Transcript evolution_evidence_2011

EVIDENCE FOR
EVOLUTION
EXAMINATION GUIDELINES GRADE 12
The theory of evolution emerges from different lines
of evidence, namely:
• Fossil record (Grade 10)
• Modification by descent (Grade 11)
• Biogeography (Grade 11)
• Genetics (Grade 12)
How fossil records substantiate the
theory of evolution
Evolutionary theory proposes that
• all living organisms share a common ancestor at some time in the
past;
• new species form from existing ones.
Fossils
• Fossils are the remains of organisms that lived in the past.
• Scientists can trace some lines of descent in the fossil record.
An example of a line of descent in the fossil record in South
Africa
• fossil mammal-like reptiles (therapsids) who display reptilian
and mammalian features. Therapsid (mammal-like reptile)
fossils from the Karoo portray the evolutionary development of
mammals from reptiles.
Evolution of mammals from reptiles
• Much of the surface of SA is covered by sedimentary
rocks known as the Karoo Supergroup which forms an
almost unbroken record that runs from the end of the
Carboniferous period through the Permian and Triassic
periods to the beginning of the Jurassic.
• This means that the various pathways of development
followed by different animal groups such as the
mammal-like reptiles can be more closely traced.
• Mammal-like reptiles represents a transitional stage
between reptiles and mammals.
• In older rocks the fossil therapsids show more reptilian
than mammalian features but in younger rocks they
show more mammalian features.
Evolution of mammals (cont)
• Two important structural changes that occurred in the
evolution of mammals can be seen in the skulls of
therapsids from SA i.e. changes in the palate and jaw
hinge.
In the mammal configuration, the quadrate and articular bones are much smaller and form
part of the middle ear. Note that in mammals the lower jaw consists of only the dentary bone.
In mammals, the quadrate and articular bones have become the incus and malleus bones in
the middle ear. Mammals use two bones for hearing that all other amniotes (fisrt fully
terrestrial vertebrates) use for eating.
Ancestry of mammals
(http://en.wikipedia.org)
Amphibians
Tetrapods
Sauropsids (including dinosaurs)
Amniotes
Pelycosaurs
Synapsids
Therapsids
Mammals
Evolution of mammals (cont)
• Reptiles are generally classified into 3 large
groups i.e. the Anapsida, Synapsida and
Diapsida.
• The Synapsida line probably gave rise to
mammals.
• Early Synapsids (including therapsids)
included plant and flesh – eating forms
which increased in numbers during the
Permian period.
Therapsids
• Gorgonopsians – dominating flesh-eater
with saber teeth
• Dicynodonts – plant eating therapsids
lived from Permian to Cretaceous.
Gorgonopsian
www.maropeng.co.za
Evolution of mammals (cont)
• Smaller flesh-eating therapsids were the
therocephalians and cynodonts.
• Members of both these fossil groups evolved
features typical of mammals e.g. bony palate,
development of cutting and crushing
surfaces on the molar teeth.
• End - Permian mass extinction – only a few
types of therapsids survived.
• After the extinction most surviving therapsids
began to evolve increasingly mammalian traits.
Thrinaxodon
These two young cynodonts called Thrinaxodon (meaning “three cusped teeth”) probably died
when the underground burrow in which they were hibernating collapsed. As adults they were
insect eating mammal-like reptiles with long bodies and short legs, perfectly adapted for living
in tunnels and burrows, much like the mongoose today.
They belong to a group of mammal-like reptiles called cynodonts meaning “dog teeth”, that are
anatomically close to becoming true mammals. Because they were active insectivores, it is
quite likely that they were warm blooded (endothermic) and hair covered.
LYSTROSAURUS
(dicynodont)
• Survived the extinction and became the
dominant terrestrial herbivore of the
EarlyTriassic.
Lystrosaurus skeleton in the
field near Bethulie
www.iziko.org.za
Evolution of mammals (cont)
• During the Late Triassic the first true dinosaurs emerged
• By the late Triassic dinosaurs became dominant.
• Therapsids and their descendants(earliest mammals)
were unable to compete with the large dinosaurs and
remained small until the dinosaurs died out about 65
million years ago.
• During the early Jurrasic(200 million years ago) the
world’s first true mammals appeared.
• Mammals and dinosaurs evolved alongside each other
but they were two separate evolutionary lines.
• The first true mammals were small creatures e.g.
Megazostrodon.
Megazostrodon
http:// dinosaurs.about.com
Modification by descent
•
•
Scientists use modification of body plans as evidence that organisms
are related.
Darwin noticed that related species tend to have some similar
structures even if they have different functions – descent with
modification. In other words he proposed that all these different
mammals evolved from a common ancestor which had a forelimb with
this basic pentadactyl pattern.
Biogeography
The modern global distribution of plants and animals
provides evidence that continental drift occurred millions
of years ago. Organisms of the same species that live in
different habitats changes in different ways over time.
This causes new species to evolve.
Genetics
• Shared genetic features is powerful evidence
that life on Earth has a common evolutionary
origin.
• DNA makes up the genes that are passed on
from generation to generation.
• Close similarities in the DNA/protein/nucleic acid
sequences of different species may indicate that
the species are related – they have inherited
DNA from a common ancestor.
• Amino acids in the Cytochrome C protein of
humans and chimpanzees are identical – closely
Evolution of horse feet
Our understanding of the
evolution of horse feet, is derived
from a scattered sampling of
horse fossils within the multibranched horse an evolutionary
tree
Transitional stages whereby the four-toed
foot of Hyracotherium, otherwise known
as Eohippus, became the single-toed foot
of Equus.
Each branch tip on
the tree of horse
evolution indicates
a different genus,
though the feet of
only a few genera
are illustrated to
show the reduction
of toes through
time.
Fossils show that
the transitional
forms predicted
by evolution did
indeed exist.
The nostril placement in Aetiocetus is
intermediate between the ancestral form
Pakicetus and the modern gray whale
An excellent example of a
transitional form in the fossil
record!