Human Evolution | ICSE Class 10 Biology Notes

This note covers the ICSE Class 10 Biology topic Human Evolution, part of the syllabus for the 2027 examination: what evolution and human evolution mean, the main kinds of evidence for evolution, Lamarck's theory of inheritance of acquired characteristics with the giraffe's neck and forelimbs and the vestigial organs of humans, why Lamarck's theory was rejected, Darwin's theory of natural selection and survival of the fittest with the peppered moth, how the two theories differ, and the human ancestors Australopithecus, Homo habilis, Homo erectus, the Neanderthals, Cro-Magnon and modern man, compared on bipedalism, cranial capacity, canine teeth, forehead and brow ridges, chin, body hair, and height and posture.

What is human evolution?

Evolutionary biology is the study of the history of life forms on earth. It tries to explain how the plants and animals around us came to be as they are, through changes that took place over millions of years.

Definition: Organic evolution is the slow and gradual change in living organisms over many generations, through which new forms of life arise from pre-existing ones.

The earth was formed about 4.5 billion years ago, and life appeared on it almost four billion years ago. The first cellular forms of life appeared about 2000 million years ago. All the life forms that exist today share similarities and share common ancestors.

Human evolution is the part of this story that traces how modern humans arose from earlier, ape-like ancestors. Humans are primates, the group of mammals that also includes monkeys and apes. Primates called Dryopithecus and Ramapithecus existed about 15 million years ago, and the human line appeared later.

From special creation to evolution

The theory of special creation, found in conventional religious literature, held that all living organisms were created as they are now, that their diversity never changes, and that the earth is about 4000 years old.

These ideas were strongly challenged during the nineteenth century. Charles Darwin concluded that life forms have changed gradually and that many earlier forms are now extinct. The earth is billions of years old, not thousands.

Theories of evolution

Two theories set out to explain how evolution happens. Lamarck's theory explains it by the inheritance of characters acquired through the use and disuse of organs. Darwin's theory explains it by the natural selection of inherited variations. Darwin's idea of natural selection forms the basis of the modern understanding of evolution.

Note: Humans did not evolve from the chimpanzees or gorillas living today. Humans and modern apes are separate branches that share common ancestors in the distant past.

How do we know that evolution has taken place?

Evidence that evolution has taken place comes from many quarters. Each kind shows that present-day organisms have descended, with change, from earlier ones.

Fossils

Fossils are the remains of hard parts of life forms preserved in rocks. Rock sediments of different ages contain fossils of different life forms, some of them extinct, such as the dinosaurs. This palaeontological evidence shows that new forms of life arose at different times. Fossil skulls, jaws and teeth are the main evidence for human evolution.

Comparative anatomy

The forelimbs of whales, bats, cheetahs and humans do different jobs, yet all of them have the same bones: humerus, radius, ulna, carpals, metacarpals and phalanges. Such organs are homologous organs, and homology indicates common ancestry. Vestigial organs, described later in this note, point the same way.

What is Lamarck's theory of inheritance of acquired characteristics?

Jean-Baptiste Lamarck (1744 to 1829) was a French naturalist. Even before Darwin, he said that life forms had evolved, and that evolution was driven by the use and disuse of organs. He set out his ideas in 1809 in his book Philosophie Zoologique.

His theory is called Lamarckism, or the theory of inheritance of acquired characteristics.

Definition: An acquired character is a change in the body that an organism develops during its own lifetime, for example through the constant use or disuse of an organ, or through the effect of its surroundings.

Main points of Lamarck's theory

1. Internal vital force: living organisms and their organs tend to increase in size.
2. Effect of environment and new needs: a change in the surroundings creates new needs, and the organism changes its habits to meet them.
3. Use and disuse of organs: an organ that is used constantly grows larger and stronger, while an organ that is not used becomes weak, reduces in size and may disappear.
4. Inheritance of acquired characters: the changes an organism acquires during its lifetime are passed on to its offspring.
5. Origin of new species: these small changes add up over many generations until a new kind of organism appears.

In Lamarck's view, effort and habit change the body, and the changed body is inherited. His best-known example is the giraffe's long neck, explained by use. The reduced organs of humans are explained by disuse. His explanation of how living forms change is no longer accepted.

How did Lamarck explain the giraffe's long neck and forelimbs?

