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Area of study 2. How old is that fossil?. Fossils tell us what kinds of organisms lived in the past, but how do scientists determine the age of a fossil? There are a number of ways to do this:
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How old is that fossil? • Fossils tell us what kinds of organisms lived in the past, but how do scientists determine the age of a fossil? There are a number of ways to do this: • dating the absolute age of rock layers above and below the layer containing the fossil to give an upper and lower range of its possible age. • knowing the age of indicator fossils or deposits found with the fossil that have been dated from similar sites elsewhere. • dating the fossil directly to determine its absolute age.
Relative age based on stratiography • Because igneous rocks are formed by the cooling of hot lava that extrudes through the Earth’s crust, they do not contain fossils. • However, they are used as reference points to compare the age of sedimentary rocks that do contain fossils and that lie above or below the igneous rock. • The rule is that the lowest sedimentary rock stratum (layer) must have been the first formed and is thus the oldest.
Upper strata must have formed later and be younger. This is the relative dating method of stratigraphy. • For example, if a layer containing fossils lies below igneous rock that is dated at 200 million years old, then the fossils must be at least that age or older
Indicator fossils • Sometimes the only way to age a fossil bed is by the use of indicator fossils together with stratigraphy. • For example, in Europe the same types of ammonoids (extinct molluscs) are found in different regions. • A species of ammonoid fossil is called an indicator fossil because it indicates that the rocks at each locality are of similar age. • An indicator fossil is a fossil of known age found in a particular type of sedimentary rock layer. It can be used to indicate the age of the deposit at any single locality in which it is found.
Applying technology—absolute dating by radiometric methods • With the discovery of radioactivity at the end of the nineteenth century, absolute measurements of geological age became possible. • These methods are based on the principle that radioactive elements decay into different forms (e.g., uranium to lead, rubidium to strontium) at rates that are constant for a particular element. • Particular radioactive elements (isotopes) are used for fossils or rocks depending on the time scale involved.
The rate of decay of the element is independent of the nature of the rocks or the environmental conditions to which they are exposed, so they act as accurate clocks. • Geologists use radiometric methods to date igneous rocks, which are formed as a result of volcanic activity but do not contain fossils.
Radiocarbon dating • Fossils that contain carbon may be analysed by radiocarbon dating, which involves calculating the amount of decay of the carbon isotope carbon-14 based on its half-life. • The half-life is the time taken for half of the atoms in a sample to decay from the time that the isotope (in this case carbon-14) was incorporated into the organism when it was alive.
Measuring the ratio of carbon-14 to carbon-12 • Carbon isotopes are forms of the element that have the same number of protons (6) and electrons (6) but a different number of neutrons (6–8) and therefore a different mass. • 14C (8 neutrons) is the rarest carbon isotope and is unstable, while 12C (6 neutrons) is the most common and is stable. • 14 C decays to nitrogen-14 (14N) and has a half-life of 5730 ± 40 years. • For example, if a sample initially had 1.0 g of 14C, then there would be 0.5 g remaining after 5730 years, 0.25g after 11 460 years, and so on.
The method of radiocarbon dating is limited to samples not older than 50 000 years, because by that age there is very little 14C left.
Thermoluminescence • The technique of thermoluminescence can be used to date objects such as pottery, cooking hearths and fire-treated tools up to 500 000 years old, older than is possible with radiocarbon dating. • Thermoluminescence is the emission of light from a mineral when it is heated. • The amount of light is proportional to the amount of radiation an object has absorbed—the older the object the more light it emits.
The geological time scale • The modern geological time scale is divided into eras, which are further subdivided into periods. Eras, from oldest to youngest, are the: • Precambrian • Palaeozoic • Mesozoic • Cenozoic. • Boundaries between eras are recognised as abrupt changes in the fossil record, marking important biological events.
The Precambrian • The Precambrian era extends from the origin of the Earth and its oceans and atmosphere, to the origin of life, with first the evolution of prokaryotic cells and then single-celled and multicellular eukaryotes. • Many Precambrian fossils have been found in black chert—rock formed from gels of silica that precipitated on the surface of ancient sea floors, trapping organisms in the process. • The oldest organisms known from such fossils are from the Pilbara district of Western Australia, dated at 3.3–3.5 billion years. • These fossils resemble modern bacteria and cyanobacteria.
