EvolutionEvolution

Theories of EvolutionGeological Time

What is evolution?What is evolution?

Evolution is the permanent genetic in a population of individuals.It does not refer to changes that occur to an individual within its own lifetime – individuals do not evolve, but populations can.The modern theory of evolution states that all living organisms share a common origin, dating back more than 4 billion years.

Theories of Evolution:17th Century ViewsTheories of Evolution:17th Century Views

View from the seventeenth century of the origin of birds and fish is summarised in the following quotation:

There is a tree, the tree of life — not, it is true, common in France, but commonly observed in Scotland. From this tree leaves fall; upon one side they strike the water and slowly turn into fishes, upon the other they strike the land and turn into birds.

Benoit De Maillet (1656–1738) De Maillet proposed that new kinds of organism could evolve as a result of changes to the structures of an existing organism, such as a fin transforming to a wing. Explaining the appearance of birds on Earth, De Maillet stated that flying fish that were chased out of the water:

… might have fallen some distance from shore among plants which, while supplying them with food, prevented them from returning to the water. Here, under the influence of the air, their anterior fins with their raised membranes transformed into wings … the ventral fi ns became limbs, the body was remodelled, the neck and beak became elongated and the fish discovered itself a bird.

Theories of Evolution:19th Century ViewsTheories of Evolution:19th Century Views

Erasmus Darwin (1731–1802) British physician and leading intellectualIn Zoonomia, published in 1794, Erasmus Darwin argued that all living organisms originated from a single common ancestor. He wrote:

Would it be too bold to imagine that, in the great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind that all warm-blooded animals have arisen from one living filament … with the power of acquiring new parts … and thus possessing the faculty of continuing to improve … and of delivering these improvements by generation to its posterity, world without end!

While Erasmus Darwin accepted that species could be transformed to produce new species, he gave no mechanism for this process.

Theories of Evolution:19th Century ViewsTheories of Evolution:19th Century ViewsJean Baptiste Lamarck (1744-1829)

French naturalistPublished in 1809 his views that:

structures in individual organisms could change in response to environmental conditions and to physiological needthese acquired structural changes would be transmitted to the next generation.

In simple terms, a giraffe has a long neck because it stretched to reach high leaves, and this long neck was then inherited by it’s children.Lamarck believed that organs appeared or disappeared according to the use made of them. He thought that use strengthened or enlarged an organ permanently and habitual disuse led to permanent loss.

Theories of Evolution:19th Century ViewsTheories of Evolution:19th Century ViewsRobert Chambers (1802-1871)

Supported the idea that species could evolve. In his best-selling book, Vestiges of the Natural History of Creation, published anonymously in 1844 Chambers wrote:

The idea, then, which I form of the progress of organic life on the globe … is that the simplest and most primitive type … gave birth to the type next above it, that this again produced the next higher, and so on to the very highest, the stages of advance being in all cases very small — namely, from one species only to another …

The cause of evolution identified by Chambers was that it occurred ‘under a law to which that of like-production is subordinate’. Since this is not a testable hypothesis, and able to be disproved, it has no scientific status.

Theories of Evolution:19th Century ViewsTheories of Evolution:19th Century Views

The views of De Maillet, Erasmus Darwin, Lamarck and Chambers were based on an assumption that species could change. This view, known as the transmutation of species, recognised that species could change over time to produce new species and were not fixed and unchanging.In contrast, the general view held by many scientists in the early 1800s, particularly in Britain, was that species were unchanging and that each species was fixed in its structure and characteristics for all time. According to this view, each species was the result of an act of creation — a view known as special creation of species. If this were the case, then each of the different kinds of fossil organisms from the past and each modern species would have been specially and individually created.These contrasting views — transmutation of species versus special creation of species — are not compatible. The view that species were fixed and each was specially created was finally dismissed as a result of the theory of evolution by natural selection developed by Charles Darwin and Alfred Wallace.

EvolutionEvolution

The concept that species can change and give rise over time to new forms is known as evolution. Evolution is also defined as descent with modification.Evolution can account for the diversity of species, past and present. Evidence from the fossil record and from many other fields of study supports the view that species can change and that evolution has occurred. Evolution provides an explanatory framework for many observations. Evolution means that past and modern species are related and that different kinds of organism living today are descended from various kinds of organism that lived in the geological past.

