16. Evolution or the gradual development of species

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16.2 The eras of life
16.3 Gallery: The beginning of life
16.4 Gallery: The Paleozoic era
16.5 Gallery: The Mesozoic era
16.6 Gallery: The Cenozoic era

16.7 The history of life – The lake
16.8 How do species evolve?
16.9 Terminology

16.1 The history of life

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There has been life on Earth for about four billion years. During this time, a huge number of species have evolved and become extinct.

Life was very simple in its primitive days. The organisms were microscopically small, single-celled archaea and bacteria. Gradually single-celled organisms developed into multicellular organisms.

These organisms gradually evolved into more complex multicellular organisms such as plants, fungi, and animals. All life was in the water until about 400 million years ago, when life began to move from sea to land.

Dinosaurs are the most well-known extinct group of animals.

The gradual process by which different kinds of living organisms are believed to have developed from earlier forms during the history of the Earth is called evolution. Studying fossils, or remains of organisms found in old rock formations, produces information about the process of evolution by showing what life looked like millions of years ago.

By comparing the structure of the organisms and the evolution of the individuals, we can see how closely or distantly the species are related to each other. In recent decades, comparisons of the genetic material of all organisms, DNA, have become the most important means of studying evolution.

16.2 The eras of life

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Study the table on the right.

The history of life on Earth can be divided into four geological eras.
They are

the Precambrian era,
the Paleozoic era,
the Mesozoic era,
the Cenozoic era.

The first cells and multicellular organisms developed during the Precambrian era. The first vertebrates, fish, began to develop in the water only about 600 million years ago. Dinosaurs ruled the Earth in the mighty Mesozoic. Birds and mammals became the dominant life during the Cenozoic Era. The modern human species has developed during the past million years.

Watch the video to find out more about the prehistoric timeline.

THE HISTORY OF LIFE
Era	Millions of years ago	Most important events	New organisms
The beginning of life	4 600	The first cells emerged.	Prokaryotes: archaea and bacteria
the Precambrian era	4 600 – 550	Photosynthesis provided energy for animals.	Eukaryotes, the first multicellular organisms
the Paleozoic era	550 – 250	In the beginning the life was only in the water, where fish evolved. Land-dwelling animals, such as amphibians and insects, developed towards the end of the era.	E.g. fish, amphibians, moss, ferns
the Mesozoic era	250 – 65	The era of reptiles.	Reptiles, birds, mammals, angiosperms (flowering plants)
the Cenozoic era	65 –	The diversification of birds and mammals.	Humans (5 million years ago)

16.3 Gallery: The beginning of life

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: Cyanobacteria are photosynthetic bacteria that are often considered the predecessors of chloroplasts. What we call "blue alga" are actually cyanobacteria.
: Stromatolites are fossilised cyanobacteria.
: The development of multicellularity and sexual reproduction were important steps in the evolution of life as we know it. Sexual reproduction makes increased hereditary variation in a species possible.
: Multicellularity made the diversification of cells possible. The picture shows the embryo of a sea urchin.

16.4 Gallery: The Paleozoic era

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: The earliest fish had no jaws. Of modern fish species, only the lamprey (pictured) retains this characteristic.
: The development of jaws rendered effective predation possible. The skeletons of cartilagenous fish, such as sharks, consist of cartilage. The picture shows a great white shark.
: The horseshoe crab has retained a similar form for millions of years. Such species are often called "living fossils".
: Toads are amphibians. They are land-dwelling animals that are still dependent on water for reproduction.
: Scorpions are an ancient group of organisms. They existed already 400 million years ago. Modern scorpions resemble their fossil predecessors.
: Dragonflies have existed on Earth for over 300 million years. They are among the oldest insects in the world.
: Horsetails are spore-bearing plants.
: The common polypody is a type of fern that likes to grow on rocky surfaces. Unlike most ferns, it is resistant to drought.

16.5 Gallery: The Mesozoic era

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: An interpretation of a dinosaur nest.
: Adaptations such as internal fertilization and embryotic growth inside an egg made reptiles better suited for life on land.
: Fossil findings have been used to recreate complete models of dinosaurs. Kentosaurus.
: Birds have descended from reptiles. Like reptiles, they reproduce by laying eggs. Birds have multiple adaptations that have made them well suited to flying, such as light bones, feathers, and a good circulatory system. The picture shows a cormorant.
: Cycads are old gymbospermic plants. They are living fossils that have remained unchanged for millenia.
: Coniferous trees are gymnosperms, seed-producing plants that have unenclosed seeds. A pine cone.
: Angiospermic plants or flowering plants reproduce via flowers and seeds. The picture shows a wood anemone.

