What is Evolution?

Main contents

  • What is Evolution?
    • Key Concepts
    • Human Evolution
    • A brief history of evolution

Evolution is the name we give to the process by which new species emerge on Earth from existing species. It is not a theoretical process; evolution has been observed in many environments, in nature and in the lab, and there are several lines of evidence that show evolution has happened in the past. There is no reason to think it will not continue in the future too. Evolution is one of the strongest and most well-supported theories in any branch of science. Over 150 years of research has strengthened the case for evolution far more than Darwin could have hoped.

Some people take evolution as something more than science, a philosophy to underpin understanding the whole universe, or as an idea to be applied to business and sociology. While this is a valid exercise, it can distract from understanding the science behind evolution. It also carries the risk that if you don’t understand the science, the conclusions you draw will be wrong.

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Key Concepts in evolution

What is an adaptation?

An adaptation is a feature of an organism that helps it function within its environment. For example, a giraffe’s long neck is an adaptation to allow it to eat from high branches. Adaptations are the result of an organism’s evolution and can be physical or behavioural.

In many ways, the ability of an organism to cope with changing environments, its adaptability, is the key to whether or not it will survive in the long-term. Animals like turtles, crocodiles and sharks have existed on Earth for hundreds of millions of years and have survived several mass extinctions. This is because they can adapt their behaviour to different environments.

What is artificial selection?

Artificial selection is when we breed plants and animals for particular characteristics. While nature selects characteristics that are of use to the organism, people select characters that are useful for themselves. We have been doing this for about 15,000 years. It is also called domestication.

For example, there are around 400 breeds of dog, all originally bred from wolves. There are even more breeds of sheep, over 1,000, bred from a common ancestral species. We don’t just breed for usefulness. We also breed animals as pets. And our gardens are full of plants bred for attractive flowers and leaves.

The remarkable range that can be achieved in this way (compare a Great Dane to a Chihuahua) shows the power of selection, even in a relatively short space of time. Given what we have done in 15,000 years, it’s no real surprise what nature has done in four billion (4,000,000,000) years.

What is Natural Selection?

If two animals are competing for the same resources, any little variation that helps is likely to be favoured. As successful animals tend to breed more, these variations become magnified, leading to the distinctive adaptations we see in the world around us. This is Natural Selection.

If we look at any species we can define the conditions it needs to survive. What it eats, where it lives, how it reproduces, these all make up it’s way of life. The specific way of life for a species is called it’s ecological niche. Within a habitat there may be hundreds or thousands of species occupying different niches, making up a community. For a potential new species to succeed, it needs to either exploit a niche that is currently unoccupied, or it needs to be better at occupying that niche than an existing species. This competition drives natural selection, and so drives evolution.

Imagine a species of butterfly with a short proboscis (the proboscis is like the butterfly’s tongue). It can only collect nectar from flat-shaped flowers. But if some of these butterflies are born with a longer proboscis, they will also be able to exploit plants with a deeper flower. If there are no deeper flowers available this won’t matter, and the new trait will disappear. But if there are deeper flowers our new butterflies have a niche to exploit and will thrive. Eventually a new species will emerge.

Big changes in the history of life, such as the first life on land, are clear examples of evolution being driven by the availability of new niches. First you have simple land plants. This creates niches for insects and other small creatures. This then creates more new niches. At such times, the progress of evolution can be rapid.

Why is extinction important for evolution?

Extinction is the key to the long history of life on Earth.

Animals compete for resources. When one species goes extinct, this leaves a hole (a niche) that other species can try fill. This is the driving force for much of evolution.

Mass Extinctions, when a large percentage of life goes extinct relatively quickly, provide an opportunity for rapid evolution because they open up new niches. Several mass extinctions have occurred in Earth’s history and they are always followed by a new wave of evolution.

What does 'Survival of the Fittest' mean?