The giraffe is the classic example of the use of organs in Lamarck's theory. It has a very long neck and long forelimbs, which let it browse on leaves high up in trees.

Lamarck's explanation, step by step

1. The ancestors of giraffes had short necks and short forelimbs, and they fed on grass and low plants.
2. Their surroundings became dry, and grass and low vegetation grew scarce, so they had to feed on the leaves of tall trees.
3. To reach the leaves, the animals constantly stretched their necks and forelimbs upwards.
4. Through this constant use, the neck and forelimbs of each animal became slightly longer during its lifetime.
5. The longer neck and forelimbs, as acquired characters, were passed on to the next generation.
6. Over many generations, the small increases added up, and giraffes came to have the long necks and forelimbs seen today.

Draw three giraffe-like animals side by side under a tall tree with leaves only near the top. The first, labelled "Ancestor", has a short neck and short forelimbs and stands below the leaves. The second, labelled "Stretching over generations", has a longer neck and forelimbs and reaches upwards. The third, labelled "Modern giraffe", has a long neck and long forelimbs and reaches the leaves. Add an arrow from left to right labelled "Acquired lengthening passed on".

The weak link is step 5, the inheritance of acquired characteristics. A neck stretched by effort during an animal's life does not change the inherited material that the animal passes on to its young. Darwin's theory explains the same long neck in a different way, compared later in this note.

What are vestigial organs, and how did Lamarck explain them?

Definition: Vestigial organs are organs present in a reduced form that have little or no function in an organism, though the same organs are functional in its ancestors or in related animals.

Lamarck explained vestigial organs by the disuse of organs. When the way of life of an animal changed and an organ was no longer used, the organ became smaller and weaker. The reduced organ was passed on, and over the generations it shrank to a small remnant.

Wisdom teeth

Wisdom teeth are the third molars, the last teeth at the back of each half of the upper and lower jaws. They are the last teeth to come out, usually at about 17 to 25 years of age.

In many people the jaw has too little room for them, so they come out partly, at an angle or not at all. Such impacted wisdom teeth may need to be removed. In ancestors with larger jaws and a diet of coarse, raw food, these teeth were useful for grinding.

Vermiform appendix

The vermiform appendix is a narrow, worm-like tube that projects from the blind end of the caecum, at the junction of the small and large intestines. In herbivores such as the rabbit, the caecum and appendix are large and help in the digestion of cellulose.

In humans the appendix is small and has no known role in digestion, so it is described as vestigial. It can become inflamed, a condition called appendicitis, and it is then usually removed by surgery.

Pinnae and their muscles

The pinna is the outer, visible flap of the ear. Animals such as dogs, cats and horses have well-developed ear muscles that turn the pinnae towards a sound. In humans these ear muscles are present but weak, and most people cannot move their pinnae, so the pinnae with their muscles are counted as vestigial.

Today vestigial organs are taken as evidence of common ancestry rather than as proof of Lamarck's theory. They show that humans descended from ancestors in which these organs were larger and did useful work.

Why is Lamarck's theory not accepted today?

Lamarck's explanation of the giraffe's neck is no longer believed. The main objection is that characters acquired during an organism's lifetime are not inherited. What passes to the next generation is the inherited material in the reproductive cells, not the changes made in the rest of the body.

Weismann's experiment

The German biologist August Weismann cut off the tails of mice and bred them for several successive generations. The young of every generation were still born with tails of normal length. Loss of the tail, an acquired change, was not inherited.

The germplasm theory

Weismann proposed the germplasm theory. The body is made of somatic cells, while the reproductive cells carry the germplasm. Changes produced in somatic cells by use or disuse do not reach the germ cells, so they are not passed on. According to this theory, only changes in the germ cells are inherited.

Everyday evidence

A wrestler develops large muscles by exercise, but the children are not born with such muscles. Muscles built by exercise are acquired characters, and they are not inherited.

Lamarck still made an important contribution. He was among the first to propose a full theory that living forms change over time and to link that change to their surroundings.

What is Darwin's theory of evolution by natural selection?

Charles Darwin (1809 to 1882) was an English naturalist. He sailed round the world on a ship called H.M.S. Beagle from 1831 to 1836, observing plants, animals and fossils. He published his theory in 1859 in the book On the Origin of Species.

