Option D - Evolution

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D.1 Origin of life on Earth

D.1.1. The Earth was formed approximately 4.6 billion years ago from a cloud of dust particles orbiting the sun. This compacted due to gravity and together with the decay of radioactive elements generated heat that caused the interior to melt and form a central dense core of iron and nickel. Lighter materials formed the mantle around the core and the lightest silicates solidified into an outer crust, the continents and the outer floors. The atmosphere was formed from gases escaping through volcanoes and consisted of H2 , H2O, CH4, NH3, N2, H2S, but no O2. This mixture of gases was strongly reducing because it contained a lot of compounds that could donate hydrogen (and electrons). The oceans formed by the condensation of water vapour.

D.1.2 Over 4 billion years ago, simple organic compounds including monomers of biological macromolecules formed in shallow warm seas forming a mixture called the ‘primordial soup’. These compounds may have been the result of reactions between the various gases of the primitive atmosphere and natural energy sources such as lightening, UV and other radiation. Muller conducted an experiment using voltage discharge. After a week, there were amino acids and small organic acids. Methane and ammonia are the only way to make organic acids. However, Muller used sterile glass, distilled water and enclosed gas. Who says the Earth was like that back then?

D.1.3 The complexity of organic molecules increased in the primordial soup as a result of polymerisation. Clay particles could have acted as catalysts by absorbing monomers and facilitating polymerisation. Polypeptides, polysaccarides and polynucleotides could have formed like this. Once formed, the first polynucleotide RNA, was capable of self-replication. It is now suggested that this so-called RNA World then invented protein synthesis. By adhering to clay particles RNA could provide a pattern for protein synthesis. Thus RNA was the first enzyme or ribozyme. Clefts in RNA helix each formed from three bases match the shapes of various amino acids and could catalyse peptide bond formation. The triplets may survive today as the genetic code.

D.1.4 RNA (ribonucleic acid) being simpler than DNA is the most likely candidate for the first replicating molecule and also template and catalyst for protein synthesis. Soon some of the proteins synthesised in this way could have had catalytic properties, the first enzymes. These primitive RNA molecules could have also have been forerunners of ribosomes as it is thought that peptide bond formation is catalysed by ribosomal RNA. At some later stage DNA replaced RNA as the carrier of the genetic code since its double-stranded structure is more stable. D.1.5 A parallel development was the formation of coacervate droplets, aggregations of hydrocarbon molecules organised into sheets and forerunners of today’s phospholipid membranes. The first cells may have been such hollow droplets that contained RNA, which could synthesise structural proteins and enzymes. These droplets can grow by incorporating more molecules from the primordial soup and spontaneously split (binary fission) when they get too big. One of the commonest metabolic pathways is glycolysis and ATP synthesis.. The result of these developments was heterotrophic anaerobic prokaryotes that absorbed substrates from primordial soup through their membranes and were forerunners of the first methanogenic bacteria. A later development was photosynthesis when the more primitive chlorophyll a molecules held in the membranes could trap light and synthesise ATP. Thus the first autotrophic bacteria came into existence whose presence assured the survival of heterotrophs which were then able to use an inexhaustible biomass supply.

D.1.6 See endosymbiotic theory (you’ve already done it in Topic One). These are the 4 eukaryotic kingdoms: Animal Kingdom- Multicellular eukaryotes which feed heterotrophically by ingestion Plant Kingdom- Multicellular eukaryotes which feed heterotrophically by ingestion. Fungus Kingdom- Multicellular eukaryotes which feed heterotrophically by absorption Protist Kingdom- Unicellular eukaryotes which feed by a variety of different methods.

D.2 The origin of species

D.2.1 Evolution- The process of slow change by which organisms alive today are descended from ancestral forms.

Lamarck’s Theory of Evolution – In 1809, Lamarck proposed a mechanism for evolution summarised as ‘evolution by the inheritance of acquired characteristics.’

D.2.2 Lamarck suggested that: - Organisms used or under-used parts of their body. - Such parts became well or under-developed. - These acquired characteristics were inherited.

There is no evidence that evolution could happen like this. There is no known way for body changes to alter the inherited DNA in the gametes of an individual. Lamarck also proposed that some internal force propelled organisms to greater complexity, with humans at the end of this process. D.2.3 The Darwin-Wallance theory of evolution- the natural selection and inheritance of favourable characteristics. This is the most accepted theory. It came about through a series of observations: - Species overproduce young - Population size remains stable - There is competition for survival - Individuals vary - Natural selection favours good variations D.2.4 In the 1950s the scientist Kettlewell released peppered and white moths onto tree trunks and observed insectivorous birds (robins, thrushes etc) feed on them. On trees, the white ones were most visible- survival of the fittest.

