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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?
Evidence for Human Evolution
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Comparative Anatomy
The examination of the gross anatomical
features of humans and apes reveals their
close similarity and evolutionary relatedness.
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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.
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Fossil Evidence
Fossil Evidence
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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.
A mud site
Ape man
What’s 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.
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
The Human Fossil Record
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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.
Human Evolution: Beginnings
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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, nonprojecting 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.
Human Evolution:First Out of Africa
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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.
1st fire
The First Europeans
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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.
Europe first
Theories of Modern Human
Origins
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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
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”).
theories
Multi Regional Evolution I
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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.
Multi Regional Evolution II
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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.
Single Origins Theory I
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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.
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.
Single Origins Theory III
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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.
Single Origins Theory IV
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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’.
Modern Human Origins
Thus, two different theories:
1) Multi Regional Evolution
2) Single Origins : “Out of Africa”
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numerous of fossils, much of the
emphasis of both theories centers on
the Neandertals.
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Neandertal Discoveries
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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.
Ape man
The Neandertals
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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.
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”.
Pity neandertal
Full Neandertal
Scene I
Neandertal Discoveries II
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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”?).
Europe II
The Skull of a Neandertal from France
La Chappelle Skull
No chin
Large brain case with a brain
often larger than those of living
humans
Modern Human Origins
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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.
Skulls I
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.
Single Origins Theory: The
Fossil Evidence
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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
Problems: Fossil Evidence for
Single Origins Theory
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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.
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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.
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We will examine this fossil evidence later on.
Single Origins Theory: Genetic
Evidence
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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.
mtDNA Results
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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.
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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.
Debates about mtDNA Results
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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.
Multi Regional Evolution:
Fossil Evidence
The evidence for multi regional evolution
is primarily centered on a number of
fossils from Asia.
пЃ¬ 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.
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Dalioblique
Northchineseneol
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.
Early Modern Humans from
Israel
At two sites in Israel, early modern human
skeletons have been excavated.
пЃ¬ Unlike the African sites, at both of these Israeli
sites, there are complete skeletons that are
directly associated with dates of between 90115,000 years.
пЃ¬ 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
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Ancestors II
Mid-East Neander
A neandertal from Iraq
(probable date: 60,000 years)
an early modern human from Israel
(date: 92,000 years)
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.
A Basic Problem: Fossil Hominids
and Archaeology in the Middle East
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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.
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.
Modern Humans
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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!
And of Course: The Essential
Question: When does �Humanness’
first appear?
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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?
Lascaux Bison
Sculpture
pechmerle
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