Survival of the Fittest is a misleading term. It was written by the sociologist Herbert Spencer in 1864. Charles Darwin didn’t like the phrase as it implies being the strongest is the best way to survive. But this isn’t accurate.

'Fittest' in evolutionary terms is relative to the environment. So an animal that may be the fittest in one set of conditions may not be so well-suited if the conditions change even slightly. Many animals that were very well adapted to their environment became extinct when conditions changed rapidly. In these circumstances, animals lower down the food chain often survived, because they were more adaptable. If we think about the dinosaurs, they were big, powerful animals with a long history on Earth. But they were so specialised that when the environment changed rapidly they couldn’t adapt and it was the smaller animals that survived (including our mammalian ancestors).

In the long term, it is the animal that better copes with change that survives, not the strongest.

Is evolution a fact or 'just a theory'?

Scientists use the word theory slightly different to most people. When a scientist speaks of a theory he’s not talking about an idea, a guess. He is talking about an explanation for a range of facts. So nothing in science is 'just a theory'; theories are the most important part of science.

Evolution is both a fact and a theory. Theories explain facts and are supported by facts, so the theory of evolution explains the facts of evolution. Scientists do not doubt evolution is true, but continue to ask questions about how it works. Any new facts test the theory and improve our understanding. This is how any branch of science works.

Charles Darwin’s theory was Evolution by Natural Selection. He thought evolution was driven by natural selection and was slow and gradual. Today we know this isn’t the complete story. Natural Selection does drive evolution. but evolution can actually move quite quickly at times. At other times life is quite stable. Darwin didn’t have all the facts we have today, so cannot be blamed for getting it slightly wrong.

A major test of the Theory of Evolution came from the discovery of genetics in the 1900s. This could have disproved evolution. Instead, it supported the theory and provided much new evidence to help explain evolution.
What is a transitional form?
Fossils that represent the evolution of a new group of plants or animals are called transitional forms. An example would be the fossils that show how birds evolved from dinosaurs.

Darwin identified a lack of transitional fossils as a major problem for his theory; he put it down to the geological record being too fragmented. Although he was right, we have since found thousands of these fossils, representing nearly every major change in life on Earth. Because all life is linked we can think of every fossil, and every living animal, as a transitional form.

Is evolution random?

Evolution is often referred to as a random, undirected, process. But is this an accurate description? It’s certainly true that the changes evolution needs to work, changes in the DNA, are a matter of chance. But there is a process by which adaptations are selected. In this sense, evolution is not random at all.

In humans, of the 7 billion proteins in our DNA, every individual will see about 175 mutations. Given much of the DNA will not affect form or function, we can see chance plays a significant part. Having said that, mutations are not entirely random. Different types of mutation are more common than others, and different parts of the genome are more prone to mutations.

Once a mutation has occurred, it is down to natural selection as to whether the change will be preserved and passed on. Natural Selection is certainly not random; it favours features that give an advantage. If the mutation carries no beneficial change, it is lost. In many cases it is the female of the species that directs the course of evolution. In most animal species the females choose who to mate with, so choose what features will be passed on.

What is the 'Evolutionary Ladder'?

Before Darwin, many scientists believed there was a hierarchy to the natural world, with humans at the top. In the Western world, the study of natural history was seen as a way to learn more about God’s creation. Animals were seen as 'higher' and 'lower' kinds, with the pinnacle of creation being humans. Scientists like Carl Linnaeus classified all of life into this hierarchy by studying degrees of similarity, without ever thinking that all animals may be related.

As scientists came to accept evolution, many couldn’t break from the notion of a hierarchy. They saw evolution as a ladder, onwards and upwards, with the more evolved, superior species at the top. Naturally, this ladder led to humans at the top. Unfortunately, some even used this logic to justify the claim that some races are better than others. But this was not Darwin’s view of evolution. He speculates in his letters that no animal should be considered 'higher' than any other. Animals are animals, and all species have just evolved to meet the needs of their environment.