The Darwin-Wallace ViewThe Darwin-Wallace View

Various naturalists in the seventeenth and early eighteenth centuries proposed that species could evolve or be transformed to produce new species, but none of these naturalists identified a testable mechanism to explain how evolution could occur. This changed when Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) proposed their theory of evolution by natural selection.The power of the Darwin–Wallace theory of evolution by natural selection was that:

the theory identified a mechanism or a cause of evolutionthis mechanism was testable by observation and experimentation.The theory also asserted that individual organisms do not evolve in their lifetimes but that evolutionary change occurs over several generations in populations.

In developing their thinking about evolution, both Darwin and Wallace were influenced by observations made on independent and separate voyages to remote islands

Darwin’s VoyageDarwin’s VoyageIn 1831, Charles Darwin was offered the unpaid position as naturalist on HMS Beagle which undertook a five year voyage around the world. Upon visiting the Galapagos Islands, a cluster of more than one dozen volcanic islands located in the Pacific Ocean, nearly 1000 kilometres west of Equador, Darwin realised that, while the islands had similar plants and animals, each island had particular and distinctive varieties. He observed that the Galapagos Islands were home to more than a dozen species of finches. These finches showed some similarities to finches he had seen in Chile in South America. After his return to England in 1836, Darwin spent many years thinking about how species could change.Darwin came to realise that each species of finch was not specially created. Instead, he recognised that each kind of finch on the Galapagos Islands had evolved from a few ancestral finches that reached there from South America.In 1844, he began to put on paper an outline of his ideas of evolution by natural selection.

Wallace’s VoyageWallace’s Voyage

Alfred Wallace also travelled widely and visited many islands. In 1858, Wallace was in the Moluccas thinking about the problem of how species are formed.Wallace wrote:

. . . checks must also act upon animals, and keep down their numbers . . . While vaguely thinking how this would affect any species, there suddenly flashed upon me the idea of the survival of the fittest — that individuals removed by these checks must be, on the whole, inferior to those that survived. Then considering the variations continually occurring in every fresh generation of animals and plants, and the changes of climate, of food, of enemies always in progress, the whole method of species modification became clear to me.

So, Wallace concluded that natural selection was a plausible mechanism for evolution of species and he wrote a manuscript outlining his ideas and ‘sent it by the next post to Mr Darwin’. Darwin realised that Wallace had independently reached the same conclusion as he had, namely, that new species could arise as a result of the action of natural selection acting over many generations on populations. They agreed to present their proposal jointly to the scientific community.

Neo-DarwinsimNeo-Darwinsim

During the first half of the twentieth century, biologists combined Darwin’s ideas with the concepts of Mendelian genetics to produce a synthesis of ideas that is known as neo-Darwinism (neo = new). Darwin and Wallace identified one mechanism to explain how species evolve — the slow and gradual accumulation over long periods of time of inherited differences through natural selection.However, this is not the only process. During the late 1900s, it was recognised that new species could also be produced by other means including:

random or chance events, known as genetic driftchanges in the number of sets of chromosomes (which is a significant and very rapid means of production of new plant species).

Intelligent DesignIntelligent Design

Is the assertion that some features of living things are best explained as the work of a designer rather than as the result of a random process like natural selection.It points to complex structures in living organisms such as the eye and systems like the mechanisms for blood clotting as evidence against natural selection, suggesting they could not have arisen through the gradual fits and starts of evolution.Gaps in the fossil record, particularly during the Cambrian period where there was an explosion of new species, are also used as evidence for intelligent design.The term "intelligent design" originated in response to a 1987 United States Supreme Court ruling involving constitutional separation of church and state – specifically its ruling that teaching creationism was unconstitutional in public school science curricula. The difference between intelligent design and creationism is that intelligent design's advocates do not try to identify the “designer”. By not naming the Judo-Christian creator God and accepting the story of Genesis as a parable, supporters of ID claim it is a scientific theory, and seek to fundamentally redefine science to accept supernatural explanations – i.e. the external “designer”.Many American high schools boards are pushing for the incorporation of intelligent design alongside or in place of traditional evolutionary theories within the science curriculum.

Comparison of excerpts from two text books for high school biology

Comparison of excerpts from two text books for high school biologyBiology(Kenneth Miller and Joseph Levine)

Darwin made bold assumptions about heritable variation, the age of Earth and relationships among organisms. New data from genetics, physics and biochemistry could have proved him wrong on many counts. They didn’t. Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past.

Of Pandas and People(Percival Davis and Dean Kenyon)

Intelligent design means that various forms of life began abruptly through an intelligent agency, with their distinctive features already intact – fish with fins and scales, birds with feathers, beaks and wings, etc. Some scientists have arrived at this view since fossil forms first appear in the rock record with their distinctive features intact, rather than gradually developing.

