Popular theories of Mass Extinction and Cradle of Humankind

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					Saturday X-tra                                                       X-Sheet: 21

      Popular theories of Mass Extinction and Cradle of Humankind

Terminology & definitions
Background extinction: the normal and natural loss of some plant and animal species
over time.
Mass extinction: when many species become extinct at the same time.
Geological time scale: a system of chronological measurements that relate to
stratigraphy and the location of fossils in the different strata over a period of time.
Stratigraphy: a branch of geology, and is the study of rock layers and stratification
Phylogenetic tree: a schematic representation that shows the links between common
ancestors. It includes how all the species are linked genetically, based on the phylum of
each species, and can be used to show possible evolutionary trends.
Hominisation: process of the gradual transition to becoming human, that refers not only
to the physical changes that took place, like diet, movement and skeletal structure, but
also the accelerated development of social behaviour.


Key Concepts
The Geological Time Scale:
Scientists generally agree that there have been six mass extinction events in the history
of life on earth, and use fossil records to determine the geological timeline/time scale. It
provides a system of chronological measurement relating stratigraphy (study of the earth’s
strata) to time. The information is used to determine the relationship between events and
when they happened relative to where the remains are located within the strata. Evidence
from radioactive dating shows that earth is about five billion years old. The geological time
scale has been organised into various units, and classified into periods. Different time
spans on the time scale are marked by major geological events, e.g., the boundary
between the Mesozoic and the Paleozoic eras is marked by the extinction of the dinosaurs
and many marine species. Older periods are defined by absolute age (radioactive dating
and carbon dating) processes. Each era is separated from the next on the scale by a
major event or change.
What Caused Mass extinction?
Some of the theories are briefly explained as follows:

MAIN                 WHAT IT CAUSED:
Continental drift    Alfred Wegener proposed that all the different continents seemed to
                     fit into one puzzle, and were originally joined to form one super-
                     continent called Pangea. Pangea split into Laurasia (North) and
                     Gondwanaland (South). Later, these two continents split further to
                     form the continents today. It is estimated that the supercontinent
                     broke up into pieces approximately 180 million years ago, in a
                     process called the continental drift. The theory is backed up by the
                     presence of the same fossils that have been found on two or more
                     continents and islands.

Volcanic theory      During the Cretaceous era, there was a lot of volcanic activity. Dust
                     and ash from the volcanic eruptions blocked out sunlight. Plants need
                     sunlight to photosynthesise food and produce oxygen. Plants would
                     have died and animals would have eventually starved. Lack of
                     sunlight would also have cause climate changes, like ecessive
                     cooling of the atmosphere etc.

Extra-terrestrial    The impact of large meteorites and asteroids (very large meteorites)
theory               on the earth. Scientist theorise that a huge asteriod smashed into
                     earth 3.5 billion years ago, and caused the extinction at the end of the
                     Cretaceous era. This is called the asteriod impact theory. This
                     would have caused huge tsunamis, acid rain, incredible heat waves,
                     world-wide fires and volcanic eruptions, causing chemical changes to
                     the earth’s atmosphere. With the lack of oxygen and huge amounts of
                     dust that blocked out sunlight, it would have killed many plants and
                     animals. It would also have decreased the temperature, causing
                     major climate change and extinction of many species. It is believed
                     that an asteroid caused the mass extinction of the dinasaurs when it
                     struck near the Gulf of Mexico.
Star Explosions      When a star explodes, it is called a supernova explosion, resulting
                     is cosmic radiation (Cosmos = our universe and beyond) that
                     impacted earth. This radiation would have caused DNA damage of
                     the plants and animals, thereby causing mass extinction. (Think of the
                     atom bomb that was dropped on Hiroshima and Nagasaki during the
                     2nd World War – people are still seeing deformities from the radiation
                     fall-out 80 years later).
6th Extinction       Human impact on the environment, pollution and disease pandemics
                     are thought possibly to bring about the earth’s sixth distinction. Many
                     scientists believe that our continued abuse of our planet will cause
                     our own final destruction of life on this planet as we know it.
A more specific geographical time scale would be to show the formation of vertebrates on
the planet. When a schematic representation is used to show the links between common
ancestors, it is called a phylogenetic tree. The tree shows how all the species are linked
genetically, based on the phylum of each species and the possible evolutionary trends.
The diagram that follows was adapted from the DoE November 2009 exam paper 2.