Today, we know Darwin was right. Evolution is an ongoing process. While there are groups that have been around for hundreds of millions of years (e.g. sharks), they are entirely different species; they may be similar, but they are not the same. They have changed to adapt to new environments. The idea that any species, variety, or race is better than another is nonsense and entirely unscientific. Evolution is better represented as a 'tree of life', with different groups of animals branching off.

What does geography tell us about evolution?

The geographical distribution of plants and animals is one of the most important pieces of evidence for evolution. Species have evolved to adapt to their habitats. Their behaviour may also change to take advantage of varying food and breeding areas.

There are many ways for populations of a species to become separate. There may be physical barriers such as mountain ranges. They may become isolated on islands. Environmental change may also split the population, for instance if a large lake partially dries up, leading to two separate lakes. Separation can even be behavioural, with different parts of a population eating different food or breeding in different places.

Because they are occupying different niches, different adaptations will be favoured, leading to variation within the species. Eventually, they will become so different they cannot breed together any more. At this point we see them as new biological species. We can see examples of this all over the world.

What is convergence?

Sometimes two animals can look the same but have very different ancestors. This is called convergent evolution.

We can find examples of this by comparing living species, or by comparing extinct species. Ichthyosaurs, which lived at the time of the dinosaurs, have been extinct for 90 million years. When alive, they looked much like dolphins. But while dolphins are mammals, ichthyosaurs were reptiles.

Hedgehogs are native to Europe, Asia and Africa. They evolved from shrew-like mammals. Echidna live in Australia and New Guinea and are part of a very special group of mammals called monotremes (mammals that lay eggs). The two have similar lifestyles and live in similar environments, but very different routes to a similar form.

We find many examples of convergence by comparing animals in Australia with animals in Europe, Asia and Africa. It is an excellent example of how natural selection works. Around 100 million years ago, the Australian landmass broke off from Africa and moved south, becoming isolated. It had it’s own unique plants and animals, which have been evolving ever since. Because you still get the same sort of environments, you get the same traits selected for, leading to similar animals with different ancestors.

What is Homology?

If you look at the wing of a bird, the flipper of a turtle, the leg of a horse and the arm of a monkey, you will see four very different limbs. Yet from a biological point of view, the difference is tiny. The same bones are present, the same basic structure. They are specialised forms of the same ancestral body part.

When body parts are the same between different animals, we call these homologs. When studying the evolution of animals, identifying homologs helps us understand how structures developed.

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Human Evolution

The human family tree shown in this exhibition is a simplified version focusing only on a few key relations. In reality we have found hundreds of fossils representing dozens of species. Many of these species have no living ancestors, but they are still part of our history. 

What makes a man?

We can group different animals together based on the number of features they share. As with any family tree, the further back you go, the more relatives you find. What this means is that the broader the definition for a group, the more organisms will fit into that group. Our species is called Homo sapiens and it can be included in several different groups including:

  • Animals which means we are multicellular organisms with distinct organs for eating and digesting food and we reproduce sexually.
  • Vertebrates which means we have a backbone.
  • Mammals which means we have mammary glands (organs that produce milk) and sweat glands. We also have hair and a four-chambered heart.
  • Primates which means we have five digits on our hands and feet and opposable thumbs. We have three main types of teeth (incisors canines and molars).
  • Hominoids or apes. We have no tail, an appendix and a specialised elbow joint that allows us to hang from trees.
  • Hominids or ‘great apes’. We are omnivores capable of walking on two legs. All hominids have the same pattern of teeth. Most live in family groups.

The Roots of our Family Tree

Primates evolved at least 65 million years ago, though genetic evidence suggests this could be as much as 85 million years ago. But it’s only about 55 million years ago that we see the first primates that we would recognise. These were lemur-like animals.

