The Tree of Life

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The Tree of Life
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Questions

• 6.5 Billion Humans
• Who were the first modern people in Africa?
• Why did humans leave Africa 50,000 years ago?
• What routes did they take?
• Did they interbreed with earlier members of the
  human family?
• When did they reach the Americas?

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DNA Evidence

• Human genome is 99.9 identical
• Random, neutral mutation is marker to
  generational relationships
• Finding same random marker is indication of
  shared ancestor
• Compare markers of different populations
• Markers shuffled in sexual reproduction
  (combining DNA from Mother and Father)

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Where is the DNA Evidence?

• Mitochondrial DNA (mtDNA) passed intact from
  mother to child
• Y chromosome passed from Father to Son

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Divergence Points

• 5-7 Million Years Ago (MYA) Divergence from the
  Chimpanzee Lineage
• 4.4 MYA Ardipithecus Ramidus first know
  Hominid
• It is not known if this was part of human descent
  chain or even actually bipedal.
• 3.9 4.2 MYA Australopithecus Anamensis
  Definitely bipedal

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From Anamensis to Sapiens

• Australopithecus Anamensis
• Australopithecus Afarensis
• Homo Habilus
• Homo Erectus 1.81 1.61 MYA
• Homo Sapiens
• Transition Species 400,000 years ago
• Homo Sapiens Sapiens 200,000 years ago
• Homo Sapiens Neanderthalensis 200,000 years ago

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Theorys of Human Evolution

• Multi-regional Model
• H. Sapiens Sapiens transitioned from H. Erectus
  across a wide portion of Africa and Eurasia.
• Fossil evidence shows that either H. Sapiens
  spread very fast after transition or transitioned
  in multiple areas. This is supported by H.
  Sapiens Sapiens and H. Sapiens Neanderthalensis.
• Genetic exchange across the area would have
  occurred to accomplish this.

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Theorys of Human Evolution

• Problems with Multiregional Model
• Would require persistent and extensive migration
  patterns to disperse the necessary genes.
• H. Erectus existed in China and Java for at least
  100,000 years after emergence of H. Sapiens
  Sapiens indicating a slow migration from the
  transition point.

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Theorys of Human Evolution

• Out of Africa Theory
• Noted from the fact that African populations have
  more differences between themselves than the rest
  of the human genotypes.
• It is thought that transition was actually from a
  small number of individuals.

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Theorys of Human Evolution

• Back to Africa Theory
• Early H. Sapieans transitional forms evolved in
  central to west Eurasia, then migrated to Africa
  and hybridized with H. Erectus and other
  hominids.
• The fact that Eurasians are more closely related
  to each other than to Africans supports this.
• This transition would still have been from an
  originally relatively small number of individuals.

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Searching for Eve

• Mitochondrial DNA Technique
• mtDNA exists outside the cell nucleus
• mtDNA is exclusively inherited along the maternal
  line

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Human Origins

• Allan Wilson at Cal-Berkeley in 1980s
• Used mtDNA to identify
• African women show 2x as much diversity
• Humans lived in Africa 2x as anywhere else
• Based on steady rate of random mutations

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mtDNA Study

• A.C. Wilson and R.L. Cann, 'The Recent African
  Genesis of Humans,' Scientific American, April
  1992, pp. 68-73
• Used 100 ethnically diverse individuals.
• The common mtDNA ancestral sequence was coalesced
  to a Eve about 150,000 years ago.
• Linked through unbroken chain of mothers
• Not necessarily only woman alive

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Searching for Adam

• The Y chromosome is exclusively passed thru the
  Paternal lineage
• The ZFY gene on the Chromosome was picked for a
  study.

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ZFY Study

• R. L. Dorit, H. Akashi, and W. Gilbert, 'Absence
  of Polymorphism at the ZFY Locus on the Human Y
  Chromosome,' Science, vol. 268, pp. 1183-1185,
  1995
• Compared 38 men of diverse geographical origins.
• Their common ZFY ancestral sequence was coalesced
  to a Adam about 270,000 years ago. Also, of
  African origin.

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Futher Growth in the Tree

• Ancestral DNA Markers in San people of Southern
  Africa and Biaka Pygmies of Central Africa
• San have click language
• Almost certain that small band (1000) humans left
  Africa for western Asia 50k to 70k years ago.
• All share unique markers

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Problems with Studies

• mtDNA is 16,000 nucleotides in size.
• Human DNA has 6,000,000,000 nucleotides
• 400,000 times as much DNA exists which could and
  would have been inherited from other individuals
  than Eve.
• mitochondria genes do not normally recombine.
• They seem to follow the mathematical rules more
  associated with bacterial genetics than human
  genetics.
• Flaws can be demonstrated in the search for
  Adam.
• The search did not include enough of the
  chromosome
• The search had too few starting candidates (as
  did the Eve study).

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Paths

• Nile River or Across Red Sea
• One population stalled in Middle East
• Another migrated toward India
• It was less like a journey and probably more
  like walking a little farther down the beach to
  get away from the crowd.
• SW Australia 45k years ago
• 40k years ago contact with Neanderatal. No
  genetic evidence of cross-breeding

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Populating the Americas

• 13k-20k years ago
• Native Americans share markers with Asians

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Genealogy

• Gene Genealogy
• A gradually coalesced towards a unique DNA
  sequence (the root of the tree)
• Individual Genealogy
• An increase by a factor of two each generation of
  ancestors which contributed to the genetic
  make-up of the individual.

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Human Diversity

• It has been estimated from analysis of the HLA
  immune genetic complex that human populations
  have never sunk down below a 50 or 100 thousand
  population Ayala Escalnte, et al1995, p205.
• The Genetic make-up of Humans would have come
  from many individuals over the course of many
  generations, not a single Eve or Adam.

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Technology

• DNA Barcoding
• mtDNA has fast mutation rate that allows
  differentiation of recent events
• A 648-bp region of the mitochondrial gene, known
  as cytochrome c oxidase I (COI), has been
  proposed as a potential 'barcode'.

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Fingerprinting

• Restrict DNA sample of interest
• Sort by size using Gel Electrophoresis
• Blot DNA

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VNTRs

• Variable number tandem repeats in intron region
• Blot probed with radioactively label VNTR in
  question. Have either mother or fathers
• D1, S1 biological offspring. D2 half. S2 unrelated

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Tree of Life
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Conclusion

• There is not any single individual at this time
  that could be identified as the Mother or Father
  of the Human race.
• We have many genetic Mothers and Fathers however
  the their genes have all been jumbled up through
  generations of sexual reproduction.
• It is possible future techniques may allow the
  discovery of a genetic Adam but it is very
  likely that this Adam might not be what we
  consider Human.
• It is doubtful that any individual Eve could
  ever be identified because of the gene scrambling
  seen thru sexual reproduction.