unit 6 history of life on earth
TRANSCRIPT
UNIT 6:HISTORY OF LIFE
ON EARTH
PALAEONTOLOGY:The study of fossil records to
discover the history of life, ancient climates and environments.
UNIT 10: BIOSPHERE, BIOMES AND ECOSYSTEMS
THE FORMATION OF MINERALISED FOSSILS
Fossils are formed in a number of different ways:
Most are formed when a plant or animal dies in a watery environment.
And is buried in mud and silt. Soft tissues quickly decompose leaving the
hard bones or shells behind. Over time sediment builds over the top And hardens into rock.
THE FORMATION OF MINERALISED FOSSILS continuous
As the encased bones decay, minerals seep in replacing the organic material cell by cell in a process called "petrification."
Alternatively the bones may completely decay leaving a cast of the organism.
The void left behind may then fill with minerals making a stone replica of the organism.
THE FORMATION OF MINERALISED FOSSILS CONTINUOUS
FOSSIL DATING:
Method used to determine the age of fossils or the strata (layer of sediment) in which they are found.
Example of methods;o Relative dating methods o Absolute dating methods like Radioactive dating.
RELATIVE DATING: The science determining the relative
order of past events, without necessarily determining their absolute age.
Determines which fossils are older or younger.
Easy to determine based on which geological deposit they come from and the Law of Superposition.
The Law of Superposition:o States that the older layer lies
underneath the younger layer in undisturbed contexts.
o Deeper layers are older than fossils from layers closer to the surface of the earth.
o The higher up you go in an undisturbed rock stratum (rock layer), the younger the rock layers become and therefore it is believed the fossils within these layers, are also younger than the fossils beneath them
Undisturbed Sedimentary Rock and its Fossils in Upper strata generally contain fossils of younger, more complex organisms, whereas, the lower strata contain fossils of simpler life forms
There is a tendency toward increasing complexity in life forms over time
ABSOLUTE RADIOACTIVE DATING: The process of determining an approximate
age for an archaeological or paleontological site or artefact.
usually based on the physical or chemical properties of the materials of artefacts.
provides a numerical age for the material tested, while relative dating can only provide a sequence of age.
Radiometric dating is based on the constant rate of decay of radioactive isotopes.
One of the most widely used and well-known absolute radiometric dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains.
Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals.
With death, the uptake of carbon-14 stops.
unstable isotope starts to decay into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain.
After yet another 5,730 years only one-eighth will be left.
Scientists can determine the date of death of an organic matter in an artefact, by measuring the proportion of carbon-14 in the organic material.
Other radiometric dating techniques include; potassium-argon dating (K-Ar
dating). Potasium-40 is a radioactive isotope
of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated.
Scientists use the results from fossil dating to make inferences about the age of a fossil, which can be used to determine which organisms lived when, and when did some organisms became extinct.
GEOLOGICAL TIMESCALES: Provides a system of chronologic
measurement relating to time that is used by earth scientists e.g geologists and palaeontologists to describe the timing and relationships between events that have occurred during the history of the Earth.
Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old.
Geological time scale is divided into different Eon’s which is divided into different Era’s, divided into different Periods, divided into different Epochs. (See geological timescale below)
What caused the change in the different era’s, and periods on the geological timescale? Climate changes e.g. increase in oxygen levels, ice ages and geological events e.g. movement of continents.
Know the three era’s: Paleozoic, Mesozoic and Coenozoic era.
Each era on the scale is separated from the next by a major geological or paleaontological event or change like mass extinctions.
E.g boundary between the Cretaceous period and the Paleocene period is defined by the Cretaceous–Tertiary extinction event, which denotes the end of the dinosaurs and of many marine species.
In the geological timescale know the major events and life forms in each era.
The Cambrian explosion is important because it is the origin of early forms of all animal groups. Life-forms have gradually changed to become present life-forms.
The Missing link between dinosaurs and birds are the Archaeopteryx, the missing link between the fish and the amphibians are the coelacanth and the missing link between the reptiles and mammals are the Thrinaxodon.
• Archaeopteryx
Thrinaxodon
MASS EXTINCTIONS:There has been five mass extinctions:1. Cretaceous–Tertiary extinction ev
ent – 65.5 Mya. About 17% of all families, 50% of all genera and 75% of species became extinct.
In the seas it reduced the percentage of sessile animals to about 33%. Mammals and birds emerged as dominant land vertebrates in the age of new life.
2. Triassic–Jurassic extinction event– 205 Mya. About 23% of all families and 48% of all genera (20% of marine families and 55% of marine genera) went extinct.
Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition.
Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments.
The Temnospondyl lineage of large amphibians also survived.
3. Permian–Triassic extinction e
vent
– 251 Mya. Earth's largest
extinction killed 57% of all families
and 83% of all genera (53% of
marine families, 84% of marine
genera, about 96% of all marine
species and an estimated 70% of
land species) including insects.
The evidence of plants is less clear,
but new taxa became dominant after
the extinction. The "Great Dying" had
enormous evolutionary significance:
on land, it ended the primacy of
mammal-like reptiles.
The recovery of vertebrates took 30 million years, but the vacant niches created the opportunity for archosaurs to become ascendant.
In the seas, the percentage of animals that were sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life, even before the "Great Dying".
4. Late Devonian extinction – 360–375 Mya. A prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and 70% of all species.
This extinction event lasted perhaps as long as 20 MY, and there is evidence for a series of extinction pulses within this period.
5 Ordovician–Silurian extinction event– 440–450 Mya. Two events occurred that killed off 27% of all families and 57% of all genera.
Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
KEY EVENTS IN LIFE’S HISTORY IN SOUTHERN AFRICA
Origins of the earliest forms of life:
evidence of fossilized bacteria (stromatolites) from caves found in the Barberton district, Mpumalanga and many other caves.
Soft bodied animals in NamibiaEarly land plants in the
Grahamstown area.Forests of primitive plants such a
Glossopteris which form most of the coal deposits in southern Africa.
The Coelacanth as a living fossil of the group that is ancestral to amphibians.
Mammal like reptiles in the Karoo e.g. Thrinaxodon and Lystrosaurus
Thrinaxodon
• Lystrosaurus
Dinosaurs found in the Drakensberg and Maluti mountains e.g. Euskylosaurus from Lady Brand in the Free State and cone bearing plants.
Euskylosaurus
First mammals found in Eastern Cape and
Lesotho
Humans fossil found in Gauteng, Free State, Kwazulu Natal, Western Cape and Limpopo
Part of the skeleton of Australopithecus sediba from the Malapa site in South Africa. Two partial skeletons unearthed in a cave belong to a previously unclassified species – which could be an early human ancestor – dating back almost 2 million years
Sterkfontein's first piece de resistance: the Australopithecus africanus Mrs Ples (now believed to be a Mister Ples), dating back 2.5-million years, found by Robert Broom in 1947. The fossil provided proof that Australopithecus could be classified as a member of the Hominidae (the family of humans) and established Africa as the Cradle of Humankind.