About 45-50 million years ago another split occurred in our family tree. This branch led to two groups, the tarsiers (a type of primate today found in South-East Asia) and the simians (the group of primates that includes monkeys and apes). The recently announced Darwinius masillae, also known as ‘Ida’, has been suggested as a common ancestor of all simians, dating to 47 million years ago. But this is far from settled and much more research is needed.

The first fossil simians were found in the Fayum, Egypt and are around 36 million years old. Today, this region is a desert, but 36 million years ago it was a sub-topical forest.

The first fossil apes were found in the region of Lake Victoria, Africa and are about 20 million years old. Proconsul africanus has ape-like arm-bones and teeth. But its skull and body are quite monkey-like. We would not expect all the distinct features of an ape to appear at once, so Proconsul is a good candidate as an early ape ancestor. Like other primates of the time it mainly lived in the trees.

Recent Ancestors

Today there are four types of great ape: humans, chimpanzees, gorillas and orangutans. The others are not our ancestors, they are relatives on our family tree, sharing common ancestors.

Our nearest relative on this tree is the chimpanzee. Genetic evidence, backed by the fossil record, tells us our common ancestor lived around 5 to 6 million years ago. At this point, one line leads to chimpanzees, another to us. We call our ancestors after this split Hominins.

You may have heard the term ‘missing link’, a hypothetical half-human, half-ape that represented the change from apes to humans. In fact, the idea of a ‘missing link’ is very poor science. Evolution is not a straight line, it is a complex branching tree with many dead ends. However, we now have enough specimens to reconstruct our evolutionary history.

To identify a fossil hominin, we have to identify physical characteristics that are unique to humans when compared to other great apes. The most obvious one is that we spend all our time walking upright on two legs (we are bipedal). While chimpanzees, gorillas and orang-utans can walk on two legs, they prefer to move on all fours (they are quadrepedal).

There are three fossil apes that may have been bipedal: Sahelanthropus tchadensis, Orrorin tugenensis and Ardipithecus ramidus. But with only limited fossil evidence, we cannot be sure if any of them were our ancestors.

The first definite human ancestors appeared around 4 million years ago. We call them Australopithecines. Careful study of bones from the skull, hips, knees and feet shows they walked on two legs. Even more remarkably, a series of footprints were found that belonged to a pair of Australopithecines.

Although they were walking on two legs, they had not abandoned the trees completely. Their hands, feet and elbow joint still show adaptations for climbing. They were vegetarians, gathering food. There is no evidence they had developed tools, or that they used fire.

Two other groups evolved from the Australopithecines. One we call the 'robust' Australopithecines, in the genus Paranthropus. The other were humans, in the genus Homo.

The First Humans

We call all animals in the genus Homo 'human'. The first of these, Homo habilis and Homo rudolfensis, lived in North East Africa, about 2.5 million years ago. They had larger brains than the Australopithecines and they were the first to use stone tools. But in many ways their features are still quite ape-like, to the extent some researchers think they should not be considered true humans. This remains a matter for debate.

There is no debate over Homo erectus, a species that is clearly human. The first specimens were found in Indonesia by a Dutch doctor. He was trying to prove that humans had evolved in Asia, not Africa. We have since found older specimens in Africa (the name Homo ergaster is sometimes used for African specimens of Homo erectus). The species first appeared around 1.8 million years ago and was still alive as recently as 50 thousand years ago. Although they look superficially like us, there are major differences in the chest and skull, in particular a smaller brain size.

It had been thought Homo erectus were the first humans to migrate out of Africa. But recent finds in Georgia suggest an older species, Homo georgicus, left a little earlier (around 2 million years). Again there is some debate about whether Homo georgicus is a separate species, or just an early population of Homo erectus.

Homo floresiensis lived on the Indonesian island of Flores till at least 18,000 years ago, perhaps even within the last 10,000 years. They may have evolved from Homo erectus, or from an earlier species of human that left Africa before Homo erectus.

There has been a lot of debate over whether they are a separate species, or just modern humans suffering from a condition called microcephaly. The evidence from the fossils currently supports the idea they were a unique species. They It is quite common in nature for animals on islands to be smaller than related species on the mainland.

