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9.8 Option- The human story: 2. Biological evidence for human evolution
Extract from Biology Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 18 Jun 08]
Prior learning: Preliminary course module
8.5.4 and 5 and module 8.4.7
HSC module 9.3.4
outline
the conditions under which fossils may form
- For fossilisation to occur a dead organism has to be
preserved and protected in some way. Examples of this type
of protection would be if the organism is quickly covered
by sediment, becomes trapped in amber or preserved in ice.
The richest fossil beds are found in caves, in ancient
lakes and the edge of the sea, in rivers and in
volcanically active areas.
- Caves are found in limestone, sandstone and as lava
tubes in volcanic rocks. Many organisms, including early
humans and the predators of early humans, would go into a
cave to eat their kill in relative safety. The bones are
discarded and become fossilised by coatings of sediments.
The caves at Sterkfontein are an example of a rich fossil
bed containing the remains of early humans.
- Lakes and seas. Fine sediments produce fossils showing
greater detail. These sediments need still conditions to be
deposited. These still conditions are found in lakes and
large seas. An example of this type of deposit containing
the skeletons of early humans is found at Koobi Fora.
- Volcanic deposits. Most volcanic eruptions are violent
events but in some instances volcanic ash is thrown up into
the air by a volcano. This ash may cover and preserve
fossils. A famous example of this is in Laetoli where
fossilised hominid footprints have been discovered.
Fossilisation 
Science and Nature,
BBC.
Becoming a Fossil (Video Resource) See
Leaving a Trail of Evidence, Unit 3 Web Resources:
What Is the Evidence for Evolution? PBS, Virginia,
USA.
relate the age of the Earth to the way in which geological time is described
- The 4.7 billion years of the Earth’s history is divided into periods depending on the succession of life. The geological time scale does not have regular subdivisions. The different periods of time lasted different lengths of time. There has been a succession of life from simple unicellular organisms through to the complex life forms that are on the Earth at the present time. There are five main geological eras: Archaean, Proterozoic, Palaeozoic, Mesozoic and Cainozoic (Cenozoic).
distinguish between and describe some relative and absolute techniques used for dating fossils
- Once a fossil is discovered it is important to be able
to place the fossil in the history of life. This is called
dating the fossil. There are two different methods of
dating fossils, relative dating and absolute dating.
- Relative dating ages fossils by comparing the fossil
with other known fossils. The date that is reached is
relative to the date of another fossil. Examples of this
type of dating are stratigraphy, palaeomagnetism and faunal
dating.
- Stratigraphy is the study of rock layers or strata.
Fossils found in lower rock levels are thought to be older
than fossils found in higher layers. This is called the
principle of stratigraphic superposition. Some fossils that
are particularly useful for stratigraphic dating are called
index fossils.
- Palaeomagnetism is based on the periodic reversals of
the magnetic field that Earth experiences. As rocks
solidify from lava they record the magnetic field of the
Earth at the time of solidification. By comparing the
patterns of magnetic fields in rocks it is possible to find
deposits that were formed at the same time.
- Faunal dating or biostratigraphy uses common fossils to date other fossils. For example in Africa the fossil pig record can be used to date the fossils of early humans.
Dating techniques Emuseum@Minnesota
State University, USA.
Relative dating Emuseum@Minnesota State
University, USA.
- Absolute dating seeks to place an actual age on a
fossil. Examples of absolute dating techniques include
dendrochronology, radiocarbon dating, thermoluminescence
and potassium-argon dating.
- Dendrochronology is the study of tree rings. Deciduous trees loose their leaves during the winter. At this time growth stops, so for each winter that a tree experiences a ring in the cross section of the stump can be seen. When there are periods of drought or good growing seasons then the size of the ring changes. Some trees such as the Giant Sequoia live for thousands of years so by comparing fossilised tree stumps it is possible to use dendrochronology to absolute age as far back as 6500 years ago.
Dendrochronology Emuseum@Minnesota State
University, USA
- Radiocarbon dating using the amount of radioactive carbon that is found in fossils such as charcoal or bones. Most naturally occur carbon is carbon -12. Carbon -14 is produced in the upper atmosphere by cosmic radiation bombarding nitrogen -14. The carbon joins with oxygen to form carbon dioxide. Plants take in this carbon dioxide during photosynthesis and it becomes part of the body structure. It moves through the food chain when an animal eats the plant. The half-life of carbon-14 is 5730 years. This means that after 5730 years half of the original material will be available. When an organism dies the carbon-14 that has been taken in during the life times decay. The original amount of carbon-14 is assumed and then compared with the remaining C-14 in the fossil. The older the fossil the less C-14 that remains.
