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The Evolutionary Origins of Human Population Variation

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Title: modern humans appear Author: alan mann Last modified by: Princeton Affiliate Created Date: 10/30/1998 10:02:12 PM Document presentation format – PowerPoint PPT presentation

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Title: The Evolutionary Origins of Human Population Variation


1
The Evolutionary Origins of Human Population
Variation
  • As we have seen, there is a geographically based
    pattern to human biological variation.
  • Later on, we will explore more of the varying
    biological features that have been studied.
  • Today, our task is to trace the evolutionary
    origins of modern human population variation
  • How did it arise?
  • What are the evolutionary and other processes
    that have brought it about?

2
Evidence for Human Evolution
  • Comparative Anatomy
  • The examination of the gross anatomical
    features of humans and apes reveals their close
    similarity and evolutionary relatedness.
  • Comparative Genetics
  • Comparisons of the genetic materials of the
    African apes and humans documents that these
    creatures are more closely related to each other
    than to any other living animal.
  • Fossil Evidence

3
Fossil Evidence
  • The direct evidence of our extinct ancestors.
  • Other evidence documents relationships only
    fossil evidence provides data on the biology and
    adaptation of our actual ancestors.
  • Because of the nature of the fossilization
    process, this evidence is often difficult to
    fully understand and interpret.

4
A mud site
5
Ape man
6
Whats in a name?
  • Although there is much debate about what terms to
    use when referring to our extinct ancestors, for
    our purposes, we will call them hominids, and
    speak of human evolution as the evolution of the
    hominids (Using this term places ourselves and
    our extinct bipedal ancestors in a separate
    family Hominidae) .
  • Thus, we can say that hominid evolution seems to
    have begun in Africa, and it is only much later
    in our evolutionary history that we find signs of
    our ancestors outside the African continent.
  • We are, more specifically, members in the genus
    Homo, of which there are a number of now extinct
    species, and we are the only living member Homo
    sapiens.

7
Human Origins
humans chimpanzees gorillas
Hominid evolution
5 - 8 myr
10 myr.
common ancestry of humans and African apes
An evolutionary diagram of human and African ape
relationships, based on various genetic studies
8
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9
The Human Fossil Record
  • With recently found fossil discoveries from
    western Kenya and Chad, the human evolutionary
    line may now begin as early as about 6 million
    years ago.
  • The earliest evidence is of animals that
    possessed some biological traits like those of
    the modern humans, some that resembled the apes,
    and many unique traits (these are not humans in
    fur suits, or bipedal apes, but a wholly extinct
    set of species with their own biology).
  • Later-in-time members of our line look more and
    more like living people.
  • A number of genera and species have been
    proposed.
  • For us, this afternoon, we will be interested in
    the last 200,000 years of human evolution and the
    emergence of modern people.

10
Human Evolution Beginnings
  • Our earliest ancestors, mainly of the group known
    as the australopithecines, are characterized by a
    combination of ape-like and human traits. They
    were bipeds, with small, non-projecting canines,
    but they had small, ape-sized brains in a skull
    that was very ape-like. Uniquely, they had
    massive back chewing teeth and huge jaw muscles.
    There were a number of species of this group. We
    know virtually nothing about their adaptation,
    diet, social organization or general behavior.
  • By about 2 million years ago, members of our own
    genus, Homo, appear on the scene, probably
    evolving from one of the later australopithecines.
    They had bigger brains and smaller back teeth,
    but were still quite different from living
    humans. The first stone tools appear in the
    fossil record about two million years ago, as
    well as indications from scratch marks on animal
    bones that meat eating was occurring, but from
    hunting or scavenging is not known.

11
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12
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13
Human EvolutionFirst Out of Africa
  • Early members of the modern human genus Homo are
    found in Africa between 2 - 1.8 million years
    ago. There is evidence of these early humans on
    the island of Java sometime after 1.8 million
    years ago Thus, about this time, there is a
    spread out of Africa, and into Eurasia. No one
    knows why this expansion of range occurred.
  • From this point in human evolution on, the Old
    World is more or less populated by human
    ancestors. Keep in mind that the glacials, or
    ice ages, periodically descended in the northern
    hemisphere, making large areas uninhabitable.
  • After our ancestors spread out of Africa, there
    begins a time of human evolution which will
    culminate with the appearance of modern humans in
    various parts of the world. How they evolved and
    the precise evolutionary pathways, are much in
    dispute.