Can you believe in God and Evolution?Can you believe in God and Evolution?

That’s up to you!!!!!Many scientists, including the director of the Human Genome Project do.

“I do not find the wording of Genesis to suggest a scientific textbook but a powerful and poetic description of God’s intentions in creating the universe. The mechanism of creation is left unspecified. If God, who is all powerful and who is not limited by space and time, chose to use the mechanism of evolution to create you and me, who are we to say that wasn’t an absolutely elegant plan? And if God has now given us the intelligence and the opportunity to discover his methods, that is something to celebrate”.

Time scales in evolutionTime scales in evolution

Time is a critical element in evolution. Estimating the age of rocks and of fossils that they contain is important in reconstructing the evolution of life on Earth.

How old is the Earth?How old is the Earth?

James Ussher (1581–1656)Anglican Archbishop of Armagh Published his calculation of the age of the Earth based on the Bible. By adding the ages of Adam and Eve and their descendants as given in the ‘Genesis’ chapter, Ussher concluded that the Earth was created on the evening of Sunday 23 October 4004 BC, making it about 6000 years old. This view prevailed until the late eighteenth century

James Hutton (1726–1797)Scottish farmer and amateur scientistPublished his conclusions about the age of the Earth in his Theory of the Earth and identified that ‘countless ages are required to form mountains, rock and soil’. Hutton recognised that repeated cycles of sedimentation, compression into rock, uplift and erosion had occurred and concluded that the Earth was millions of years old. He recognised that the processes of building layers of sedimentary rock were slow and yet, in spite of this, some of these layers were several kilometres thick.

How old is the Earth?How old is the Earth?Charles Lyell (1797–1875)

Published the three-volume Principles of Geology n 1830–33Lyell recognised that present-day cycles of sedimentation, compression into rock, uplift and erosion must have been repeated many times in the past and that the same geological forces acting today would also have acted in the past.

Lord Kelvin (1824–1907) Calculated the age of the Earth based on an estimated rate of cooling of Earth from an original molten state His first estimate (in 1862) was 98 million years old. (In 1897, he amended this estimate to 20 to 40 million years.) Kelvin’s estimates were too low because at that time, no-one knew about radioactive elements that are a source of internal heat in the Earth’s crust and so affect the rate of cooling.

John Joly (1899)Irish scientist Estimated the age of the Earth from salt content in the oceans. He calculated that it would have taken about 90 million years for the oceans to accumulate the present-day salt levels from inflow of river waters. This method was an underestimate of Earth’s age because it failed to account for salt in other locations, such as salt deposits.

How old is the Earth?How old is the Earth?

Other attempts to date the Earth were based on estimating rates at which sediments were deposited under the sea and then measuring the total thickness of sedimentary rock strata. Based on average sedimentation rates of 0.3 metres per 1000 years, one estimate of the Earth’s age as about 1600 million years was obtained in 1910.From about 1910 onwards, estimates using simple radiometric methods were made of the age of the Earth. By the mid-twentieth century, modern radiometric dating techniques had been developed. In 1956, Clair C. Patterson (1922–1995) used these techniques to calculate an accurate and reliable estimate of the age of the Earth based on the age of an iron meteorite. This meteorite had an estimated age of about 4500 million years. Since meteorites came into existence at the time of formation of the solar system, their age also identifies the age of the Earth.Since then, the age of many more meteorites has been determined and all have been found to have ages between 4400 and 4500 million years. This value has been confirmed by the dating of some moon rocks at 4500 million years old.

Geologic TimeGeologic Time

Our understanding of geologic time or deep time is recent.Based on modern techniques of radioactive dating, we now know that the Earth is about 4500 million years old — an interval that provides sufficient time for evolutionary and geological processes to occur.The geologic history of the Earth is divided into various time intervals as a hierarchy that includes eons, eras and periods.This geologic time scale developed in the eighteenth and nineteenth centuries when scientists organised sedimentary rock strata in the same region into groups of similar relative ages and also recognised similarities in rock strata in different regions because they contained identical fossils. By about 1840, the major divisions of the time scale based on relative ages were established but the absolute ages were not identified until during the twentieth century.