On the Galapagos Islands, Darwin found many varieties of small black birds, later called Darwin's finches, with beaks suited to different foods. The naturalist Alfred Wallace, who worked in the Malay Archipelago, reached similar conclusions at about the same time. Darwin may also have been influenced by the work of Thomas Malthus on populations.

Main points of Darwin's theory

1. Overproduction: organisms produce more offspring than can survive. If every individual reproduced as much as possible, a population would grow exponentially, as a growing population of bacteria does.
2. Limited resources: food, space and other natural resources are limited, and in reality the size of a population stays fairly stable, apart from seasonal changes.
3. Struggle for existence: because more are born than resources can support, individuals compete for resources. The struggle can be with members of the same species, with other species, or with harsh conditions of the environment.
4. Variation: members of a population vary in their characteristics, and no two individuals are exactly alike. Most of these variations are inherited.
5. Natural selection: individuals with heritable variations that let them use resources better, and so suit their surroundings better, survive and leave more offspring than the others. Nature thus selects them.
6. Origin of new species: over many generations, the useful variations accumulate, the characteristics of the population change, and new forms appear to arise.

Darwin's theory rests on two key ideas. Branching descent means that all living forms have come down from common ancestors, and it accounts for homology. Natural selection is the mechanism that changes populations over time.

What does survival of the fittest mean?

The phrase survival of the fittest was coined by the philosopher Herbert Spencer, and Darwin later used it to describe natural selection. It means that the individuals best suited to their surroundings survive and reproduce more than the others.

Definition: Fitness, in Darwin's sense, refers ultimately to reproductive fitness: those who are better fit in an environment leave more offspring than others.

Fitness does not mean being the strongest, biggest or fastest. A well-hidden moth may be fitter than a conspicuous one. What counts is how many offspring an individual leaves.

A bacterial example

Suppose a colony of bacteria, A, is growing on a medium. The bacteria vary in their ability to use one food component. If the medium changes, only part of the population, B, can survive under the new conditions. In time, B outgrows the others and appears as a new species.

Bacteria divide so fast that this can happen within days. The same change in a fish or a fowl would take millions of years, because their life spans are in years. Under the new conditions, B is fitter than A, and nature selects for fitness.

Fitness must be inherited

Selection can change a population only when the useful character is inherited. Adaptive ability has a genetic basis, and fitness is the end result of the ability to adapt and to be selected by nature.

Natural selection can be seen today. Excess use of herbicides and pesticides has selected resistant varieties, and antibiotics have selected resistant microbes, within months or years rather than centuries. Evolution is not directed towards a goal; it depends on chance events and chance mutations.

How does the peppered moth show natural selection at work?

The best-known example of natural selection in action is the peppered moth, Biston betularia, of England. It occurs in a light form with white wings and a dark, or melanised, form with dark wings. The moths rest on tree trunks, where birds hunt them.

Definition: Industrial melanism is the increase in the proportion of dark, melanised individuals of a species in areas where industrial smoke and soot have darkened the surroundings.

What was observed

In a collection of moths made in the 1850s, before industrialisation set in, there were more white-winged moths on the trees than dark-winged ones. In a collection from the same area in 1920, after industrialisation, there were more dark-winged moths. The proportion had been reversed.

How natural selection explains it

1. Before industrialisation, the tree trunks were covered by a thick growth of almost white lichens.
2. Against this pale background, the white-winged moths were camouflaged and survived, while the dark moths stood out and were picked out by predators.
3. After industrialisation, industrial smoke and soot darkened the tree trunks, and lichens did not grow in the polluted areas.
4. Against the dark bark, the white-winged moths were now easily spotted by predators, while the dark moths were hidden and survived.
5. The surviving dark moths left more offspring, so over the generations the proportion of dark moths rose.

Two panels each show the same pair of moths resting with wings spread on a tree trunk: a dark, orange-brown moth on the left and a white moth on the right. In panel (a), the unpolluted area, the trunk surface is pale yellowish-brown. In panel (b), the polluted area, the trunk surface is a much darker blackish-brown, and the white moth stands out sharply against it.

See Fig. 6.4 in your NCERT textbook

In rural areas, where industrialisation did not occur, the count of melanic moths stayed low. This supports the explanation. In a mixed population, the forms that are better adapted survive and increase, but no variant is completely wiped out.