The next significant species was Homo heidelbergensis. One of the most important things about this species is the development of certain parts of the brain, and also the area where the voice box is. These changes suggest Homo heidelbergensis may have had the capability of very basic speech.

There have been several other species of human named, all of which probably represent populations of Homo heidelbergensis. These include Homo rhodesiensis and Homo mauritanicus in Africa and Homo cepranensis and Homo antecessor in Europe. Scientists think Homo heidelbergensis evolved into Homo neanderthalensis in Europe, and into Homo sapiens in Africa.

Homo neanderthalensis was, until just 20,000 years ago, our nearest living relative. When first discovered some scientists thought they were our ancestor, but we now know this is incorrect. Though similar in appearance to modern humans, Neanderthals had a heavier build, with thicker bones. Their arms and legs were shorter and they had larger muscles. Their nose was wider than ours. All these differences made the Neanderthals well-suited to a hard life in a cold climate.

The Neanderthals are the only other species we know to have buried their dead in a ritual way. Bodies have been found surrounded by objects and often showing signs of having been painted. It is generally accepted by anthropologists that this demonstrates a belief in an afterlife, and probably primitive religion.

The Neanderthals were wiped out by a combination of changing climate and competition for resources with a more adaptable rival: Homo sapiens. There had been suggestions that the two species inter-bred, but this is not supported by the fossil evidence or the genetic evidence.

Today there is just one known species of human, our species, Homo sapiens. We evolved around 250,000 years ago in Africa, and gradually spread across the globe.

All humans outside of Africa originate from just one population of Africans that left Africa, then spread across the globe. Effectively, all of us outside Africa are immigrants! We can use fossils to track this emigration. Humans first entered Asia 125,000 years ago, then on into Australasia 55,000 years ago and the Americas at least 15,000 years ago. The first modern humans in Europe occur around 40,000 years ago.

The Human Brain

Humans have unusually large and complicated brains. Languages, culture and self-awareness are all a result of this development.

Walking upright has given us many advantages. It freed our hands to make tools. It allowed us to survive as the African environment changed. It has allowed us to spread across the world. But it is also responsible for allowing our brains to develop.

Most animals are born quite late in their development, meaning their brains are fully-formed and they reach adulthood quickly. But in apes, and especially in humans, the babies are born earlier. This means the infants have to stay within their families for longer. It also means their brains can get bigger than would otherwise be possible.

A big brain needs a lot of energy, around one quarter of your bodies total consumption (despite the brain taking up just 2% of your total body mass). When the first humans started to eat meat 2 million years ago, this provided a source of protein and therefore more energy. Hunting needs co-ordinated behaviour, which meant there was an evolutionary advantage to being better communicators, better planners, and better tool-makers. Better hunters get more meat, which means more protein, which can fuel still bigger brains.

We can see that there is a connection there, fuelling our development. But where does our consciousness and culture then come from?

The parts of the brain that allow us to plan are the same parts that let us think more about the world around us. Our self-awareness is a direct result of our evolution, and our culture comes from that, re-enforcing the social bonds necessary in hunter-gatherer communities. It is this development more than any other that is responsible for the success of humans.

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A brief history of evolution

Evolution and Philosophy

Throughout human history, people have wondered about the world around them. Where did all the plant and animal life come from? How did the planet form?

Many cultures collected fossils and speculated what they were. Often they formed the basis for myths and legends (e.g. dragons, giants). But many ancient philosophers correctly identified they were the remains of long-dead animal life. Nearly 2,600 years ago, the Greek philosopher Anaximander set out ideas on the origin and variation of life. Although he never used the words ‘evolution’ or ‘extinction’, this is what he was describing. Over the next 700 years many Greek and Roman thinkers followed his lead. Aristotle wrote about the natural world in the mid 4th Century B.C. His writings on zoology and geology would influence Western thinking on nature right into the 1500s.