Geologic TimeGeologic Time

Geologic ages almost defy comprehension. We can gain some understanding by representing the age of Earth as a 100-metre track, with the starting line being the present time and the finishing line being the time of formation of Earth more than 4500 million years (Myr) ago. A human life of 72 years on this scale would be an undetectable 0.002 millimetres. Just the tip of a fingernail (one millimetre) over the starting line would take us back 45 000 years.Two steps (two metres) would take us to about 90 million years ago, to the time when Australia was part of the Gondwana super-continent and dinosaurs dominated life on Earth.Perhaps the most remarkable fact about this geologic time scale is that the first indirect evidence of life on Earth appeared about 3850 million years ago, but it was not until the Ediacaran period about 620 million years ago that the first multicellular animals appeared in the fossil record. The first members of the genus Homo appeared only 2.4 million years ago and the first modern human (H. sapiens, our species) first appeared about 190 000 years ago.

Geological ErasGeological Eras

The boundaries of eras are marked by major evolutionary events, such as mass extinctions. The four eras are (from oldest to youngest):

PrecambrianPalaeozoicMesozoicCenozoic

Precambrian Era 4560 – 570 million years agoPrecambrian Era 4560 – 570 million years ago

Extends from the origin of Earth and its oceans and atmosphere, to the origin of life, with first the evolution of prokaryotic cells and then single-celled and multicellular organisms.Multicellular organism only appeared 680 mya towards the end of the Precambrian era. Their fossils are referred to as Ediacaran fauna.Fossils of Edicaran fauna are nearly all small (~3cm in diameter. Some were small and worm-like and probably burrowed through soft sand and mud on the bottom of the sea. Others are like jellyfishes and sea anemones as well as some extinct species.

Palaezoic Era570 – 245 million years agoPalaezoic Era570 – 245 million years ago

Beginning of this era is marked by the appearance of a diversity of animals (the Cambrian explosion) and the its end is marked by the great Permian extinction.Palaezoic era is divided into six periods:

Cambrian – marked by appearance of invertebrates, trilobites were most common marine multicellular animals of the early CambrianOrdovician – widespread shallow seas, diverse marine life, first vertebrates (jawed fish)Silurian – first invasion of land by plants, fungi and scorpion like arthropodsDevonian - first trees and land vertebrates, radiation of fishCarboniferous – ferns and amphibians dominant, first reptilesPermian – reptiles dominant, Pangaea formed and mass extinction occurred as a result of reduced rainfall and extremes of temperature.

Mesozoic Era245 – 65 million years agoMesozoic Era245 – 65 million years ago

Often described as the “Age of Reptiles”Divided into three periods:

Triassic – marine and dinosaurs, including flying pterosaursJurassic – dinosaurs dominant, first birdsCretaceous – flowering plants originate, first marsupials and placental mammals

Cenozoic Era65 million years ago to present

Cenozoic Era65 million years ago to present

The Cretaceous extinction (included dinosaurs) marked the start of the Cenozoic era, leading to modern organisms we are familiar with today.Divided into two periods:

Tertiary – flowering plants and mammals increase, Homo genus appearsQuaternary – major ice ages, human range increases

Relative age versus absolute ageRelative age versus absolute age

The age of a person can be stated in two ways — by relative age or by absolute age. For example:

the statement ‘Kym is older than Tran who is younger than Shane’ gives the relative ages of these persons but does not give their chronological ages.Relative age allows us to place the persons in order of age: Kym then Shane then Tran.the statement ‘Kym is 34 years, Shane is 18 years and Tran is 16 years old’ gives the absolute ages of these people.

Likewise, the ages of geological structures, such as a deposit of mudstone or a layer of solidified volcanic ash (tuff), can be given in either relative or absolute terms.

Relative ageRelative age

The capacity to estimate the age of rocks enables the age of fossils to be inferred when they are embedded in a rock layer of known age or are located between layers of solidified volcanic ash that can be dated.The stratigraphic method of dating rocks gives the relative age of rock strata and uses the principle of superposition. This principle simply states that, for rock layers or strata (singular: stratum), the oldest stratum is at the bottom and progressively younger layers lie above it.A problem with this method is that sometimes layers of rocks are eroded away, buckled, moved or reburied and the original geological sequence is destroyed.

Relative ageRelative age

Another method for dating rocks uses the principle of correlation and is based on identifying particular fossils, known as index fossils.These are geologically short-lived species whose fossils have a limited occurrence in the fossil record and so are found only in a restricted depth of sedimentary rock strata. The presence of these fossils in rock strata, even in widely separated regions of the world, can be used to identify these rocks as having the same age.These two methods give the relative ages of rock layers throughout the world. Rocks of the same age are identified by the name of a geological period, such as Devonian or Jurassic.