14
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15
1st fire
16
The First Europeans
  • Some archaeologists claim that the very earliest
    sites in Europe are in southern Spain and central
    France and are dated to more than one million
    years ago.
  • The earliest well documented site in Europe,
    however, is in central Spain, near Burgos, where
    human fossils are dated to about 780,000 years.
    There is no agreement as to the species of these
    fossils.
  • This is considerably later than Homo expansion
    into Asia, and was probably dictated by glacial
    activity and the limited routes into Europe.
  • Other sites in Europe, in Germany, France, Italy
    and Greece are all probably 500,000 years or
    later.

17
Europe first
18
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19
Theories of Modern Human Origins
  • Two major theories attempt to explain the latter
    phases of human evolution and the development of
    modern human population variation (human races)
  • They view human origins very differently, with
    the differences based primarily on how isolated
    hominid populations were after spreading out
    from Africa around 1.8myr.
  • Both theories have long histories, and in one
    guise or another, have been around since the
    recognition of the essential non-modern human
    qualities of the neandertals in the middle of the
    19th century

20
Competing Models of Human Origins
  • The two competing models are known as
  • 1. The Multi Regional Evolutionary Model.
  • 2. The Single Origins Model (usually called Out
    of Africa).

21
theories
22
Multi Regional Evolution I
  • With expansion of early Homo into Eurasia,
    hominid populations moved into new environments
    and began to evolve biological features for life
    in those places.
  • In this model, hominid populations were
    continuously distributed over the continents, and
    were in more or less constant contact with other
    populations, thus sharing genes.
  • This gene flow insured that the hominids remained
    one evolving species.
  • By about 700,-400,00 years ago, archaic members
    of H. sapiens had appeared.

23
Multi Regional Evolution II
  • These archaic H. sapiens populations in the
    different areas eventually evolve into living
    human regional populations (races).
  • Thus, human races have a long antiquity in their
    local environments, having evolved from earlier
    archaic sapiens, and before that, from the local
    early Homo populations.
  • Multi regional evolution stresses the ebb and
    flow of gene flow as a crucial factor in human
    evolution and in modern human origins.

24
Single Origins Theory I
  • Begins in the same fashion as multi regional
    evolution with the spread of early Homo out of
    Africa into Eurasia. Hominid populations move
    into new environments and begin to evolve
    biological features for life in those places.
  • In this theory, hominids lived in small, isolated
    populations and, lacking genetic contact, evolved
    into a number of new species.
  • In Europe, this new species will eventually
    evolve into the neandertals, who become extinct
    toward the end of human evolution.

25
Single Origins Theory II
  • While in Europe these now isolated hominids
    evolve into a new species, the Neandertals, In
    Africa and Asia, other species of Homo were also
    evolving. Like the Neandertals in Europe, they
    also possess low sloping brain cases, and large
    projecting faces lacking a chin. They had large
    brains, often within the range of living humans.

26
Single Origins Theory III
  • Between about 200,-100,000 years ago, modern
    humans, Homo sapiens, evolved from an earlier
    Homo ancestor.
  • This evolutionary origin apparently took place in
    one locale, most probably somewhere in sub-
    Saharan Africa.
  • Soon after this origin, these modern humans begin
    to expand out of Africa, marking a second
    expansion out of Africa.
  • These modern humans move into all parts of the
    Old World, replacing earlier species of Homo,
    like the Neandertals, in those areas.

27
Single Origins Theory IV
  • Thus, in this theory, modern humans, Homo
    sapiens, evolve relatively recently in one locale
    and spread out from there.
  • Modern human races all have a relatively recent
    origin in Africa.
  • Earlier humans in other parts of the Old World
    were separate species from modern humans. They
    were not part of the ancestry of modern humans
    but an extinct side branch, replaced by these
    newcomers who moved out of Africa.

28
Modern Human Origins
  • Thus, two different theories
  • 1) Multi Regional Evolution
  • 2) Single Origins Out of Africa
  • Because they are amongst the most numerous of
    fossils, much of the emphasis of both theories
    centers on the Neandertals.