16.6 Gallery: The Cenozoic era

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: The great mammal species of African savannahs, such as the elephant, the African buffalo, the rhinoceros, the lion and the cheetah, are among the most well-known mammals in the world.
: Over 9 000 bird species exist in the world today.
: Platypuses are exceptional mammals, as they lay eggs and have no mammary glands.
: Mammals developed some 200 million years ago. Kangaroo.
: Mammals are well adapted to different environments. For example, they have thick fur, good instincts and highly developed social skills. Birds and mammals filled the ecological niche that was left open after dinosaurs became extinct.
: The modern human has developed only in the last 5 million years.

16.7 The history of life - The lake

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Although millions of species have become extinct over the course of life's history, some species have always survived. Echoes of these species can still be found, even when one looks at a nearby lake.

The very first organisms were prokaryotic bacteria and archaea. As prokaryotic organisms, they consisted of only one cell which had no nucleus or specialized organelles. More complex cell structures emerged as bacteria and archaea cells were combined. For example, the chloroplasts of plant cells were originally photosynthetic bacteria that began to live inside other cells. Protists were primitive unicellular organisms. They are not fungi, animals, or plants.

The traditional categorization of living organisms.

Cyanobacteria are among the oldest organisms one can see when studying a lake. These photosynthetic bacteria emerged over 3,5 billion years ago. They were among the first organisms that could produce energy from sunlight using photosynthesis. As a result of photosynthesis, oxygen was gradually introduced into the atmosphere.

Cyanobacteria are photosynthetic bacteria.

Our lakes are home to a large variety of microscopically small unicellular algae and protists. Their cells are more developed than those of bacteria, as they have nuclei and organelles. All multicellular organisms, such as plants and animals, have descended from these kinds of organisms.

Plants moved to land before animals could. The oldest fossils of land plants date back to over 400 million years ago. They were a type of moss that would grow in shallow shore water. Plants diversified and grew in size drastically during the Carboniferous period. During this time, the lake shores of Finland were dominated by large fern forest.

Most species that inhabit our lakes are invertebrates. Many groups of invertebrates are quite old. For example, zooplankton such as water fleas have existed over 600 million years ago.

Flying insects developed during the same time as land plants grew more common. Dragonflies are among the oldest insect groups one can see in a lake. They developed 300 million years ago.

Fish are the earliest group of vertebrates. Most fish that can be found in Finnish lakes belong to the group of bony fish. Their fossils have been dated back over 450 million years.

Amphibians were the first vertebrates to move to the land some 400 million years ago. However, although they could live on the land, their reproduction was still dependent on water environments. Reptiles were the first vertebrates that could reproduce on dry land.

Angiosperm or flowering plants are a relatively new group of plants that emerged under 65 million years ago. Most of the plants you see on the lake shore are angiosperms. During the same time as angiosperms developed, the animal kingdom diversified through birds and mammals.

16.8 How do species evolve?

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In the wild, species are constantly changing, although it is usually difficult to see the change because this happens so very slowly. The basis for the evolution of species is that mutations occur in the genomic DNA of an organism. This is constantly happening in all organisms. Most of these mutations are repaired or insignificant. However, some mutations persist and affect the survival and progeny of individuals.

The picture below shows a white peacock. It has a mutation in its genome that prevents the development of coloured feathers. When the mutation is also present in the germ cells, it will be passed onto offspring.

The peacock's feathers do not produce colour pigment as a result of a genetic mutation.

Harmful mutations disappear because only individuals carrying the mutation precede poorly. If the mutation is favorable, eg. protects the organism against predator or helps them survive the winter, it will be passed on to the next generation. In this way, species become slightly different from generation to generation. The elimination of harmful traits and the passing on favourable traits to the next generation is called natural selection. Over time this process allows organisms to adapt to their environment.

How can new species arise from existing species? For species to separate from one another, different reproductive barriers, which prevent mating or reproduction of viable, fertile offspring, are formed. This may happen, for example, if one part of a previously unified population is isolated from the other parts of the populations. As there is no contact between these two populations, they can then gradually evolve in different directions. Over time, the differences become so great that individuals of different populations can no longer reproduce with each other. When this happens, a new species has emerged.

16.9 Terminology

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Evolution is the gradual development of species as the result of natural selection.
A fossil is the remains of an ancient organism.
A mutation is a change in an individual's genome.
Natural selection is a phenomenon where the fittest individuals reproduce most effectively.
Extinction is a phenomenon where a species disappears completely.
Archaea and bacteria are primitive, unicellular, prokaryotic organisms.
Eukaryotic organisms are an evolutionary group that consist of protists, plants, animals, and fungi. Their cells have nuclei and specialized organelles.
Cyanobacteria are water-dwelling bacteria capable of photosynthesis.
Angiosperm plants or flowering plants reproduce via flowers. Most plants one can see in Finnish nature are angiosperms.

16. Evolution or the gradual development of species

Contents

16.1 The history of life

16.2 The eras of life

16.3 Gallery: The beginning of life

16.4 Gallery: The Paleozoic era

16.5 Gallery: The Mesozoic era

16.6 Gallery: The Cenozoic era

16.7 The history of life - The lake

16.8 How do species evolve?

16.9 Terminology