It is the change in the inherited characteristics of biological populations over successive generations. Evolutionary processes give rise to diversity at every level of biological organization, including species, individual organisms and molecules such as DNA and proteins.

All life on Earth is descended from a last universal ancestor that lived approximately 3.8 billion years ago. Repeated speciation and the divergence of life can be inferred from shared sets of biochemical and morphological traits, or by shared DNA sequences. These homologous traits and sequences are more similar among species that share a more recent common ancestor, and can be used to reconstruct evolutionary histories, using both existing species and the fossil record. Existing patterns of biodiversity have been shaped both by speciation and by extinction.

Charles Darwin was the first to formulate a scientific argument for the theory of evolution by means of natural selection. Evolution by natural selection is a process inferred from three facts about populations: 1) more offspring are produced that can possibly survive, 2) traits vary among individuals, leading to different rates of survival and reproduction, and 3) trait differences are heritable. Thus, when members of a population die they are replaced by the progeny of parents better adapted to survive and reproduce in the environment in which natural selection takes place. This process creates and preserves traits that are seemingly fitted for the functional roles they perform. Natural selection is the only known cause of adaptation, but not the only known cause of evolution. Other, non-adaptive causes of evolution include mutation and genetic drift.

Photo courtesy: scientific American (nature.com)

Animal Evolution

All animals and plants are classified as multicellular eukaryotes. Their bodies are made up of large numbers of cells, and microscopic inspection of these cells reveals that they contain a nucleus and a number of other organelles. Compared to prokaryotic organisms such as bacteria, plants and animals have a relatively recent evolutionary origin. DNA evidence suggests that the first eukaryotes evolved from prokaryotes, between 2500 and 1000 million years ago. That is, eukaryotes as a taxon date from the Proterozoic Era, the final Era of the Precambrian. Fossils of both simple unicellular and more complex multicellular organisms are found in abundance in rocks from this period of time. In fact, the name "Proterozoic" means "early life".

Plants and animals both owe their origins to endosymbiosis, a process where one cell ingests another, but for some reason then fails to digest it. The evidence for this lies in the way their cells function. Both plant and animal rely on structures called mitochondria to release energy in their cells, using aerobic respiration to produce the energy-carrying molecule ATP. There is considerable evidence that mitochondria evolved from free-living aerobic bacteria. They are the size of bacterial cells. They divide independently of the cell by binary fission. They have their own genome in the form of a single circular DNA molecule. Their ribosomes are more similar to those of bacteria than to the ribosomes found in the eukaryote cell’s cytoplasm. And like chloroplasts they are enclosed by a double membrane as would be expected if they derived from bacterial cells engulfed by another cell.

Like the plants, animals evolved in the sea. And that is where they remained for at least 600 million years. This is because, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation. Once photosynthesis had raised atmospheric oxygen levels high enough, the ozone layer formed, meaning that it was then possible for living things to venture onto the land.

The oldest fossil evidence of multicellular animals, or metazoans, is burrows that appear to have been made by smooth, wormlike organisms. Such trace fossils have been found in rocks from China, Canada, and India, but they tell us little about the animals that made them apart from their basic shape.

A foot on the land

Whatever the origins, animals may have ventured onto land early in the Cambrian. Previously scientists believed that animals did not begin to colonize the land until the Silurian (440-410 million years ago). However, the 2002 discovery of the footprints of the animals that scuttled about on sand dunes about 530 million years ago has changed this view. These animals were arthropods, and resembled centipedes about the size of crayfish. They probably didn’t live on land, instead coming ashore to mate or evade predators. At this time the only land plants appear to have resembled mosses.

Problem Encountered in the move to land

These early land animals had to solve the same problems that plants faced when they moved to the land. Water conservation, gas exchange, reproduction and dispersal, and the fact that water no longer buoyed them up against the pull of gravity. Like plants, animals evolved waterproof external layers, internal gas exchange systems, ways of reproducing that did not involve water, and strong support systems (endoskeletons and exoskeletons) that allowed them to move about on land. Remember that not all animal taxa were equally successful in solving these problems.

Species and Speciation

An animal species is a group of animals that is able to interbreed in the same area to produce fertile offspring, in order to keep the species going. Speciation is the dividing within an animal species, producing sub-species of animals within one group. Different sub-species of animals are usually separated by a natural barrier such as water or land, and can eventually adapt to their surroundings so effectively that they become too different from their original species that the two can no longer interbreed.


Despite the sad thoughts of losing animal species, species extinction is part of the natural course of evolution, as in order for one species to survive, another species must fail. It is thought that 99% of the Earth's animal species have become extinct since the beginning of the Earth, and that the bulk of these have happened in the last 60 million years. Today, the extinction rate of animal species is rapidly increasing due to the impact made on animals and their habitats by humans.

Species Similarities

Despite being unrelated, animal species often develop similarities which can make too completely different species of animal look very similar. This is known as convergence and makes evolutionary tracing very difficult as it can throw scientists off course. For example, dolphins which are mammals and sharks which are fish, are very similar in appearance and the two can often be confused, but they are not closely related.


The constant movement of continents and changes beneath the surface of the Earth are constantly reshaping the planet and these changes often have a dramatic effect on animal life as they split animal groups up and create completely new habitats. Animals were originally split up with the dividing of continents and the isolation of islands and are brought back together again when the land collides or with the introduction of new species to new places by humans.


Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by co-operating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

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