29
Neandertal Discoveries
  • Earliest of the fossil hominids to be found and
    identified.
  • First recognized in 1856 from a quarry find in
    the Neander Valley of Germany.
  • Because it was the first discovery of a fossil
    hominid, and because this find came just 3 years
    before the publication of The Origin of Species,
    it quickly became part of the controversy
    surrounding Darwinian evolution.

30
Ape man
31
The Neandertals
  • Fossil hominids who occupied Europe and the
    Middle East from about 150,000-30,000 years ago,
    when they disappear from the scene.
  • Some view them as a subspecies of Homo sapiens,
    while others place them in their own species H.
    neanderthalensis.
  • Morphologically, they possessed large brain
    cases, with low and long skulls and large
    projecting faces lacking a chin.
  • They were capable of very complex behavior,
    including the deliberate burial of their dead.

32
The Neandertals
  • Said one English anatomist of the first find in
    the Neander Valley
  • It may have been one of those wild men,
    half-crazed, half idiotic, cruel and strong, who
    are always more or less to be found living on the
    outskirts of barbarous tribes, and who now and
    then appear in civilized communities to be
    consigned perhaps to the penitentiary or the
    gallows, when their murderous propensities
    manifest themselves.

33
Pity neandertal
34
Full Neandertal
35
Scene I
36
Neandertal Discoveries II
  • After the initial discovery, many other fossils
    similar to the original Neander Valley find were
    excavated in many parts of Western and Eastern
    Europe, in the Middle East and as far East as
    Iraqi Kurdistan and Uzbekistan .
  • Early on, they became stereotypes of the brutish,
    primitive, bestial ancestors of humans....and the
    term Congresspeople often hurl at each other
    when they wish to convey their opponents lack of
    humanity (did the Neandertals, one wonders, call
    their adversaries Congresspeople?).

37
Europe II
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39
La Chappelle Skull
The Skull of a Neandertal from France
Large brain case with a brain often larger than
those of living humans
No chin
40
Modern Human Origins
  • So, what are the relationships between the
    Neandertals (and their contemporaries in other
    parts of the Old World) and living humans?
  • Multi Regional evolution and Single Origins
    theories rely on different sorts of evidence.
  • 1) Multi regional evolution relies primarily on
    fossil evidence from Asia.
  • 2) Single Origins emphasizes fossil evidence
    from Africa and comparative genetic evidence from
    living human populations.

41
Skulls I
42
Single Origins Theory Fossil Evidence
  • This theory would be acceptable if fossils were
    found that were modern human in form, but dated
    earlier in time than Neandertals. Clearly, it
    would be difficult to support multi-regional
    evolution if modern humans were around either
    before or at the same time as their presumed
    ancestors, the Neandertals.

43
Single Origins Theory The Fossil Evidence
  • Three sites in Africa appear to have modern human
    fossil remains, and each seems dated to the time
    of the Neandertals, or earlier.
  • Unfortunately, each of these sites has problems
    associated with either the dates or the
    interpretation of the anatomy.
  • The Three sites are located in
  • 1) southern Ethiopia
  • 2) on the border between South Africa and
    Swaziland
  • 3) on the very southern most coast of South
    Africa

44
Problems Fossil Evidence for Single Origins
Theory
  • Unfortunately, as these three sites in Africa all
    have a variety of difficulties associated with
    their total acceptance as evidence for the early
    appearance of modern humans in Africa, the
    evidence has to be carefully evaluated.
  • There are other sites, however, not in Africa,
    but in Israel, in the Middle East, which would
    appear to offer much better evidence for the very
    early appearance of modern humans.
  • We will examine this fossil evidence later on.

45
Single Origins Theory Genetic Evidence
  • At the moment, this is the strongest evidence for
    a recent origin of modern humans in Africa.
  • It is based on the analysis of DNA, but not
    primarily the DNA found on the chromosomes in the
    neucleus. Other genetic material is found in
    structures called mitochondria (known as mtDNA).
  • Mitochondria (singular mitochondrion) are cell
    structures responsible for carrying out the
    conversion of the sugar glucose into a form
    usable to the cell for energy.