Lichens can be used as indicators of industrial pollution, because they do not grow in polluted areas. Where air pollution was later reduced, the light-coloured moths became common again.

Pollution did not create the dark moths. Both forms were already present, and pollution changed which of them survived better. This is natural selection acting on existing variation, as Darwin proposed.

How do Lamarck's and Darwin's theories differ?

Lamarck and Darwin both accepted that living forms evolve. They differed on how the change happens and on what is passed on to the offspring.

Darwin's explanation of the giraffe

In Darwin's view, the ancestors of giraffes varied in the length of their necks and forelimbs. When low food grew scarce, those with slightly longer necks reached more leaves, survived better and left more offspring. Their offspring inherited the longer necks, and over many generations the long-necked form prevailed.

What Darwin could not explain

Darwin could not say where variations come from or how they are inherited. Mendel had described inheritable "factors", but Darwin either ignored these observations or kept silent about them. Early in the twentieth century, Hugo de Vries, working on the evening primrose, proposed that sudden large changes called mutations cause evolution. Later studies in population genetics brought some clarity.

Who were the earliest ancestors of humans?

Human ancestors are known mainly from fossil bones, skulls and teeth, and from their tools. The members of the human line are called hominids (many scientists now use the term hominins for this line). They are described in the order in which they appeared, though they did not form a single straight line of descent.

Dryopithecus and Ramapithecus

About 15 million years ago, primates called Dryopithecus and Ramapithecus existed. They were hairy and walked like gorillas and chimpanzees. Ramapithecus was more man-like, while Dryopithecus was more ape-like.

Australopithecus

Fossils of man-like bones found in Ethiopia and Tanzania show that, about 3 to 4 million years ago, man-like primates walked upright in eastern Africa. They were probably not taller than 4 feet. About 2 million years ago, the Australopithecines probably lived in the grasslands of East Africa.

The name Australopithecus means "southern ape", and the first fossil of this group was found in South Africa in 1924. Australopithecus walked on two legs, though its long arms suggest that it also climbed trees. Its cranial capacity was about 400 to 550 cc, only a little larger than that of a chimpanzee.

It had a low, sloping forehead with prominent brow ridges and no chin. The Australopithecines hunted with stone weapons but essentially ate fruit.

Homo habilis

Among the bones discovered, some were different. They belonged to the first human-like being, the hominid called Homo habilis, meaning "handy man". Its brain capacity was 650 to 800 cc. Its fossils were found at Olduvai Gorge in Tanzania, along with simple stone tools. It probably did not eat meat.

Homo erectus

Fossils discovered in Java in 1891, often called Java man, revealed the next stage, Homo erectus ("upright man"), about 1.5 million years ago. Fossils known as Peking man were later found in China.

Homo erectus had a large brain of around 900 cc and stood fully upright. It probably ate meat, made stone hand-axes and is thought to have used fire.

Who were the Neanderthals, Cro-Magnon and modern humans?

Neanderthal man

Neanderthal man is named after the Neander Valley in Germany, where its fossils were found in 1856. The Neanderthals, with a brain size of 1400 cc, lived in the Near East and Central Asia between 1,00,000 and 40,000 years ago. They also lived in Europe.

They used hides to protect their bodies and buried their dead. They were short and heavily built, with very heavy brow ridges and a weak or absent chin.

Cro-Magnon man

Cro-Magnon man is the name given to the early modern humans of Europe, first found in 1868 in the Cro-Magnon rock shelter in France. They lived from about 40,000 to 10,000 years ago and belonged to our own species, Homo sapiens.

Cro-Magnons were tall, with a cranial capacity of about 1600 cc, a high forehead and a well-developed chin. They made fine tools of stone and bone and painted animals on the walls of caves.

Homo sapiens sapiens (modern man)

Homo sapiens arose in Africa and moved across the continents. Modern humans are named Homo sapiens sapiens, with a cranial capacity that averages about 1350 to 1450 cc.

The evolution of modern man, with language skills and self-consciousness, is the most successful story of evolution, and it appears to parallel the evolution of the human brain and of language.