Evidence for evolution can be found anywhere in the world, so it would be a surprise if the idea had only arisen in the West. When we study other civilisations we see they too thought of evolution. In China, in the late 4th Century B.C., the philosopher Zhuangzi wrote a number of Taoist texts. In one, he argues that animals have the ability to change from simple to complex forms.

More recently, around A.D. 830, the Muslim writer Al-Jahiz produced The Book of Animals. In it he speculates that species can change under pressure from the environment. Al-Jahiz was based at the Bayt al Hikma, Baghdad. At the time, it was one of the greatest centres of learning anywhere in the world. The idea of evolution was discussed there several times over the next hundred years.

Although all these early scientists explored the same ideas as Darwin, they were not an influence on him, or on science in the 1800s. It is only in more recent years their work has been appreciated.

The birth of modern science

From the 1500s onwards there was a huge interest in natural history in Europe. Amateur scientists, mainly doctors, members of the clergy, or rich gentlemen, thought by studying nature they could understand God’s creation (there were no professional scientists at the time). This is what we call Natural Theology. But discoveries in anatomy, geology and palaeontology raised many questions.

Comparative anatomy showed that most animals were built to the same basic body plan. All vertebrates (animals with backbones) are essentially the same, with the same organs and skeletal structure. Some of these features seemed to serve no purpose.

Palaeontology, the study of fossils, was a new science in the 1600s. Prior to this, fossils were dismissed as odd-shaped rocks, few thought of them as the remains of living creatures. But now several scientists started to study fossils properly, working out what type of animals they belonged to.

By the 1700s the idea of evolution (or ‘transmutation’), based on observations from nature, was being advanced as an alternative, or an accompaniment, to creation. Scientists such as Carolus Linnaeus, Pierre-Louis Maupertuis, Georges-Louis LeClerc and Erasmus Darwin all wrote on the idea that species were not fixed, but could instead change.

LeClerc also suggested that fossils represented forms of life that no longer existed; they had become extinct. This was a major development in understanding the natural world. LeClerc speculated that the Earth must be much older than previously thought to explain all these long-gone animals.

Geology was another new branch of science. In the late 1700s and early 1800s researchers such as James Hutton, William Smith and Charles Lyell studied rocks and fossils. Smith used fossils to link layers of rock in different places. From this, he mapped the geology of England and Wales. This was important as scientists could now see the history of life on Earth laid out in different chapters, defined by the layers of rock.

Hutton and Lyell thought about how rocks formed and how so many repeating layers could exist. Lyell, drawing on the earlier work of Hutton, showed that you could identify the processes that formed a particular set of layers. You could then suggest how long it would take to make all that rock. He concluded the Earth was of an unimaginably vast age, hundreds of millions of years, as opposed to a few thousand years.

In the 1800s Jean-Baptist Lamarck, Geoffroy St Hilaire, Alexander von Humboldt, Leopold von Buch, Augustin de Candolle, William Charles Wells and Patrick Matthew developed the idea of evolution further, producing theories that described how evolution could work. Lamarck’s theory, where the offspring inherited characteristics a parent acquired in life, was particularly popular. In his notebook of 1838 Darwin names Lamarck, St Hilaire, von Humboldt and von Buch as men who had the idea of natural selection before him (though he is quick to clarify they thought of it independently).

It wasn’t just science that explored the idea of natural selection. Studying human populations, the sociologists Thomas Malthus (late 1700s into the early 1800s) and Adam Smith (late 1700s) both talked about competition for resources limiting population growth and selecting for stronger individuals. Malthus’ work was a definite influence on Charles Darwin.

So we can see that by the mid 1800s evolution and natural selection were not new ideas. Far from living in a time of oppressive religious fundamentalism, Charles Darwin lived in an age when nothing was taken for granted and the very nature of the world was open to question. But as Darwin would discover one assumption was beyond question; the special nature of humanity.