Virtual dating exercise California State
University, Los Angeles, USA.
- Thermoluminescence uses the property of minerals such as quartz, feldspar and calcite to emit light when they are heated. The amount of light shows when the object was last heated. A piece of pottery when fired has its thermoluminescence clock set to zero. Over time natural radiation causes the amount of thermoluminescence to increase. This technique is useful for inorganic items up to 200,000 years of age. It is used to date campfire stones, pottery and figurines.
Thermoluminesence Emuseum@Minnesota
State University, USA.
- Potassium argon is another radiometric measuring system that is similar to carbon dating. Naturally occurring radioactive potassium decays to argon and calcium over time. The higher the potassium to argon ratio the older the sample. It is used to age volcanic flows that lie between fossil beds.
describe relative dating techniques using fossil sequence in strata
- If a fossil is lower in the rock layers then it is said to be older than more recent fossils found in higher strata. For example, if a fossil such as a trilobite is known to be 375 million years old and other fossils are found in the same layer then the second fossils can be dated the same as the trilobite. If the same trilobite is found in another location then the fossils found with it can be dated as the same age. If a fossil is found lower down in the sediments than it is older the fossil trilobite.
Model stratigraphic methods University
of California Berkeley, USA.
discuss the difficulty of interpreting the past from the fossil record alone, including:
- conflicting dates based on different technologies
- the paucity of the fossil record
- different interpretations of the same evidence
Conflicting dates based on different technologies
- New evidence and techniques become available at different times. The different technologies will each have a different margin of accuracy. Some methods may give results that are accurate to thousands of year, others say, to tens of thousands of years. Therefore each method can lead to a different interpretation of the fossil age.
The paucity of the fossil record
- Fossils are a rare occurrence. There are gaps in the fossil record and the fossil record is heavily weighted towards the organisms that have hard parts such as the shells of molluscs. Soft-bodied animals are not fossilised as often. Fossils tend to be incomplete (an entire skeleton is unusual to find due to the problems of time and the fossilisation process) and this can make it difficult to interpret some features of a fossil or a species. The hardest parts of primate bodies are the teeth and jaws. These are found more often and some species are only known from a single specimen of a tooth. Additionally, the destruction of fossils can occur through earth processes such as the weathering of sediments. Many important hominid fossils have been found lying on the surface. If someone who knew what they were had not discovered them they may have been lost forever.
Different interpretations of the same evidence
- Different scientists may specialise in different areas and therefore interpret the evidence differently. Different people/scientists have different pre-conceived ideas that can affect their interpretations. There are a lot of anthropologists working in the area of humans evolution and so it is not surprising that there are different interpretations of the same evidence. Also, a fossil may be unusual in some way, e.g. through disease, age or the fossilisation process. If the only evidence you have for a species is a single tooth then the rest of the anatomy and physiology of the fossil is open to the interpretation of each of the people working on it.
process and analyse information from secondary sources to model karyotype analysis
- Use the following Internet sites as a source of
secondary information. Process the
information by looking for trends and patterns in the
information.
- Analyse the information by using models to explain the phenomena.
Primate Cytogenetics Network Washington,
USA.
Karyotyping activity , University of
Arizona, USA.
process information from secondary sources to model DNA-DNA hybridisation in order to demonstrate its use in determining relationships between organisms
- Process the information by assessing the accuracy of any measurements. The following Internet sites contain secondary information to model DNA-DNA hybridisation.
DNA-DNA hybridisation Dr John W. Kimball
Andover, MA, USA.
Living hominids Dr John W. Kimball
Andover, MA, USA .