Prehistoric cave art developed about 18,000 years ago. Cave paintings by prehistoric humans can be seen at the Bhimbetka rock shelter in Raisen district of Madhya Pradesh. Agriculture came about 10,000 years ago, and human settlements began.

A timeline of human ancestors

How did the key human features change during evolution?

As the human line evolved, seven features changed in a clear direction, as the skulls and skeletons of the ancestors show.

Three rows each show a skull in side view beside a drawing of the head. The adult human skull, at the top, has a large rounded brain case, a nearly vertical face and a short jaw with an even row of teeth. The baby chimpanzee skull, in the middle, also has a large rounded brain case and a small jaw. The adult chimpanzee skull, at the bottom, has a low, flat brain case and long jaws projecting far forward with large, pointed canine teeth. The baby chimpanzee skull is more like the adult human skull than the adult chimpanzee skull.

See Fig. 6.11 in your NCERT textbook

Bipedalism

Bipedalism is walking upright on the two hind limbs. Dryopithecus and Ramapithecus walked like gorillas and chimpanzees, but man-like primates were already walking upright in eastern Africa 3 to 4 million years ago. Homo erectus and later forms walked fully upright, as humans do.

Bipedalism freed the hands for carrying and for using tools. In the skeleton, the opening for the spinal cord, the foramen magnum, moved to the underside of the skull, and the backbone became S-shaped.

Increasing cranial capacity

Cranial capacity is the volume of the brain case of the skull, measured in cubic centimetres (cc). It rose from about 400 to 550 cc in Australopithecus to 650 to 800 cc in Homo habilis, around 900 cc in Homo erectus and 1400 cc in the Neanderthals.

The Neanderthals and Cro-Magnons had cranial capacities as large as, or larger than, the average of people today, so the steep rise took place in the earlier ancestors.

Reduction in the size of canine teeth

Apes have long, pointed canine teeth that project beyond the other teeth. In the human line the canines became smaller, until in modern humans they are level with the other teeth. The jaws also became shorter, so the face projects less. The change is linked with the use of tools and fire to prepare food.

Forehead and brow ridges

Brow ridges are bony ridges above the eye sockets. Early ancestors had a low, sloping forehead with heavy brow ridges. As the brain case grew larger and higher, the forehead became high and vertical, and the brow ridges became smaller. In modern humans the brow ridges are very small or absent.

Development of the chin

Early ancestors had projecting jaws with a lower jaw that sloped backwards, so they had no chin. The Neanderthals had at most a weak chin. A well-developed chin appears in Cro-Magnon and modern humans, and it is a feature of Homo sapiens.

Reduction in body hair

Dryopithecus and Ramapithecus were hairy, and early hominids are thought to have been covered with hair, like apes. Modern humans have very little body hair. Hair does not fossilise, so this change is inferred rather than seen directly in fossils.

Height and posture

Early hominids were short. The man-like primates of eastern Africa were probably not taller than 4 feet, and Homo habilis was about 1.0 to 1.3 m tall. Homo erectus reached about 1.5 to 1.8 m, and Cro-Magnons were about 1.7 to 1.8 m tall. Posture changed from partly ape-like to fully upright.

Glossary

• Organic evolution — The slow and gradual change in living organisms over many generations, through which new forms of life arise from pre-existing ones.
• Acquired character — A change in the body that develops during an organism's own lifetime through use, disuse or its surroundings; such changes are not inherited.
• Vestigial organ — An organ present in a reduced form with little or no function, though it is functional in the ancestors or in related animals.
• Vermiform appendix — A narrow, worm-like tube projecting from the blind end of the caecum; vestigial in humans but large in herbivores such as the rabbit.
• Struggle for existence — Competition among individuals for limited food, space and other resources, because more offspring are produced than can survive.
• Natural selection — The process by which individuals with heritable variations suited to their surroundings survive better and leave more offspring than others.
• Fitness — In Darwin's sense, reproductive fitness: the better fit individuals in an environment leave more offspring than others do.
• Industrial melanism — The rise in the proportion of dark, melanised forms of a species, such as the peppered moth, in areas darkened by industrial smoke and soot.
• Bipedalism — Walking upright on the two hind limbs, which freed the hands of human ancestors for carrying and for making and using tools.
• Cranial capacity — The volume of the brain case of the skull in cubic centimetres; it rose from 650 to 800 cc in Homo habilis to 1400 cc in Neanderthals.
• Brow ridges — Bony ridges above the eye sockets, heavy in early ancestors such as Homo erectus and the Neanderthals but very small or absent in modern humans.