D.2.5 Another theory for the origin of life is Panspermia- the theory that is concerned with the arrival of material from outer space, which has evolved. Another theory is Special creation- the belief held by many religions in a mythology which explains the origin of the species ready formed (and unchangeable) by a supernatural being/beings.

D.2.6 The scientific method (hypothesis,, experiment, analysis, and reformulation of the hypothesis) is largely inapplicable to both beliefs and evolution. Because we are unsure of past conditions, it is impossible to try and rerun evolution experimentally. We can not be sure that Miller or Kettlewell’s simulations had occurred in the past. Apart from the study of fossils, evidence for evolution is circumstantial; it is based on deductions made from the observation of differences between living organisms.

D.3 Evidence for evolution

D.3.1. Biogeography- geographical distribution of species. Why the strange distribution? Why don’t areas with similar climates necessarily have similar animals? Why does Australia have many marsupials, but almost no placental mammals (which it can support)? We find species where they are because they evolved from ancestors that inhabited certain regions. Australia has no placental mammals because they never evolved there and there wasn’t any contact with other places where they had evolved. It is thought that ancestral mammals originated in Eurasia and migrated to the southern continents via land bridges, which were later cut off by continental drift and sea level changes. Mammals reaching each continent then evolved into many new species to occupy the various niches. This process is called Radiative Adaptation.

D.3.2 Fossil- any preserved remnant or impression left by a past organism. If an organism happens to die somewhere where it can be preserved (acid bogs, ice, amber), whole specimens including soft tissue can be found. Mainly fossils are the only mineral rich parts (like bones) of animals which have been buried. Especially aquatic things often settle into water where sand and mud made sandstone and shale (respectively) over them, preserving them. Sometimes mineral rich parts become petrified (made into stone) when more minerals come in to replace organic material. Sometimes fossils are just rocks which have formed in spaces the shape of animals where organic material has dissolved away and other things (like hard minerals) have taken its place.

D.3.3 Radiocarbon dating can be used to date fossils containing the remains of organic compounds. A small proportion of carbon atoms exists as the heavier 14C isotope. These become incorporated into organisms during life but not after death when 14C atoms slowly decay to the lighter 12C and release radiation in the process. The range of time that this technique can be effectively used is dependant on the half-life. Pg 778 Yellow Book

D.3.4 Half life- The time during which the radioactivity falls to half its original level.

D.3.5 Find current amount of element. Find normal amount of element (the amount it would have had in its environment). Already know half-life. Find out how many half-lives it took to get to the current levels. Convert into corresponding number of years.

D.3.6 Pg 779-781

D.3.7 All organisms use DNA as their hereditary material and together with RNA carry out protein synthesis in the same way. The 3 base triplet code for each amino acid is also the universally used genetic code. This is strong evidence for a common ancestor for all living things on this planet. Similarly the use of proteins as structural units and enzymes is common to all living things and specific proteins (e.g. haemoglobin) are found among groups of organisms further suggesting common ancestry. Furthermore, proteins are built from a selection of only twenty specific amino acids out of a potentially infinite variety.

D.3.8 By comparing nucleic acids and specific proteins from different organisms, it is possible to construct phylogenetic (evolutionary) trees. A lot of different changes in one place are usually an indication of a species coming from one common ancestor.

D.3.9 Techniques are now available for comparing the nucleotide differences between DNA taken from different organisms, For example, human and chimpanzee is 1.2% different. These differences are due to mutations; accidental uncorrected changes to DNA. By relating these genetic diseases to dates of fossil remains, a mutation rate is obtained which is the basis of an evolutionary clock.

D.3.10 Closely related organisms go through similar stages in their embryonic development. For example, all vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats. In humans, these structures change into other things. The same skeletal elements make up the forearms of humans, cats, whales, bats etc, though they have different functions. This doesn’t make any sense unless they share some common ancestor. Homology- similarity in characteristic resulting from common ancestry. Pg 770- Yellow Book Fig 42.12

D.3.11 Heavy metal tolerance in plants- One of the most convincing examples of natural selection is provided by the evolution in certain grasses of tolerance to heavy metals, such as copper, zinc and lead, enabling them to flourish on the spoil from mines. Various physiological mechanisms have evolved to allow these plants to grow and reproduce in soils where heavy metals are present at concentrations that kill normal plants. In some species, the toxic metals bind to organic molecules in the cell walls where they remain trapped, unable to harm the cell’s contents. In other species, the metals are stored in the vacuole, out of harm’s way etc etc. Indeed, the plants have become so well adapted to living in such conditions that on unpolluted soil they do not fare so well and are out-competed by normal plants. Antibiotic resistance in bacteria- bacteria have very short generation times, are haploid and can reproduce sexually in a variety of ways. These features enable bacteria to evolve rapidly in response to changes in their environment. With the widespread use of antibiotics over the last 40 years, many bacteria have evolved cellular mechanisms enabling them to grow in the presence of a great many antibiotics- some have even developed to feed off certain antibiotics.