46
mtDNA Results
  • Comparisons based on segments of the mtDNA from a
    number of human populations
  • 1) Documents a greater amount of mtDNA
    variation in Africans in comparison to human
    populations in other parts of the world.
  • 2) Discovered unique variations in Africa.
  • Conclusions drawn from this data
  • 1) Modern humans originated in Africa.
  • 2) There was a subsequent spread to other parts
    of the Old World, replacing earlier hominid
    populations.

47
Debates about mtDNA Results
  • Many scientists believe that these results are
    simplistic and do not reflect the realities of
    human origins.
  • Some suggest that because Africa was an optimal
    environment for earlier hominids, population size
    was always larger there than elsewhere thus
    there was a greater number of mutations, and more
    variability.
  • Others argue that if there was significant
    evolutionary selection on the mtDNA genes, then
    it would be very difficult to predict the nature
    of this evolution.

48
Multi Regional Evolution Fossil Evidence
  • The evidence for multi regional evolution is
    primarily centered on a number of fossils from
    Asia.
  • In China, for example, there is fossil evidence
    that the distinctive facial features of living
    Asian peoples had already appeared early in Asian
    human evolution, before any possibility of modern
    human migration out of Africa.

49
Dalioblique
50
Northchineseneol
51
Problems with both Theories
  • Fossil and archaeological evidence from the
    Middle East would appear to be damaging to both
    theories.
  • 1) Human fossil evidence argues against multi
    regional evolution.
  • 2) Archaeological evidence argues against a
    Single Origins theory.

52
Early Modern Humans from Israel
  • At two sites in Israel, early modern human
    skeletons have been excavated.
  • Unlike the African sites, at both of these
    Israeli sites, there are complete skeletons that
    are directly associated with dates of between
    90-115,000 years.
  • In contrast, at other Israeli sites,
    Neandertal-like fossils have been found. These
    are dated both earlier and later than the early
    modern human fossils

53
Ancestors II
54
Mid-East Neander
A neandertal from Iraq an
early modern human from Israel (probable date
60,000 years) (date 92,000 years)
55
Implications for Human Evolution
  • This fossil evidence would seem to argue strongly
    against the multi regional evolution theory.
  • If Neandertals are known both before, as well as
    after, well dated occurrences of modern humans in
    the Middle East, it would appear unreasonable to
    suggest that there was regional evolutionary
    continuity between Neandertals and early modern
    humans.

56
A Basic Problem Fossil Hominids and Archaeology
in the Middle East
  • All of the fossil hominids in the Middle East,
    including the Neandertals, and the early modern
    humans from Israel, are all found with the very
    same sort of stone tools.
  • Moreover, the archaeological sites contain the
    same sorts of hunted animal bones and the
    arrangement and organization of the sites is the
    same.
  • Both the early modern humans and the Neandertals
    buried their dead, and in the same fashion,
    without any grave offerings.

57
How Can the differences between Neandertals and
Modern Humans be explained?
  • If Neandertals and modern humans are different
    species, with a long evolutionary separation, how
    can the common use of the same stone tools be
    explained? The process of speciation requires
    reproductive isolation, which would have
    prevented separated populations from sharing
    knowledge of tools.

58
Modern Humans
  • By 30,000 years ago, modern humans, associated
    with more sophisticated stone and bone tools, are
    found in parts of Central and Eastern Europe.
  • At the same time, there are well dated
    Neandertals at sites in Spain and France.
  • What were the relationships between the
    Neandertals and modern humans
  • were they a separate species?
  • did neandertals contribute to the
    ancestry of living humans?
  • How did modern humans evolve?
    Quickly in one place or slowly over the Old
    World?
  • Stay Tuned!

59
And of Course The Essential Question When does
Humanness first appear?
  • What are the unique features of a modern human?
  • When did these unique traits develop, and under
    what circumstances?
  • Did they appear suddenly, at once, or gradually
    over a long time?
  • Were the Neandertals, and their contemporaries,
    also human? Did they share these traits with us?
  • Were the origins of these features the reason
    modern humans were able to spread over the whole
    world?

60
Lascaux Bison
61
Sculpture
62
pechmerle
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