Transcript Out-of-Africa Theory: The Origin Of Modern Humans

Background Information
 First species of Homo, Homo habilis, evolved in Africa
around 2 million years ago.
 Later, a descendant of Homo habilis, Homo erectus
evolved (along with other hominids), and spread out
of Africa.
 Homo erectus (and its lineage) gave rise to Homo
sapiens around 100,000 to 200,000 years ago.
Two Main Theories
 Out of Africa Theory (OOA) – suggests that H erectus
migrated from Africa about 1mya and formed additional
populations in Europe and Asia. Only the African
population evolved into H sapiens and again migrated into
Europe and Asia replacing H erectus as they went.
 Multi-regional Evolution Theory –suggests that H
erectus migrated from Africa about 1mya and formed
additional populations in Europe and Asia. Parallel
evolution occurred and they all became H sapiens with
regional differences.
Why migrate?
 Scientists think the reason for migration was because of numerous ice ages.
These caused Africa to dry and minimised food supply.
 The options were to stay and survive, or migrate and look for more opportunity.
Migrating also reduces competition.
 These ice ages assisted migration as sea levels were lower so land bridges were
created. Straights between islands would have been smaller (small boats could
be made to cross these?).
 Following the coast meant food supply was high.
 Migration would have been fast
 NCEA 2007
Explain the impact a change in climate from wet tropical to cooler drier
conditions would have on migration out of Africa by H. erectus.
Discuss the impact a change in climate from wet tropical to cooler drier
conditions would have on migration out of Africa by H. erectus.
Modern humans reached Australia as long as 60 000 years ago. This was at much the
same time as they were moving into Europe, despite the fact that Europe is much closer
to Africa. Explain why modern humans were relatively slow to colonise Europe.
1. The route into Asia may have been easier to travel and to access food
2. Europe, being in an ice age would have been heavily glaciated
3. Neadertals were already present in Europe, increasing competition
4. Humans adaptations were not suited to cold climates.
Genetic Tools to Find the Answer
 Fossil records
 DNA sequencing
 Mitochondrial DNA analysis (mtDNA)
 Maternally inherited, therefore telling the story from the female
side of human history
 Y Chromosome analysis
 Inherited down the paternal line, complementing the mtDNA
 Microsatellite DNA analysis
 Segments of tandemly repeated DNA with a short repeat length,
usually 2-5 nucleotides
NCEA 2011
 Investigations into the patterns of genetic variation in modern human
populations have been used to support the view that the origin of Homo
sapiens is the result of a recent dispersal event known as the “Out of Africa”
model.
 Supporting evidence for this model has come from studies of mitochondrial
DNA (mtDNA), which is inherited through the maternal line.
 There are fourteen ancestral populations in Africa, but only one outside Africa.
 Discuss the pattern of dispersal this evidence supports and why the evidence
has been used.
 In your answer:
• describe the “Out of Africa” model
• explain why mtDNA was used to provide evidence for this model
• explain, using the “Out of Africa” model, why more diversity is found in the
African Homo sapiens populations compared to those of Asia and Europe.
What is mitochondrial DNA?
 Mitochondria are structures within cells that convert the energy from food into
a form that cells can use. Although most DNA is packaged in chromosomes
within the nucleus, mitochondria also have a small amount of their own DNA.
This genetic material is known as mitochondrial DNA or mtDNA. In humans,
mitochondrial DNA spans about 16,500 DNA building base pairs, representing
a small fraction of the total DNA in cells.
 Mitochondrial DNA contains 37 genes, all of which are essential for normal
mitochondrial function. Thirteen of these genes provide instructions for
making enzymes involved in oxidative phosphorylation. Oxidative
phosphorylation is a process that uses oxygen and simple sugars to create
adenosine triphosphate (ATP), the cell's main energy source. The remaining
genes provide instructions for making molecules called transfer RNA (tRNA)
and ribosomal RNA (rRNA), which are chemical cousins of DNA. These types
of RNA help assemble protein building blocks (amino acids) into functioning
proteins
Mitochondrial DNA
(mtDNA) is only inherited
from ones mother. After
10 generations a person
only has mtDNA from 1
person out of 210!