D.4 Human Evolution

D.4.1 Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Mammalia Sub-class: Placental mammals Order: Primates Family: Hominidae Genus: Homo Species: Sapiens Sub-species: Sapiens

D.4.2. - Long limbd and gripping hands and feet with opposable thumbs and big toes - Fingers with nails, not claws, and fingertips sensitive to touch - Large forward facing eyes for stereovision and distance judgment - Colour vision to identify other primates and identify food - Larger brains than other mammal groups for motor-coordination and social behaviour.

D.4.3. Anotomical changes- Apes can only stand for short periods of time because their lower backs and legs are not designed to support the weight of the upper body. Humans are neotenous apes (neoteny is when infant characteristics persist into the adult). Adult humans have chimp features. Baby chimps have large brains, relative to body size, protected by bulbous craniums, small jaws and no brow ridges. The spinal cord enters the skull by a hole under the centre of the skull. Neotony can be explained in terms of small hormonal changes controlling bone growth and the onset of sexual development. A delayed onset of puberty has lead to an increased period of parental care. In the lower body, the spine and hip joint have been strengthened to support and transfer weight to the legs. The knee can be locked straight and the feet have become support platforms, with the toes all in a straight line. Molecular similarities- Differences between human and ape DNA and particular proteins can be used to estimate when we shared a common ancestor. Nucleotide sequence in human and chimp DNA differ by little over 1%. When species undergo divergent evolution, accidental changes in the nucleotide order (i.e. mutations) occur to the chromosomes. As time progresses, the nucleotide sequence of a particular shared gene becomes increasingly different. When compared to fossil evidence an approximate data, and thus a rate of change can be estimated.

D.4.4 Australopithecines- human-like hands and teeth. Walked upright. Brain was 1/3 size. (Lived about 1.5-3.7 million years ago) A. afarensis- a different species of Australopithecine (2.8 – 3.7 million years ago) and gave rise to A. africanus- African ape (2 – 3 million years ago) A. robustus- (1.3 –2.4 million years ago) overlapped with H. habilis for about ½ a million years, but may have not competed directly. H. Erectus- (300,00 – 1.6 million years ago) was taller and had a bigger brain; almost full size) H. Sapiens- Us (130,000 years ago).

D.4.5 Global changes resulting in ecological changes in Africa may have prompted the evolution of new hominid species. Between 7 and 5 million, there was a general cooling and drying of the earth’s climate. In Africa, the tropical rainforests shrank and were largely replaced by savannah. The apes retreated with the forests except for a group that took this ecological opportunity to adapt to open woodland on the grassland fringes. There were the first Australopithecines and rapidly evolved in bipedalism. Again 3 million years ago another deterioration in global climate may have prompted the evolution of the first Homo species characterised by an increase in brain size. In the past two million years, the Quaternary, the world has experienced more or less regular cycles of glaciation each lasting 100,000 years.

D.4.6 By arranging extinct animals and plants into some kind of geological sequence, it is possible to suggest how one group may have evolved into another.

D.4.7 Bidepalism- may be an adaptation to open woodland and savannah grassland. Energetically, bipedalism is more efficient than knuckle-walking and it may have evolved as a way to cover the large distances between food more quickly. An upright stance is cooler due to faster air movements and only the head is expected to the sun retaining protective hair. Brain expansion- The primates show a brain size increase from prosimians through monkeys and apes. The primates are intensely social, political animals and extra brain tissue may be learning to be socially adept with other animals of the group. The Australopithecine had brains no larger than their ape ancestors did. The expansion of Savannah about three million years ago, due to another global cooling and drying, may have prompted the evolution of the first Homo line with significantly larger brains suggesting an increase in lifestyle complexity.

D.4.8 The evolution of speech and the development of the reflective mind (consciousness) occurred at some time in the Homo lineage.

D.4.9 Cultural evolution is the passing down of knowledge and ideas between generations, Examples: art /agriculture/ language / technology It has been argued to be more important than genetic evolution. There may be an inherited capacity to acquire culture knowledge. Education increases survival chances. Cultural evolution is rapid, human lifestyles therefore change rapidly, and some traditions have been lost. Cultural evolution was not as important in early evolution. Different cultural evolutions: Tool making revolution- 20, 000 years ago. Associated with the use of tools which allowed improvements in hunting and food-gathering techniques. Agricultural revolution- 10,000 – 6, 000 years ago, Improvements in farming and widespread domestication of animals and plants. Scientific- industrial revolution- 300 years ago. Improvements in food production, industry and medicine.

D.4.10 Genetic evolution is a change in traits that are heritable through DNA. It happens relatively slowly (over generations) Cultural evolution is the accumulation of learned things.

D.4.11. Bullshit alert