See also the section on DNA-DNA hybridisation below. Use
the table given to draw a graph to model the relationships
between the primates.
compare living primates to hypothesise about relationships between groups of primates using evidence from:
- karyotype analysis
- DNA–DNA hybridisation
- comparison of haemoglobins
- DNA sequencing
- mitochondrial DNA as a molecular clock
Karyotype analysis
- Karyotype analysis looks at the number, shape, size and banding patterns of the chromosomes in an organism. During mitosis the chromosomes thicken and become visible. At this point a picture can be taken of the chromosomes paired up. A karyotype is defined as the appearance of chromosomes, their size, shape and number. Each organism has a different karyotype. The table below illustrates the different chromosome numbers of a range of organisms.
| Organism | Chromosome number |
|---|---|
|
human
|
46
|
|
gorilla
|
48
|
|
chimpanzee
|
48
|
|
beans
|
22
|
|
goldfish
|
94
|
DNA–DNA hybridisation
- DNA is found in almost every living cell. It consists
of four bases arranged in a spiral helix. The four bases
combine to form the genetic code. From generation to
generation there should only be small changes in the
genetic make-up of a species. The difference between two
species should indicate how long ago they shared a common
ancestor. The closer two species are the more similar will
be their DNA.
- DNA hybridisation is used to work out how closely two species are related. DNA is extracted from cells. It is heated and the double strands of DNA separate. Restriction enzymes are used to snip the strands into smaller pieces. Then the DNA of two different species is mixed together. As the DNA cools the strands collide into each other and realign to form double stranded DNA. If the DNA is similar then the code will be similar and there will be strong bonds formed. The mixture is then heated and the temperature required to separate the strands again will indicate how closely related the two species are. The table below shows the similarity between different primates based on this technique.
| Primate | % similarity of DNA with human DNA |
|---|---|
|
human
|
100
|
|
chimpanzee
|
97.6
|
|
gibbon
|
94.7
|
|
rhesus monkey
|
91.1
|
|
capuchin
|
84.2
|
Comparison of haemoglobins
- Haemoglobins are proteins found in the blood. They are important for the transport of gases in the blood. If two species have similar haemoglobin, their DNA must be similar and they have shared a common ancestor recently. To test for haemoglobin similarity blood serum from humans is injected into rabbits and the rabbits respond by producing antibodies to the human blood proteins. The antibodies are extracted from the rabbits. These antibodies are added to the sample of blood taken from other species. A precipitate forms. The greater the reaction to the human antibodies the more similar to humans is the species. So human blood would have a 100% reaction while a dog may have no reaction. Some results are summarised in the table below.
| Species | Reaction to human antibody (%) |
|---|---|
|
human
|
100
|
|
chimpanzee
|
97
|
|
baboon
|
50
|
|
dog
|
0
|
DNA sequencing
- The order of bases along a DNA strand is called its sequence. If the sequences are similar then the two organisms have shared a common ancestor in recent times. In DNA sequencing genetic and biochemical tests are used to sequence the bases in a portion of DNA. These are then compared with different species.
Mitochondrial DNA as a molecular clock
- Most of the DNA in a cell is found in the nucleus but a small amount of DNA is found in the mitochondria. The mitochondria were once free-living organisms that have been incorporated into eucaryotic cells. Mitochondrial DNA (mtDNA) is much simpler than nuclear DNA and consists of a single circle of DNA. During fertilisation the female egg cell provides the mitochondria for the new organism. This means that mitochondrial DNA is always through the female line. By comparing the mitochondrial DNA from living primates it is possible to calculate a molecular clock based on a constant rate of mutation. So depending on how different the mtDNA is an indication of how many years has passed since two organisms shared a common ancestor.
identify
data sources, gather, process and
present
information from secondary sources about the maternal
inheritance of mitochondrial DNA and its importance in
tracing human evolution
Background
The evolutionary sequence for the primates is encoded in their genes. The fossil record gives some evidence but it is very patchy. By looking at the genetic similarities between species a different classification occurs than the traditional classification based on anatomy.
- Identify your data sources to
determine appropriate ways in which aspect may be
researched.
- Gather your information from a range
of resources including popular scientific journals, CD ROMs
and the Internet.
- Process the accuracy and the relative
importance of the information gathered.
- Present the information by using a text type such as a report.
The following Internet sites provide examples of new information and technology changing understandings. They are a useful starting point.
Mitochondrial DNA J Kimball, Biology
- The Living Science, Kenneth R. Miller, Professor of
Biology, Brown University ,Providence, Rhode Island, USA.
Molecular analysis of Neanderthal DNA from
the northern Caucasus Gor V. Ovchinnikov et al,
Nature 2000. University of California, Los Angeles,
USA.
|