Common errors and misconceptions

• Misconception: Humans evolved from the chimpanzees living today. Correct: Humans and modern apes evolved along separate branches from common ancestors that lived millions of years ago.
• Misconception: A giraffe that stretches its neck passes the longer neck on to its young. Correct: Characters acquired during a lifetime are not inherited; Weismann's tail-cutting experiment with mice showed this.
• Misconception: Survival of the fittest means that the strongest animals survive. Correct: Fitness means reproductive fitness; the fittest are those that suit their surroundings and leave the most offspring.
• Misconception: Industrial pollution turned white peppered moths dark. Correct: Both forms already existed; the darkened bark changed which form predators caught, so the proportion of dark moths rose.
• Misconception: Natural selection completely wipes out the less fit form. Correct: It changes the proportions of the forms; in the peppered moth, no variant was completely wiped out.
• Misconception: Vestigial organs prove Lamarck's idea of disuse. Correct: Vestigial organs are evidence of common ancestry; the idea that shrinking caused by disuse is inherited is not accepted.
• Misconception: Human cranial capacity has kept rising right up to the present. Correct: It rose steeply from Australopithecus to the Neanderthals; Neanderthals and Cro-Magnons had cranial capacities as large as people today.
• Misconception: Darwin explained how variations arise and are inherited. Correct: Darwin could not explain the origin of variations; mutations and the study of inheritance supplied this later.

Exam-style questions with model answers

Q1. Justify the statement: "Vermiform appendix is a vestigial organ in humans." [2 marks]

1. The vermiform appendix is a narrow, worm-like tube that projects from the blind end of the caecum.
2. In herbivores such as the rabbit it is large and helps in digesting cellulose, but in humans it is small and has no known role in digestion. It is a reduced organ with little or no function, so it is vestigial.

Q2. What is industrial melanism? Name the insect in which it was observed. [2 marks]

1. Industrial melanism is the increase in the proportion of dark, melanised individuals of a species in areas where industrial smoke and soot have darkened the surroundings.
2. It was observed in the peppered moth, Biston betularia, in England.

Q3. Explain Lamarck's theory of inheritance of acquired characteristics with the example of the giraffe. [3 marks]

1. Lamarck held that organs used constantly grow larger, organs not used become smaller, and such acquired changes are passed on to the offspring.
2. The ancestors of giraffes had short necks and forelimbs. When grass and low plants became scarce, they had to stretch their necks and forelimbs to reach the leaves of tall trees.
3. Constant use lengthened the neck and forelimbs slightly in each animal's lifetime, and this acquired lengthening was inherited by the young.
4. Over many generations the small gains added up, giving the long neck and forelimbs of the modern giraffe.

Q4. Name three vestigial organs in humans. How did Lamarck explain their presence? [3 marks]

1. Three vestigial organs in humans are the wisdom teeth (third molars), the vermiform appendix and the pinnae with their ear muscles.
2. Lamarck explained them by the disuse of organs: when an organ was no longer used, it became weaker and smaller during the lifetime of the organism.
3. The reduced organ was passed on to the offspring, and over many generations it shrank to the vestigial form seen today.

Q5. Give four differences between Lamarck's theory and Darwin's theory of evolution. [4 marks]

1. Cause of change: Lamarck held that the use and disuse of organs changed the body, while Darwin held that natural selection acted on inherited variations.
2. What is inherited: for Lamarck, characters acquired during the lifetime; for Darwin, inborn, heritable variations.
3. Role of the environment: for Lamarck, it created new needs that changed the body; for Darwin, it selected among variations already present.
4. Status: Lamarck's theory is no longer accepted, while natural selection is accepted as a main mechanism of evolution.

Q6. Explain survival of the fittest with reference to the peppered moth. [4 marks]

1. Survival of the fittest means that the individuals best suited to their surroundings survive and leave more offspring than others.
2. The peppered moth of England occurs in a white-winged form and a dark, melanised form. Before industrialisation, white lichens covered the tree trunks, so the white moths were camouflaged while the dark moths were picked out by predators.
3. After industrialisation, smoke and soot darkened the trunks. The white moths were now easily spotted and eaten, while the dark moths were hidden and survived.
4. The dark moths left more offspring, so by 1920 dark moths outnumbered white ones in the same area, reversing the proportion found in the 1850s.