From this diagram you can see the origin of each of the vertebrates groups. The larger
size of the bubble represents when most of the species lived. Most fish species were in the
Devonian period. Most amphibian species were present in the Permian period, reptiles
lived in the Jurrassic period, and so on. Point A on the schematic diagram shows the
common ancestor split that gave rise to mammals, and the line that leads to the common
ancestor that gave rise to birds and reptiles.
Characteristics that humans share with other primates
 opposable thumb with power grip and precision grip (we are able to wrap the fingers
  around objects while the thumb stands loose to wrap around the other side in order to
  hold the object).
 bare finger tips – for a better sense of touch.
 long arms (primates have much longer arms than humans to enable them to swing in
  trees, and for a more fluid movement).
 freely rotating arms and hands - owing to the ball-and-socket joint in the shoulder,
  and the gliding joint in the wrist, both joints are able to rotate through 180o.
 stereoscopic vision – where two eyes are able to focus on one object and perceive
 visual acuity – eyes have an increased number of rod AND cone cells with their own
  nerve cells where cone cells enable us to see colour.
 large brain when compared to body mass – allowing for intelligence and thinking
 brain centres that are able to process information from the senses, are enlarged and
  function well - sense of touch and sight especially.
 olfactory centre (sense of smell) in the brain is reduced.
 few offspring – humans and primates have longer gestation periods, less offspring
  and increased parental care.
 upright posture and bipedalism (bi = two and pedal = walk, so bipedalism means
  walking on two legs). Primates sometimes move on two legs but often use their arms to
  assist them.
 social dependency – group cohesion and living together ,enjoying shared activities

Characteristics that make humans different from other primates
 humans are always bipedal as we always only walk on two legs and never use our
  hands on the ground. Walking on two legs has implications beyond those affecting the
  skeleton and muscles, as scientists theorise that the upright posture and subsequent
  changes to the nervous system resulted in the enlargement of the cerebral
  hemispheres. (see figure 21-7 below).
 a human face and skull is flat with no prognathous (protruding jaw structure) (see
  diagram a below).
 dentition (teeth) is similar to that of monkeys and apes, but different from that of older
  primates, like the gorilla, with smaller canines, since humans do not require large
  canines to rip flesh to eat, or for defense. Teeth are aligned into the jaws in a gentle
  curve/’u’ shape (see diagram c below).
 larger brain than primates (brain size varies from 1200ml to 1800ml with the average
  size being 1400ml) (see diagram b below).
 humans have learned to communicate through language.

The Cradle of Humankind
The Cradle of Humankind is a World Heritage Site first named by UNESCO in 1999. It is
located about 50 km northwest of Johannesburg, South Africa. Hominid remains have
been excavated at the Cradle of Humankind. Many anthropologists believe that hominids
lived all over Africa, but their remains are only found at sites where their bones were
preserved into fossils, as at the Cradle of Humankind. Archaeological caves in the
Makapan Valley show traces of human occupation and evolution dating back about 3,3
million years. There is evidence that links the origin and evolution of humankind to
fossils of several specimens of early hominids, dating back to between 4,5 million and 2,5
million years.
Fossil evidence

Who found it:     Date:   What was found:        Where was it       Relevance:
Raymond Dart      1924    Juvenile               Taung is a small   The Taung
                          Australopithecus       town in the        Child’s skull
                          africanus skull,       North West         shows that it
                          called the Taung       Province.          was positioned
                          Child.                                    directly above
                                                                    the spine,
                                                                    indicating an
                                                                    upright posture.
                                                                    This is a trait
                                                                    seen in
                                                                    humans, but not
                                                                    other primates.

Dr Robert         1947     2,3-million year Sterkfontein            The uncovering
Broom and                 old fossil of     Caves                   of Mrs Ples
John Robinson             Australopithecus                          provided further
                          africanus,                                proof of the
                          commonly know as                          development of
                          Mrs Ples.                                 humankind, and
                                                                    supported the
                                                                    findings of the
                                                                    Taung Child.
Team: Maurice     1974    Excavated 40% of       Hadar in the       Lucy’s skull
Taieb, Donald             a 3.2 million year     Awash Valley of    capacity was
Johnson, Mary             old skeleton of an     Ethiopia’s Afar    small like apes,
Leakey and                Australopithecus       Depression         but showed
Yves Coppens              afarensis, called                         bipedalism, as
                          Lucy                                      in humans,
                                                                    proving the
                                                                    theory that
                                                                    preceded the
                                                                    increase of the
                                                                    human brain
Dr Ronald J       1997    The near-complete      Swartkrans in      Also found
Clarke                    3.3 million year old   the Cradle of      evidence for the
                          Australopithecus       Humankind          controlled use
                          skeleton, called                          of fire dating
                          Little Foot                               back to 1 million
                                                                    years ago.
X-ample Questions
Question 1
The graph below shows how the number of families of species has changed over time.
The arrows indicate periods of mass extinctions.

