Human Evolution

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Chapter 19

• Human Evolution

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On the Origin of The Species

• Sold out in 1 day, had only a small section on
  the origin of man.
• 12 years later The descent of Man, and Selection
  in Relation to Sex.
• The last chapter concerns the origin and
  evolution of our species

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On the Origin of The Species

• It is a much debated topic with different camps
  of researchers drawing different conclusions
• Scopes Monkey Trial 1925 in TN
• Prohibited teaching evolution
• 1995 AL state board of education ruled all
  textbooks must state Evolution is a theory not
  fact.

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On the Origin of The Species

• I have taught you that evolution is a process,
  and this process is occurring around us.
• Utilizing evolutionary theory we will examine
  physical, morphological, molecular, and
  behavioral evidence to test hypotheses about
  human origins

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Origin of Homo sapians

• Ever since Bishop Wilberforce famously ridiculed
  the possibility that man was descended from apes,
  and T. H. Huxley bravely chose primate ancestry
  rather than ignorance, the debate over our
  origins has claimed a special place in
  evolutionary theory.
• With the acceptance by most of us that we are
  indeed a product of natural selection,
  discussions surrounding the issue have cooled
  somewhat.
• But exactly how natural selection acted to
  produce the modern human form has remained hotly
  contested.

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Relationships of Humans and Extant Apes

• Order Primates
• Suborder Catarrhini Old World monkeys and apes
• Superfamily Hominoidea great and lesser apes
• Family Hominidae great apes and humans
• Homo sapiens
• Our closest relatives are Pongo pygmaeus, Gorilla
  gorilla, Pan troglodytes, and Pan paniscus
• Orangutan, gorilla, standard chimpanzee, bonobo

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Relationships of Humans and Extant Apes

• These taxonomic classifications are not
  universally accepted
• It is accepted that humans belong to the same
  clade as African great apes
• Synapomorphies
• Relatively large brain, elongated skulls,
  enlarged brow ridges, shortened canines, wrist
  bone fusion, reduced hair, absence of a tail,
  great flexibility of hips and ankles
• Which is our closest relative?
• Chimpanzees or gorilla?

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Relationships of Humans and Extant Apes

• Molecular phylogeny of Sarich and Wilson had
  polytomy of chimpanzees, gorilla, and man
• Timeline showed that split between apes and Old
  World monkeys was 30 Mya
• Split between African great apes and humans 5 Mya

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Relationships of Humans and African Apes

• Gorillas and chimpanzees share knucklewalking
  which humans lack
• Humans and two chimpanzees share features of
  skull, teeth, limbs, and delayed sexual maturity
• Some molecular analyses place humans and gorillas
  as sister taxa
• Conflicting evidence makes interpretation of
  correct branching order difficult

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Relationships of Humans and African Apes

• Difficult to identify which characters are
  ancestral or derived
• Dryopithecus is an extinct European ape 10 My old
• Shares many traits with gorillas that humans and
  chimps lack
• If these characters are ancestral and humans and
  chimps dont have them, it is most parsimonious
  for humans and chimps to be sisters

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Relationships of Humans and African Apes

• Using DNA sequences, researchers attempt to
  answer human/chimp/ gorilla question
• Three different genes gave different tree
  topologies
• mtDNA, Y chromosome, nuclear gene
• All trees support humans as sister to chimpanzees

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Relationships of Humans and African Apes

• Criticism of these reconstructions because they
  are gene trees and not species trees
• If ancestor was genetically variable for a
  particular locus, the gene tree may not
  accurately represent the species tree
• Ancestor may pass on a subset of its alleles for
  a gene to each descendent species
• That gene will not give an accurate estimate of
  phylogeny

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Gene Tree vs Species Tree
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Relationships of Humans and African Apes

• Ruvolo wanted to address this issue directly
• She exhaustively sampled human and ape
  individuals
• If the gene tree and species tree is incongruent,
  individuals of a species would not group together
• They always did group together in her study so it
  was not a problem for that gene

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Relationships of Humans and African Apes

• Another way to test this issue is use as many
  genes as possible and see which ones are
  congruent
• In 14 independent genes
• 11 showed humans and chimps together
• 2 showed gorillas and chimps
• 1 showed humans and gorillas
• Human/chimp relationship most likely

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Genome Projects

• Humans 23 prs of chromosomes while chimps and
  gorillas have 24

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Timing the Split
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Recent Ancestry of Humans

• Humans and chimps last shared a common ancestor
  5.4 Mya
• Most recent ancestor was a knucklewalker, ate
  mostly fruit, lived in many habitats, had complex
  social groups, and made tools
• What intermediate ancestors lived between
  chimp/human ancestor and modern Homo and Pan?