Q7. Describe the main points of Darwin's theory of natural selection. [5 marks]

1. Overproduction: organisms produce more offspring than can survive; if all reproduced fully, populations would grow exponentially.
2. Limited resources: food and space are limited, so the size of a population stays fairly stable apart from seasonal changes.
3. Struggle for existence: individuals compete for the limited resources, with members of their own species, with other species and with the environment.
4. Variation: no two individuals are exactly alike, and most variations are inherited.
5. Natural selection, or survival of the fittest: individuals with heritable variations that suit their surroundings survive and leave more offspring; fitness means reproductive fitness.
6. Origin of new species: useful variations accumulate over many generations, the characteristics of the population change, and new forms arise.

Q8. Describe how each of these features changed during human evolution: (a) bipedalism, (b) cranial capacity, (c) canine teeth, (d) forehead and brow ridges, (e) chin. [5 marks]

1. (a) Bipedalism: early primates walked like gorillas and chimpanzees; man-like primates walked upright 3 to 4 million years ago, and later forms walked fully upright, freeing the hands.
2. (b) Cranial capacity: it increased from about 400 to 550 cc in Australopithecus to 650 to 800 cc in Homo habilis, about 900 cc in Homo erectus and about 1400 cc in the Neanderthals.
3. (c) Canine teeth: large, projecting canines became smaller until they were level with the other teeth, and the jaws became shorter.
4. (d) Forehead and brow ridges: the low, sloping forehead with heavy brow ridges became high and vertical, and the brow ridges became very small or disappeared.
5. (e) Chin: absent in early forms and weak or absent in the Neanderthals, it is well developed in Cro-Magnon and modern humans.

Key takeaways

• Organic evolution is the gradual change of living organisms over many generations; humans evolved from earlier ape-like primates and share common ancestors with modern apes.
• Lamarck proposed that organs grow with use and shrink with disuse, and that such acquired characters are inherited, as in his explanation of the giraffe's long neck and forelimbs.
• Wisdom teeth, the vermiform appendix and the pinnae with their muscles are vestigial in humans, which Lamarck explained by the disuse of organs.
• Lamarck's theory is rejected because acquired characters are not inherited, as Weismann's experiment of cutting the tails of mice over successive generations showed.
• Darwin's theory rests on overproduction, limited resources, struggle for existence, inherited variation and natural selection, with fitness meaning reproductive fitness.
• In the peppered moth, soot-darkened bark let dark moths escape predators, so dark moths outnumbered white ones by 1920, reversing the 1850s proportion.
• The human ancestors and their relatives are described in the order in which they appeared: Australopithecus, Homo habilis, Homo erectus, the Neanderthals, Cro-Magnon and Homo sapiens sapiens, though they did not form a single straight line of descent.
• During human evolution, posture became fully upright, cranial capacity rose, canines and brow ridges became smaller, the forehead became vertical, a chin developed and body hair decreased.

Test yourself

Which organ use did Lamarck give as the reason for the giraffe's long neck?

The constant stretching of the neck and forelimbs to reach leaves on tall trees, with the acquired lengthening passed on to the young.

From which part of the gut does the vermiform appendix project?

It projects from the blind end of the caecum, at the junction of the small and large intestines.

What did Weismann's experiment with mice show?

Mice whose tails were cut off over several successive generations still produced young with normal tails, so the acquired loss of the tail was not inherited.

What does fitness mean in Darwin's theory?

Fitness refers ultimately to reproductive fitness: individuals better fit in an environment leave more offspring than others.

Why did dark-winged peppered moths survive better after industrialisation?

Smoke and soot darkened the tree trunks, so dark moths were camouflaged and hidden from predators, while white moths were easily spotted.

What was the cranial capacity of Homo erectus, and where were its fossils found in 1891?

Homo erectus had a cranial capacity of around 900 cc, and its fossils were found in Java in 1891.

Which human ancestors had a well-developed chin?

A well-developed chin is found in Cro-Magnon man and modern man, Homo sapiens sapiens; earlier ancestors had no chin or a weak one.