The only source of
variation in mtDNA is by
mutation.
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mtDNA mutates 10 times faster
than nuclear DNA so is useful
for studying short-term
evolutionary change.
By comparing mtDNA they
found as expected, closely
related people had similar
mtDNA.
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Therefore an evolutionary tree
could be constructed which
showed two main branches;
the African and everyone else.
This suggests two main groups;
those who stayed in Africa and
those who left.
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By knowing how quickly mtDNA
mutates it is possible to date
the split at about 200 000ya (or
10 000 generations ago).
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mtDNA analysis suggests that
we evolved in Africa.
Modern humans are very similar
genetically, more so than
chimpanzees. This opposes
parallel evolution which should
produce greater genetic
variation.
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Genetic and fossil evidence seems
to support the replacement
hypothesis.
The physical differences between
fossil remains of hominin species
in Asia vs Africa (and Europe)
would have only occurred if they
evolved independantly.
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Out of Africa summary
Evidence that supports
Evidence that does not
support
Oldest H.sapiens fossils found in
Africa.
Transitional fossils found in China
Modern humans appeared Africa
before Neanderthals disappeared.
mtDNA can be interpreted in
different ways
mtDNA can be used to trace
lineage back to Africa
More variability in DNA/genes in
Africans
Multiregional theory
Evidence that supports
Evidence that does not
support
Asian fossils show clear transition
from older hominids to modern
H.sapiens
Would require large amounts of
gene flow
The oldest H.sapiens fossils are
found in Australia dated at 60000
years
mtDNA suggests H.sapiens evolved
from Africa.
China fossils show no evidence of
African features ever replaced the
ancient Chinese.
Polymorphisms
 Polymorphism - Existence of a gene in several allelic
forms.
 Polymorphic regions provide a very unique set of
genetic markers for studying human origin and
migratory patterns.
 Used to construct a global evolutionary tree of modern
man
Mitochondrial DNA
 Out-of-Africa hypothesis was first sketched out in
1987, based on mitochondrial DNA analysis
 Suggested that modern man first appeared on the
scene in eastern Africa about 150,000 years ago, and
left between 35,000 and 89,000 years ago, eventually
conquering the globe.
Y Chromosomal DNA Study
 Researchers looked at DNA samples from 12,000 male
Y chromosomes in Asia.
 Looking for 3 specific mutations on the Y chromosome
known to have originated in Africa.
 Researches found that every one of the 12,000 samples
carried one of the three mutations or polymorphism
Conclusion to the Y Chromosome Study
 Little or no interbreeding of Homo erectus and
Homo sapiens.
 Individuals are descendants from Africa
 Likely that the early African man emigrated to
North Africa and made the leap to Asia and then to
the rest of the world.
 Indicates that modern humans of African origin
completely replaced earlier populations in East
Asia.
Microsatellite DNA Analysis
 Researchers tried to find the estimated time of the
deepest split of the human population.
 Applied a genetic distance measurement to 30
microsatelite regions to construct a phylogenetic tree
for 14 world-wide human populations
What did they find?
 In the tree obtained, the deepest root separated
Africans from non-Africans.
 Their calculations suggest the split happened an en
estimated 115,000 to 156,000 years ago.
mtDNA Analysis
 Study on the complete mitochondrial genome.
 16,500 base pairs in each sequence
 53 people diverse from different geographical, racial,
and linguistic backgrounds.
Results
 A tree rooted in Africa
 Tree suggests that some Africans are closer to
Europeans and Asians than to other Africans.
Conclusion
 DNA sequencing evidence shows that modern humans
originated in Africa and migrated north out of African,
then eventually to the rest of the world.
 Oldest fossils of modern humans are found in Africa
dating around 160,000 years old.