dinosaurs began about 200 million years ago. Mammals remained small and lived underground to escape the dinosaurs. The dinosaur age came to an abrupt end 65 million years ago when a giant asteroid collision triggered another volcanic storm that destroyed all surface life. An ice age followed which, on melting, formed the landscape as we know it today with large areas savannah grassland surrounded by dense forest.
The extinction of the dinosaurs unleashed the rapid evolution of the surviving mammals into the rich variety of species that we see today. The first primates developed in the great forests of Africa about 50 million years ago, and about 5 million years ago the great apes appeared. The first upright walking apes arrived about 3 million years ago. About 2.4 million years ago the first primitive human-like primates (homo habilis) emerged from the forests and became hunter-gatherers on the great savannahs of Africa.
Homo habilis developed the ability to make flint tools and weapons. The Stone Age had begun. Over the next 500 000 years Homo habilis slowly became more adept, learning new skills like making fire and cooking. They evolved into Homo erectus who began to migrate out of Africa. They were hunter-gatherers, there was only a small population, and there was plenty of food around so they thrived. Their era lasted until about 70 000 years ago. There were many local variations in climate during this age and Homo erectus developed into at least five different species of human types including the Neanderthals in Europe about 350 000 years ago, and Homo sapiens (our ancestors) who appeared in North Africa about 200 000 years ago.
About 70 000 years ago the climate changed again into a very severe glacial period that lasted until 15 000 years ago when there was a rapid melt. The Neanderthals and most other human species did not survive this ice age, but Homo sapiens developed survival skills and became the dominant human species. They started to spread throughout the world colonising Russia about 50 000 years ago, moving into Europe about 30 000 years ago and finally the Americas about 25 000 years ago. Most of the large animal species also did not survive the ice age and so Homo sapiens were forced to become farmers to maintain their food supplies. Villages developed and the Neolithic Age had begun with the germs of modern civilisation.
Simplified scheme of human evolution