The Precambrian • Marine animals of the Ediacaran fauna : • The oldest fossils of multicellular marine animals are found in Precambrian rocks, with excellent examples in South Australia. These fossils are known as the Ediacaran fauna. They are preserved in hardened and baked sandstones (quartzites). They are impressions and moulds of animals that were all soft- bodied, with no traces of hard skeletons. • The fossils are nearly all small, averaging 3 cm in diameter. Some are worm-like, and probably burrowed through soft sand and mud on the bottom of the sea, feeding on organic detritus (small particles of dead organisms).
The Palaeozoic—an ancient time • The beginning of the Palaeozoic era is marked by the appearance of a diversity of animals (the Cambrian explosion) from 545 million years ago. • Its end is marked by the great Permian extinction, 250 million years ago. • Multicellular animals with organ systems were alive at least 600 million years ago.
The Palaeozoic • Trilobites were the most common marine multicellular animals of the early Cambrian era. • These are an extinct group of arthropods. Other fossils include echinoderms, living examples of which are sea stars.
The Palaeozoic • During much of the Ordovician period, shallow seas were widespread on the continents and animals included crinoids, corals and cephalopod molluscs, such as nautiloids, related to the living Nautilus. • Later another group of molluscs, the ammonoids, became dominant. Ordovician jawless fishes, only a few centimetres in length, were the first vertebrate animals.
The Palaeozoic • During the Silurian period, scorpion-like arthropods first invaded the land, together with simple plants and fungi. • In the seas, large armouredfishes, both jawless and jawed, evolved by the Devonian period, often called the ‘Age of Fishes’. • It was a time of rapid evolution of sharks and bony fishes. Among the bony fishes were fleshy-finnedfishes.
The Palaeozoic • During the Carboniferous period, forests of large tree-sized plants with woody stems (such as lycopods) evolved, providing habitats for a variety of terrestrial animals. • In swampy areas many of these forests formed extensive coal beds.
However, times were about to change and the Permian marked the close of the Palaeozoic. • It was a time when all of the continents came together as one single super-continent, called Pangaea. • This large land mass caused reduced rainfall, extremes of temperature and the death of many species, called a mass extinction.
At the end of the Permian period, animal groups such as trilobites and some groups of ammonoids died out forever. • Reptiles became more abundant but amphibians, more dependent on water, declined in dominance. • Seed plants had a great advantage, their dormant tough seeds allowing them to survive harsher conditions.
The Mesozoic—middle life • The Mesozoic era (‘middle life’, 250–65 million years ago) includes the Triassic, Jurassic and Cretaceous periods, and is often described as the ‘Age of Reptiles’.
The Mesozoic • During much of the Triassic period, the dominant vertebrates were a diverse group of bulky, large-headed reptiles with a sprawling posture. • Living alongside these reptiles were forms that had limbs under the body, allowing more support of body weight and greater mobility. • They included flying pterosaurs, crocodiles and dinosaurs.
The Mesozoic • Dinosaurs, which include some of the largest animals ever to have lived on Earth, reigned over the land throughout the Jurassic and Cretaceous. • They became extinct rather abruptly at the end of the Cretaceous period. The reason for this is uncertain. • Some scientists point to evidence of a large asteroid that struck the earth 65 million years ago. • They consider that the impact sent enough debris into the atmosphere to block sunlight and plunge the Earth into long-term cold and dark, which led to extinctions.
The Mesozoic • Although mammals first evolved in the Triassic period, as evidenced by a few teeth and jaw fragments, they do not become abundant and diverse in the fossil record for almost another 100 million years. • Fossils recognisable as marsupials (the most common) and placental (eutherian) mammals first appear in the Cretaceous period. • In marine communities, large turtles and predatory, dolphin-like ichthyosaurs lived.
The Mesozoic • Land plants of the Triassic and Jurassic were ferns, seed ferns, cycads, Ginkgo and conifers. • By the Late Cretaceous, flowering plants were more abundant throughout the world.
The Cenozoic—the beginning of modern life • The Cretaceous extinction ended the Mesozoic era and marked the start of the Cenozoic era, leading to modern organisms we are familiar with today. • The Cenozoic era (‘modern life’, 65 million years ago to the present) includes the Tertiary period (now officially divided into the Palaeogene and Neogene) and the most recent period, the Quaternary.
The Cenozoic • Throughout the Cenozoic we can recognise among the fossils more and more modern groups of flowering plants. • Mammals became abundant. In Australia, the environment changed from wet rainforest to more arid conditions during the Miocene period. • Kangaroos evolved that could survive dry conditions and feed on tough grasses that had also evolved. • The fossil record for kangaroos extends back about 25 million years.
