Theories of evolution
Darwin's theory of evolution explains how life on Earth has changed over geological time. Scientists believe this is the reason why all living things on Earth exist today. The theory is supported by evidence from fossils, and by the rapid changes that can be seen to occur in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past and the extinction of species continues to happen.
The basic idea behind the theory of evolution is that different species have developed over time from simple life forms. These simple life forms first developed more than three billion years ago (as the Earth is believed to be about 4.5 billion years old).
This timeline shows key events in the evolution of life forms on Earth, from the first bacteria to the human species.
You can see a more detailed history of life timeline on BBC Nature.
Download and print Tree of Life poster (pdf file, 2.6Mb)
Charles Darwin is famous for his theory of evolution, but he was not the only person to develop such a theory.
Charles Darwin was an English naturalist. He studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas about evolution in a book called On the Origin of Species, which was published in 1859.
Darwin's theory caused controversy among his contemporaries and his ideas were only gradually accepted, although some people still do not believe them today. The reasons for skepticism include:
Jean-Baptiste Lamarck was a French scientist who developed an alternative theory of evolution at the beginning of the 19th century. His theory involved two ideas:
However, we now know that in most cases this type of inheritance cannot happen.
Lamarck's theory cannot account for all the observations made about life on Earth. For instance, his theory implies that all organisms would gradually become complex, and simple organisms disappear. On the other hand, Darwin's theory can account for the continued presence of simple organisms.
Different organisms can be classified by studying their similarities and differences. These studies also help us to understand the evolutionary relationships between different organisms.
Evolutionary trees are used to represent the relationships between organisms. The diagram shows an evolutionary tree.
In this evolutionary tree, species A and B share a common ancestor. Species F and G share a common ancestor, which itself shared a common ancestor with species E. All seven species share a common ancestor, probably from the distant past.
The theory of evolution states that evolution happens by natural selection. The key points are that:
Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed on to the next generation. Given enough time, a species will gradually evolve.
You need to remember that variation can be caused by both genes and the environment. But it is only variation caused by genes that can be passed on to the next generation.
Life on Earth today exists because of the conditions that were present when life was evolving. If in the past the conditions had been different, evolution by natural selectionnatural selection: the natural process whereby the best-adapted individuals survive longer and have more offspring, thereby spreading their characteristics through a population. 'Survival of the fittest'. could have produced some very different results. For example, if the Earth had been hotter, colder, bigger or smaller, our bodies would have changed to suit those conditions.
Mutationsmutation: An error in DNA replication which results in a change in an organism's genetic blueprint. Some mutations are random, others are caused by environmental factors cause changes in genes. When new forms of a gene appear because of mutation, a rapid change in a species may happen if the environment changes. Such rapid changes support the theory of evolution by natural selection.
Before the industrial revolution in Britain, most peppered moths were of the pale variety, meaning that they were camouflaged against the pale birch trees that they rest on. Moths with a mutant black colouring were easily spotted and eaten by birds. This gave the white variety an advantage and they were more likely to survive to reproduce.
Airborne pollution in industrial areas then blackened the birch tree bark with soot. This meant that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. The black variety now had the advantage and were more likely to survive and reproduce. Over time, black peppered moths have become far more numerous in urban areas.
Microorganisms such as bacteria and viruses reproduce very rapidly. They can evolve in a relatively short time. One example is the bacterium E. coli. Its DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] can be damaged or changed when it reproduces. Most of the time this causes the death of the bacterial cell.
Occasionally, the mutation is beneficial for the bacterial cell. For example, it may allow resistance to an antibiotic. When that antibiotic is present, the resistant bacteria have an advantage over bacteria that are not resistant.
Antibiotic-resistant strains of bacteria are an increasing problem in hospitals.