1.1   Explain the term “mass extinction”                                           (2)
1.2   When did the Mesozoic era start?                                             (2)
1.3   Approximately how many families of species died out at the end of the
      Paleozoic era? Show ALL working.                                             (3)
1.4   What happened to the number of families after each mass extinction?          (1)
1.5   Scientists have come to the conclusion that the dinosaurs became extinct
      about 65 million years ago. What is currently the most accepted theory for
      this mass extinction?                                                        (1)
1.6   Describe the theory mentioned in QUESTION 1.5 that is proposed as a
      possible cause of extinction of the dinosaurs.                               (5)
      (Taken from DoE Wester Cape September 2009 Paper 2)

Question 2
The diagrams below represent the skull, the upper jaw and lower jaw of three organisms,
the Taung child (Australopithecus africanus), a modern human (Homo sapiens) and a
gorilla (Gorilla gorilla). The arrow indicates the position of the foramen magnum (the
opening that allows the spinal cord to connect with the brain). Study the diagrams and
answer the questions that follow.
2.1.   Identify the organisms that are represented by each of A, B and C.         (3)
2.2.   Assuming that the diagrams were drawn to scale, name TWO observable
       differences between the skulls of organisms A and B.                       (4)
2.3.   Which organism (A, B or C) represents a carnivore?                         (1)
2.4.   Explain your answer to QUESTION 2.3 using features visible in the diagram.
2.5.   Name TWO organisms that are best adapted for walking on two legs rather
        than four legs, by looking at the position of the foramen magnum (indicated
        by the arrows).                                                           (2)
       (Taken from DoE Exemplar 2008 Paper 2)

Various options are provided as possible answers to the following questions. Choose the
correct answer.
1. Most scientists agree that the number of mass extinctions that have occurred in the
   history of life on earth is …
       A        3
       B        7
       C        6
       D        5
2. According to the theory of continental drift, all the land masses were joined together to
   form one super-continent called …
       A     Gondwanaland.
       B     Pangaea.
       C     Eurasia.
       D     Laurasia.
3. Which of the following are used to explain mass extinction?
       i.     Ice ages
      ii.     Outbreeding
     iii.     Volcanic activity
     iv.      Disease
       A      (i) and (ii) only
       B      (i), (iii) and (iv) only
       C      (i), (ii) and (iii) only
       D      (ii), (iii) and (iv) only
4.    The evidence that related species in similar biomes across the world developed
      from a common ancestor, is obtained from …
      A      micro-evolution
      B      embryology
      C      biochemistry
      D      biogeography
5.    Study the diagram below showing stages in continental drift.

      The correct sequence of events during continental drift is …
      A     1, 2, 3, 4
      B     1, 4, 2, 3
      C     1, 3, 4, 2
      D     1, 2, 4, 3
6.    Extinction occurs….
      A. When a species is unable to survive in its environment.
      B. When only a few of a species survives.
      C. When a species goes into hiding until conditions improve.
      D. With the gradual depletion of a population.
7.    Discontinuous distribution is…
      A. When the organisms live in one area and distribute within their environment.
      B. Organisms originate in one area and disperse outward.
      C. When organisms originate in different areas and migrate towards one another.
      D. When different species are distributed evenly among one another.
8.    Pangea is said to be…
      A. The continents which made up two large masses of land.
      B. The continents, which made up one large mass of land.
      C. The seven continents that exist today.
      D. The continents, which made up three masses of land.
9.    What is an Ice Age?
      A. It is when it gets cold in winter time
      B. When there is a very long period of time with very low temperatures
      C. The Artic and Antarctic poles
      D. When it takes ages to get warm
10.   The fossil records reflect that the first meteor collision occurred ….
      A. About 5 billion years ago
      B. About 8 billion years ago
      C. About 3,5 billion years ago
      D. About 2,5 Billion years ago

1     D
2     B
3     B
4     D
5     C
6     A
7     B
8     B
9     B
10    C