Theory of Evolution

Origins of life

Living organisms on Earth today, and the chemicals which work together to keep them alive, are extremely complex, even in single-celled micro-organisms. Some people find it hard to accept that such complexity could have evolved through natural selection. Some religious people believe that all living things on Earth were made by God, or that life was begun by God but then evolved through natural selection. We will probably never be absolutely certain about how life began, as no one was there to observe it. But scientists must base their theories on evidence.

Lamarck

 

Darwin was not the only person to develop a theory of evolution. Jean-Baptiste Lamarck was a French scientist who developed an alternative theory at the beginning of the 19th century. His theory involved two ideas. These are:

• the law of use and disuse
• the law of inheritance of acquired characteristics

His theory stated that a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring.
This table summarises the two different explanations for giraffes having long necks.

Darwin's theory of evolution

Charles Darwin (1809 - 1882)

Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859.

Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures in it.

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria to the first modern humans.

Selective breeding

Natural selection and selective breeding can both produce changes in animals and plants. The difference between the two is that natural selection occurs in nature, but selective breeding only occurs when humans intervene.
Selective breeding is a process where we choose the characteristics we want in an animal. We then breed together a male and female that show some of those characteristics. From the offspring produced we select those that show the characteristic the most, and breed them together.
This process is repeated over many generations, each time selecting and breeding together those animals that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.

All of these dogs were selectively bred from a common ancestor

Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.

Causes and effects

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, for example the chemicals in cigarette smoke.
Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Very rarely the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.
It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.

Bacteria

Bacteria

The word bacteria is the plural of bacterium. Grammatically the headline should just say "What are bacteria?" The incorrect usage has been included in the headline to remind readers that it is wrong - and hopefully help correct an increasingly common mistake in the English language. Bacteria are tiny living beings (microorganisms) - they are neither plants nor animals - they belong to a group all by themselves. Bacteria are tiny single-cell microorganisms, usually a few micrometers in length that normally exist together in millions.

A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.

Bacteria come in three main shapes:

• Spherical (like a ball)
  
  These are usually the simplest ones. Bacteria shaped like this are called cocci (singular coccus).

• Rod shaped
  
  These are known as bacilli (singular bacillus).
  Some of the rod-shaped bacteria are curved; these are known as vibrio.

• Spiral
  
  These known are as spirilla (singular spirillus).
  If their coil is very tight they are known as spirochetes.

There are many variations within each shape group. 

This is a file from Wikimedia Commons

 

Bacteria are found everywhere

Bacteria can be found in:

• Soil
• Radioactive waste
• Water
• Plants
• Animals
• Deep in the earth's crust
• Organic material
• Arctic ice
• Glaciers
• Hot springs
• The stratosphere (between 6 to 30 miles up in the atmosphere)
• Ocean depths - they have been found deep in ocean canyons and trenches over 32,800 feet (10,000 meters) deep. They live in total darkness by thermal vents at incredible pressure. They make their own food by oxidizing sulfur that oozes from deep inside the earth.

Scientists who specialize in bacteria - bacteriologists - say bacteria are found absolutely everywhere except for places that humans have sterilized. Even the most unlikely places where temperatures may be extreme, or where there may be a high concentration of toxic chemicals have bacteria - these are known as extremophiles (an extremophile is any organism adapted to living in conditions of extreme temperature, pressure, or/and chemical concentrations) - these bacteria can survive where no other organism can. 

The cells of bacteria

A bacterial cell differs somewhat from the cell of a plant or animal. Bacterial cells have no nucleus and other organelles (sub-units within a cell with a specific function) bound by a membrane, except for ribosomes. Bacteria have pili, flagella, and a cell capsule (most of them), unlike animal or plant cells. An organism without a nucleus is called a prokaryote.

A bacterial cell includes:

• Basal body - this anchors the base of the flagellum, allowing it to rotate.

• Capsule - a layer on the outside of the cell wall. Some bacteria don't have a capsule.

• Cell wall - a thin layer (membrane) outside the plasma membrane, and within the capsule.

• DNA (Deoxyribonucleic acid) - contains all the genetic instructions used in the development and functioning of the bacterium. It is inside the cytoplasm.

• Cytoplasm - a gelatinous substance inside the plasma membrane. Genetic material and ribosomes lie inside.

• Flagellum - this is used for movement; to propel the cell. Some bacterial cells have more than one.

• Pili (singular: pilus) - these spikes allow the cell to stick to surfaces and transfer genetic material to other cells. A study revealed that pili are involved in causing traveler's diarrhea.

• Plasma membrane - it generates energy and transports chemicals. Substances can pass through the membrane (permeable). It is located within the cell wall.

• Ribosomes - this is where protein is made (synthesized). Ribosomes are small organelles made up of RNA-rich granules.

 

This is a file from Wikimedia Commons