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Recent Ancestry of Humans

• Fossils provide some answers
• Fossil record is incomplete
• Scientists disagree on placement of fossils on
  hominid tree
• Sahelanthropus tchadensis

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Recent Ancestry of Humans

• Sahelanthropus older end of window for split.
  Looks like chip due to brain case but from front
  looks like Australopithecine.
• Australopithecines are one of the earliest humans
• Two main body forms
• Robust and gracile
• Bipedal like modern humans
• Known from limb structure and footprints

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Earliest known hominid

• At between 6 and 7 million years old, this is the
  earliest known record of the human family.
• Discovered in Chad in Central Africa, the new
  find, nicknamed 'Toumaï', comes from a crucial
  yet little-known interval when the human lineage
  was becoming distinct from that of chimpanzees.
  The skull's combination of primitive and advanced
  features suggests remarkable past diversity in
  the human family tree, and should ensure that the
  term 'missing link' is consigned to history.

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Earliest known hominids

• Figure 2 The known fossil record of hominids,
  including S. tchadensis, also showing ourselves
  (top left) and the chimpanzee (top right).
  Extinct species are indicated with the dates of
  the earliest and latest fossil evidence, but
  these are likely to increase and decrease,
  respectively, especially for the less well-known
  examples. Species are assigned to one of four
  categories, based on brain and cheek-tooth size,
  and inferred posture and locomotion (we are
  obligately bipedal facultative bipedalism is the
  ability to walk or run on two legs, or as a
  quadruped, according to circumstances). A fifth
  category is for 'insufficient evidence'. The
  species marked with an asterisk were all unknown
  a decade or so ago, an indication of the paucity
  of evidence, until recently, of hominid evolution
  between 1 and 4 million years ago. This
  comparatively rich record contrasts with the
  earlier part of the hominid fossil record. There
  are likely to be many 'undiscovered' species in
  the fossil record between 7 and 4 million years
  ago, and in reconstructing the early stages of
  human evolution in particular the incompleteness
  of data should always be acknowledged.

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• The timescale and phylogeny of hominids.
• Ape relationships are shown in grey for the
  chimpanzee (Pan troglodytes), bonobo (P.
  paniscus), gorilla and orangutan (Pongo
  pygmaeus).
• The approximate times of divergence are derived
  from molecular data.
• The phylogenetic relationships among hominins
  (shaded) are uncertain.
• The solid red bars denote the time span of the
  fossil species and/or the uncertainty of fossil
  ages.
• The identity of the last common ancestor of
  chimpanzees and humans (LCA) is not known.
• Note that the estimated age of Sahelanthropus
  tchadensis predates molecular estimates of the
  time of the chimpanzeehuman divergence. This
  species could pre- or postdate the LCA.
• Also note that Homo sapiens represent only the
  last 3 of the time span of hominin evolution.

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Recent Ancestry of Humans

• Australopithecines were short with small brain
  cases
• Modern human structure more similar to gracile
  australopithecines
• Robust australopithecines had enormous teeth and
  very strong jaw muscles for biting tree branches
  (like gorillas do today)

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Australopithecines
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Fossil Timeline
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Recent Ancestry of Humans

• First Homo was H. habilis
• Homo had larger brain cases and smaller teeth
  than Australopithecus
• Homo was also taller with less sexual dimorphism
  in size
• Homo rudolfensis may be the same species as H.
  habilis but have some skeletal differences
• H. ergaster is more recent ancestor from Africa

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Recent Ancestry of Humans

• H. erectus was first human to leave Africa
• H. heidelbergenesis is its descendent
• H. neanderthalensis may be our ancestor or may
  have been a sister species that died out
• Modern Homo sapiens appeared approximately
  100,000 ya

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Recent Ancestry of Humans

• Strait reconstructed a cladogram based on fossils
• Such a cladogram is difficult to read because
  ancestor-descendent relationships are not
  adequately depicted
• All species appear as terminal taxa

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Recent Ancestry of Humans

• Strait also constructed hypothesis of descendent
  relationships
• Time scale allows testing by fossil record
• New fossils should be found from ages depicted in
  correct hypothesis

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Recent Ancestry of Humans

• Determining trait polarities is difficult so that
  deciding on which hypothesis is more likely is
  difficult
• In the past as many as five human species
  coexisted
• We are the sole surviving species of a much
  larger human radiation

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Origin of Homo sapiens

• Paleontologists disagree about the taxonomic
  status of H. ergaster
• Same species as H. erectus or different?
• New H. erectus fossil recently found in Georgia
  from 1.75 Mya
• There is also disagreement about status of H.
  neanderthalensis
• Species or subspecies of H. sapiens?
• H. antecessor may be ancestor of H. sapiens and
  H. neanderthalensis