Absolute ageAbsolute age

During the twentieth century, techniques for identifying the absolute age of rocks were developed. The most important method of estimating the absolute age of rocks is the radiometric dating technique that is based on the decay of certain radioactive elements. The radioactive elements concerned are those present in minerals in igneous rocks and each element decays at a particular rate. Igneous rocks form when molten rock solidifies, either below the Earth’s surface (for example, granite) or on the Earth’s surface (for example, basalt, tuff). This form of dating cannot be applied to sedimentary rocks that are derived from the erosion of pre-existing rocks because the minerals that they contain were formed prior to the rocks themselves.The principle of radiometric dating depends on the fact that various elements, known as parents, contain radioactive isotopes that spontaneously decay or break down to form stable daughter products.

Absolute ageAbsolute age

The time for the decay is specific for each radioactive isotope and the half life is the time taken for half the original radioactive isotope to decay.Each technique can be used over a particular age range that depends on the half life of the parent radioactive isotope.

Idealised decay of a radioactive parent element over a time period of five half lives.

Idealised decay of a radioactive parent element over a time period of five half lives.

Example: Potassium-Argon DatingK40 → Ar40

Example: Potassium-Argon DatingK40 → Ar40

When lava erupts at the surface, it loses all its argon.With time K40 decays to Ar40. The longer the time, the more Ar40. Thus by precisely measuring the amount of Ar40 and K40 in a rock, geoscientists can make precise estimates of its age.

← K40

← Ar40

Example: carbon 14 datingC14 → N14

Example: carbon 14 datingC14 → N14

All living organisms are built of carbon-containing organic matter, such as proteins (for example, the keratin of hair, nails, hooves, claws and the collagen of bones) and structural carbohydrates (for example, cellulose of plant tissues). When an organism is alive, the carbon in its organic matter is a mixture of two isotopes:

the stable isotope, carbon-12 (C-12)the radioactive isotope, carbon-14 (C-14) which decays to N-14.

In life, the proportion of these two isotopes is constant and matches that in the carbon dioxide in the atmosphere. This proportion remains constant during the life of an organism.After one half life of 5730 years, half the original amount of C-14 present in the organic material at the time of death will have disappeared, and so on for each successive half-life period. By measuring the ratio of C-14 to C-12 in a sample of organic material, an estimate of the time since death of the organism that produced this material can be obtained.C-14 dating can date bones and also artefacts, such as the wooden handles of tools or localised collections of ash and fragments of burnt wood. C-14 dating can estimate the absolute age of this material provided it is not older than about 60000 years.

ThermoluminescenceThermoluminescence

Thermoluminescence is the emission of light from a mineral when it is heated.It can be used to date objects such as pottery, cooking hearths and fire treated tools up to 500000 years old.It measures the accumulated radiation dose of material containing crystalline minerals since it was either heated (lava, ceramics) or exposed to sunlight (sediments). The older the object the more radiation it will have accumulated.The material is heated during measurements, and a weak light signal is produced that is proportional to the radiation dose.

Electron-spin resonance (ESR)Electron-spin resonance (ESR)

Electron-spin resonance (ESR) is a useful dating technique for ages from about 50 000 years ago to 500 000 years old. This period encompasses much of the evolutionary history of the genus Homo.The ESR dating technique depends on the fact that when objects are buried they are bombarded by natural radiation from the soil.For objects are composed of minerals (e.g. flint tools and fossil teeth) this bombardment causes some of the electrons in the minerals to move from the ground state to a higher energy level and some remain trapped at higher energy levels. The rate at which high energy electrons become trapped is determined by the background radiation; the longer the material has been buried, the greater the accumulation of higher-energy electrons.

Electron-spin resonance (ESR)Electron-spin resonance (ESR)

When material is exposed to heat, fire or bright sunlight, all the electrons return to the ground state so that the ‘electron clock’ is re-set to zero. As a result, ESR can be used to estimate the time since material under investigation was last heated, such as when a flint instrument was burned in a fire or when a tooth last lay exposed on the ground in sunlight.In ESR, scientists make direct measurements of:

the number of high-energy electrons trapped in the material under investigationthe radiation owing to any unstable isotopes in the material itselfthe background radiation in the soil in which the sample was buried.

Using these measures, it is possible to calculate a date that indicates the time that has passed since the ‘electron clock’ was last set to zero.