The Cenozoic • During the Cenozoic period, monkeys and apes also evolved, including in the last 3 million years our genus Homo. • The evolutionary history of modern humans is a story of increased use of tools, language, culture as well as the development of agriculture and domestication of animals.
Biogeography • Biogeography is the study of the distribution of organisms and is another type of evidence for the theory of evolution. Biogeographers ask questions such as: • Why are placental mammals (such as tigers, lemurs and orang-utans) found in Malaysian rainforests but not in Australia and New Guinea (which have marsupials such as tree kangaroos and possums instead)? • Why are Asian birds (such as the Streaked Weaver bird) found on the island of Bali, whereas the island of Lombok, only 25 kilometres away, has cockatoos familiar to the Australian region?
Biogeography • One of the early biogeographers, Alfred Russel Wallace (co-discoverer with Charles Darwin of the theory of evolution by natural selection), asked such questions as he travelled through the Malay region collecting and studying the distributions of plants and animals. • From his observations and those of other naturalists he recognised that the world may be divided up into a number of biogeographic regions
Explanation by continental drift • What do these geographic patterns mean in evolutionary terms? • They suggest that evolution within each of these three taxonomic groups may have involved the geographic split of ancestral species and consequent isolation of populations on different land masses. • For these particular organisms, the continents drifting apart (based on the geological theory of plate tectonics) is one explanation. • The separating land masses carried a diversity of living organisms that gradually evolved in isolation from one another. • Knowing the dates of the separation of land masses is an indication of the age of plant and animal groups that evolved on the separate regions.
Take notes from the following slides under these headings: • Evidence of evolution from comparative anatomy • Homologous features • Analogous features • Comparative embryology • von Baer’s law • Vestigial structures
Evidence of evolution from comparative anatomy • If you compare the human body to that of a chimpanzee, you can see a striking resemblance in structure. • This is an example of comparative anatomy (also referred to as comparative morphology). • Humans and chimpanzees both have two arms (forelimbs), each with a hand and fivedigits (fingers). They also have two legs (hindlimbs), each with a foot and fivetoes. Guinea pigs, dogs, cats, kangaroos, seals and dugongs (all mammals like us).
For this reason all of these organisms are called tetrapods (literally meaning ‘four-footed’). • Tetrapodlimbs all have the same basic structure, even though they are modifiedfor different functions, such as the bird’s wings for flying, the cat’s legs for running and the seal’s flippersfor swimming.
Homologous features • Features of organisms that have a fundamental similarity of structure are called homologous features. • Homologous features are evidence of an evolutionary relationship. • They indicate that organisms, such as tetrapods, have had a common evolutionary origin. of common ancestry.
Homologous features • Close examination of tetrapod forelimbs shows that the same series of bones is present in each, but the ‘blueprint’ pattern has been modifiedso that different results occur.
Homologous features • In a bat, the hand bones (metacarpals) and fingerbones (phalanges) are elongated, providing essential support for the bat’s membranous wing. In the flipperof a seal, these bones are short and thick. • In humans, the hands are intermediate in proportion and are highly manipulative.
Homologous features • It is reasonable to conclude from these observations that mammals evolved from a common ancestor that possessed this ‘blueprint’ pattern of bones in its forelimb.
Homologous features • Homologous features are evident in all groups of organisms. In land plants, the seeds of cycads, Ginkgo, conifer trees and floweringplants show a variety of shapes and sizes but they are the same basic structure. • The seed of most conifers is winged and blown about by the wind, whereas the seed of an acacia lacks a wing, has a tough outer coat and a coloured nutritious appendage to attract ants that do the job of dispersal.
Homologous features • Despite the variation among seeds, these plants all reproduce by seeds, and it can be argued that they have evolved from a common ancestral group.
Analogous features • Organisms can also show similarity that is not due to common ancestry. Flying animals such as butterfliesand birds have wings. • Anatomical structures that are found in different groups of organisms, such as wings in birds and butterflies, are described as analogous features; that is, they serve the same function but have evolved independently.
Thus, when biologists attempt to work out evolutionary relationships, they must distinguish between homologous and analogous features. • They only use comparisons based on homologous structures. Identifying features as analogous provides evidence of a convergent pattern of evolution.
Comparative embryology • Sometimes it is difficultto determine whether particular anatomical features are homologous or not because the appearances of the adults are so different. • For example, the circulatory system of adult fishesand mammals are not particularly alike.
Comparative embryology • However, an examination of these circulatory systems during embryonic development shows clearly that they are based on the same pattern. This is an example of comparative embryology.