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Origin of Homo sapiens

• Hypotheses about transition from H.
  erectus/ergaster to H. sapiens
• African Replacement (Out of Africa)
• H. sapiens evolved in Africa, migrated to Europe
  and Asia, and replaced H. erectus and H.
  neanderthalensis without interbreeding
• Candelabra
• H. sapiens evolved separately in Europe, Africa,
  and Asia without gene flow among regions
• Two extreme models

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Origin of Homo sapiens

• Hypotheses about transition from H.
  erectus/ergaster to H. sapiens
• Other models between two extremes
• Hybridization and Assimilation
• H. sapiens evolved in Africa and migrated to
  Europe and Asia with some amount of hybridization
  
• Some European and Asian genes were assimilated
  and persist into modern humans

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Origin of Homo sapiens

• Hypotheses about transition from H.
  erectus/ergaster to H. sapiens
• Multiregional Evolution
• H. sapiens evolved concurrently in Europe,
  Africa, and Asia with sufficient gene flow to
  maintain their continuity as a single species
• Current gene pools are a mixture of all of these
  regional variants

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Origin of Homo sapiens

• Which hypothesis has most supporting evidence?
• Are human racial differences due to separate
  evolution 1.52 Mya (Candelabra) or are races
  recently evolved 100,000 200,000 ya (African
  Replacement)?
• Candelabra has been widely rejected
• A single species could not evolve independently
  in three places at the same time

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Origin of Homo sapiens

• Frayer argues that the only way African
  Replacement can be correct is if African H.
  sapiens had superior tools to allow him to defeat
  H. erectus
• No such archaeological tool record has been found
• Frayer also argues that Australian aborigines
  have similar features to Java Man (Homo erectus)
• Rejects African Replacement

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Origin of Homo sapiens

• Lieberman and Waddle predict cladograms based on
  African Replacement and Multiregional Evolution
  models
• Predicts the relationships of modern humans
• Can be tested with molecular evidence

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Origin of Homo sapiens

• Paabo recovered a mtDNA sequence from a H.
  neanderthalensis skeleton
• From Germany 30,000100,000 ya
• Also sequenced 663 modern humans, 7 chimpanzees,
  and 2 bonobos
• Found that modern humans from Europe, Africa,
  Asia, America, Australia, and Oceania are more
  closely related to each other than any is to H.
  neanderthalensis
• Estimated divergence of Neandertal to modern
  humans to be 317,000741,000 ya
• Supports African Replacement model

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Origin of Homo sapiens

• Researchers test models with modern sequences
  only
• Problem is that prediction in the same for
  African Replacement and Multiregional Evolution
• Just time scale is different
• Out of Africa model says differentiation of races
  began 200,000 ya
• Multiregional Evolution model says it began 1.8
  Mya
• Quantitative rather than qualitative differences

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Origin of Homo sapiens

• Hedges tested models with mtDNA
• Remember mtDNA is maternally transmitted
• 189 people produced phylogeny which lead back to
  a single woman from Africa
• Mitochondrial Eve
• Of course there was not a single woman that gave
  rise to all of these 189 people
• Many indirect female ancestors who lived at the
  same time

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Origin of Homo sapiens

• Hedges study does not only support African
  Replacement model
• All models predict ultimate ancestor to be from
  Africa
• Vigilant estimated common ancestor of all
  present-day mtDNA lived 166,000249,000 ya
• Ruvolo estimated it to be 129,000 536,000 my
• Horai estimated from entire mtDNA genome to be
  125,000161,000 ya

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Ingman, M., Kaessmann, H., Pääbo, S.
Gyllensten, U. Nature 408, 708-713 (2000). 

• Figure 1 The origin and dispersal of modern
  humans, Homo sapiens. The time of origin of
  modern humans is not well known but may have been
  about 200,000 (130,000465,000) years ago. New
  evidence from mitochondrial genomes5 bolsters the
  hypothesis that the place of origin was
  sub-Saharan Africa and that the dispersal from
  Africa occurred within the past 100,000 years.
  The earliest known fossil and archaeological
  evidence on each continent14, shown on the map,
  is consistent with this view.

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Origin of Homo sapiens

• All of these dates are similar and seem to
  support African Replacement model
• Because mtDNA is effectively all a single gene,
  we need independent estimates of age of split
• mtDNA may be estimating the split to be too young
• Populations connected by gene flow may diverge
  after their alleles do

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Origin of Homo sapiens

• Bowcock examined 30 nuclear microsatellite loci
  from 14 human populations
• Deepest split in their phylogeny is African from
  non-African populations
• Split occurred 75,000287,000 ya
• Support for African Replacement model

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Origin of Homo sapiens

• Tishkoff examined allelic variation on Chromosome
  12
• Short tandem repeat polymorphism
• Examined amount of allelic diversity in
  populations around the world
• Found African populations had highest allelic
  diversity
• Dated split to be 102,000450,000 ya
• Consistent with African Replacment

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Templeton 2003Out of Africa Again and Again!

• A new model of recent human evolution.
• Major expansions of human populations are
  indicated by red arrows. Genetic descent is
  indicated by vertical lines, and gene flow by
  diagonal lines.
• The timing of inferences lacking resolution at
  the 5 level and/or not validated by more than
  one locus are indicated by question marks.

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Evolution of Uniquely Human Traits

• Humans walk bipedally, have very large brains,
  use complex tools, and use language
• Chimpanzees and other animals make and use tools
• Only humans make and use complex tools
• First tools were stone flakes and choppers
• Oldowan Industry

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Evolution of Uniquely Human Traits

• Oldest Oldowan tools from Gona, Ethiopia 2.52.6
  My old
• Who made these tools?
• Oldest Homo fossil is 2.3 My old
• Homo habilis
• Maybe robust australopithecine
• They had three hand muscles that chimpanzees lack
• Like modern humans do
• Increased grasping ability

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Evolution of Uniquely Human Traits

• Susman hypothesized that new muscles evolved in
  response to selection for complex tool use
• A. afarensis did not have these specialized
  muscles
• We cannot know for certain whether Homo was first
  tool user or a robust australopithecine

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Evolution of Uniquely Human Traits

• Which hominid first used language?
• Because language is a behavior, it cannot
  fossilize
• Language is a complex adaptation involving neural
  circuitry in brain
• Appears to be an innate trait in humans
• Also involves adaptations of throat
• In apes and human babies larynx is high in the
  throat and seals with nasal cavity to prevent
  choking

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Evolution of Uniquely Human Traits

• Which hominid first used language?
• In mature humans, larynx is lower to allow
  greater diversity of sounds
• Increases choking risk
• Some archaeologists argue that language cannot be
  proven without arbitrary symbols
• Cave paintings in Europe 32,000 y old
• But humans built boats and travelled to Australia
  by 40,000 ya
• Must have had language to accomplish this

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Evolution of Uniquely Human Traits

• Which hominid first used language?
• Fossils of larynx offer other evidence
• Neandertal larynx appears to have been
  undescended
• A 60,000 y old Neandertal skeleton from Israel
  had an intact hyoid bone
• Anchors throat muscles in larynx
• Found hyoid bone to be nearly identical to modern
  humans and completely different from chimpanzees
• Indicates a descended larynx
• Probably could speak

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Evolution of Uniquely Human Traits

• Which hominid first used language?
• What about brain ability?
• Brain size increased steadily in Homo
• Enabled early man to speak
• Costly because large brains require much energy
  to maintain
• Endocasts reveal similar brain structure to
  modern humans
• Including speech centers of brain
• Language may be 2 My old

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Recent evidence and hypotheses for Human origins

• How closely related are we to neanderthals?
• New evidence for the persistence of an ancestor
  of H. erectus that evolved specifically to island
  life and lived until recently!

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• Origin of our species. The figure shows the
  geographical and temporal distribution of hominid
  populations, based on fossil finds, using
  different taxonomic schemes. The new finds from
  Herto4, 5 (H) represent early Homo sapiens. a,
  This reflects the view that both Neanderthals and
  modern humans derived from a widespread ancestral
  species called H. heidelbergensis2. b, However,
  evidence is growing that Neanderthal features
  have deep roots in Europe2, 8, so H.
  neanderthalensis might extend back over 400,000
  years. The roots of H. sapiens might be similarly
  deep in Africa, but this figure represents the
  alternative view that the ancestor was a separate
  African species called H. rhodesiensis. Different
  views of early human evolution are also shown.
  Some workers prefer to lump the earlier records
  together and recognize only one widespread
  species, H. erectus2 (shown in a). Others
  recognize several species, with H. ergaster and
  H. antecessor (or H. mauritanicus) in the West,
  and H. erectus only in the Far East8 (shown in
  b). Adapted with permission from refs 8, 11.

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• Species considered to be part of the tribe
  Hominini, or hominins, as opposed to chimpanzee
  ancestors, or panins.
• The horizontal axis spreads the species out
  according to the relative size of their chewing
  teeth and brain size taxa with large molar and
  premolar crowns are to the right, and those with
  smaller post-canine teeth are to the left.
  O'Connell and colleagues1 suggest that the grade
  shift between Australopithecus and Homo 8 was
  initiated by the systematic exploitation of
  subterranean food sources, rather than by
  hunting, as was traditionally thought. The
  hypothetical taxa (?) are a reminder that the
  number of taxa will probably increase.

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Chew on this

• The jaw muscles of apes, such as this mountain
  gorilla, and humans could reflect a profound
  evolutionary divergence.
• Based on a single mutation and gene duplication
  in myosin genes..

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2004 Homo floresiensis

• We are the only living species of the genus Homo.
  Given the startling results of a cave excavation
  in Southeast Asia, it seems that we coexisted
  with another species until much more recently
  than had been thought.
• Skeletal remains show that these hominins were
• only one meter tall
• had a brain one-third the size of that of modern
  humans
• and lived on an isolated island long after Homo
  sapiens had migrated through the South Pacific
  region.

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2004 Homo floresiensis
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Ingman, M., Kaessmann, H., Pääbo, S.
Gyllensten, U. Nature 408, 708-713 (2000). 

• Figure 1 The origin and dispersal of modern
  humans, Homo sapiens. The time of origin of
  modern humans is not well known but may have been
  about 200,000 (130,000465,000) years ago. New
  evidence from mitochondrial genomes5 bolsters the
  hypothesis that the place of origin was
  sub-Saharan Africa and that the dispersal from
  Africa occurred within the past 100,000 years.
  The earliest known fossil and archaeological
  evidence on each continent14, shown on the map,
  is consistent with this view.

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2004 Homo floresiensis

• Until around 12,000 years ago, when a volcanic
  eruption seems to have ended the party, Flores
  was a looking-glass garden of Komodo dragons and
  even larger lizards, giant tortoises and enormous
  rats. Alongside them were tiny, primitive
  elephants and, as we now know, tiny, primitive
  people
• Probably descended from full-sized Homo erectus
  that made landfall on Flores as much as 900,000
  years ago, the islanders dodged the dragons and
  hunted the elephants.
• This population became smaller with each
  generation, instances of the well-known
  phenomenon of endemic dwarfing in small, inbred
  island populations, until they were transformed
  into new species.
• Homo erectus became Homo floresiensis.

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2004 Homo floresiensis

• The species' diminutive stature indicates that
  humans are subject to the same evolutionary
  forces that made other mammals shrink to dwarf
  size when in genetic isolation and under
  ecological pressure, such as on an island with
  limited resources.
• The discovery is prompting increased scrutiny of
  sites on other Southeast Asian islands, both to
  look for more of the same species and to place it
  in context with Homo sapiens and Homo erectus,
  our closest relative.
• Homo erectus was found to have lived on the
  nearby island of Java as long as 1.6 million
  years ago the team suggests that the Flores
  hominins may be their descendants.
• These astonishing little people made tools,
  hunted tiny elephants and lived at the same time
  as modern humans who were colonizing the area.

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2004 Homo floresiensis

• Homo floresiensis in the context of the evolution
  and dispersal of the genus Homo.
• Solid lines show probable evolutionary
  relationships dashed lines, possible
  alternatives.

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Quotes from the authors

• MM My own feeling is that future archaeological
  discoveries in Southeast Asia will show that
  human dispersal and cultural change were much
  more complex than previously believed, and that
  Asia may have played a much more prominent role
  in these issues than adherents of the simplistic
  'Out of Africa' explanation for everything would
  have us believe
• PB Although it was a member of our genus, H.
  floresiensis is unlikely to have contributed to
  the gene pool of H. sapiens. So for me, its
  importance is not in the evolutionary story of
  modern humans, but in how the broad group from
  which modern humans evolved may have adapted and
  evolved to different ecosystems. Prior to this
  finding it would not have been thought that a
  hominin with the brain size, and possibly limited
  cognitive ability, of H. floresiensis could make
  the type of tools associated with the skeleton,
  or even get to Flores at all. I suppose that this
  is what challenges existing notions of what it is
  to be human the most.

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The Future of Science and Human Evolution

• With the impending completion of the project to
  sequence the chimpanzee genome, the tantalizing
  prospect of whole-genome comparisons between
  humans and our closest living relative is not too
  far away.
• It has been suggested that such a comparison
  could throw up around 40 million nucleotide
  differences between humans and chimpanzees.
  Identifying which of these differences encode the
  essential elements of being human is a daunting
  task.
• So. Hold on to your seats as we learn in coming
  years what really makes us . Human

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