Human Evolution

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Transcript Human Evolution
Human Evolution
The Questions & the Best Current Answers
The rules of science
Hypotheses must be testable
Must be couched in previous knowledge (like
a scientific theory).
What makes a theory scientific?
Why is the theory of evolution scientific?
What makes the theory of gravity scientific?
Can we prove a theory?
Are theories perfect?
Do they provide truth?
 Should they be “believed” in?
What does the theory state?
Biological evolution = a shift over time in the
proportion of organisms differing genetically
in one or more traits.
Includes the following concepts
Natural selection
Genetic drift
Gene flow
What are the questions?
Where did all of the different living
(today) species come from?
Have extinctions occurred in the past?
If the answers appear to be yes…
Then have humans evolved?
What are evolutionary hypotheses?
1)If life is static, no different fossil organisms
should be found.
2)If life changes, fossils should be found of
organisms that do not exist today.
We can disconfirm no. 1.
Does that mean we have “proven” two?
Does the fossil record disconfirm the ToE?
What evidence would disconfirm it?
What are evolutionary hypotheses
about humans?
1)If humans did not evolve, then fossil forms in the
past should be the same as today.
2)If humans did evolve, then different human-like
forms should occur in the fossil record.
We can disconfirm no. 1?
But does this mean no. 2 is “proven?”
Does the fossil record disconfirm the ToE?
What evidence would disconfirm it?
Some other questions
Why do biologists think apes & humans
share a common ancestor?
This is a very important question,
because biologists & archaeologists
expect that older human fossils should
look more ape-like.
Some other hypotheses
1)If humans & apes do not share a common
ancestor, they should not have similar DNA.
2)If humans & apes share a common ancestor,
they should have DNA more similar than
between humans & other mammals.
We can disconfirm no 1.
But does this mean no. 2 is “proven?”
Does the fossil record disconfirm the theory?
What evidence would disconfirm it?
More hypotheses
1)If humans & apes do not share a common
ancestor, fossils should not look somewhat
ape-like & somewhat human-like.
2)If humans & apes share a common ancestor,
progressively older fossils should be more
We can disconfirm no 1.
But does this mean no. 2 is “proven?”
Does the fossil record disconfirm the theory?
What evidence would disconfirm it?
In every single case…
Our conclusions are based on the
disconfirmation of hypotheses.
It is important to base our conclusions on
what we do not find.
The ToE is a scientific theory because it
has never been disconfirmed following the
rules of science.
When we look…
At the modern biological world, what do we find?
At the fossil record, what do we find?
We find information that disconfirms stasis.
This is a carefully considered process that
arrives at the “best current answer.”
It is not a personal opinion
It is not proof or truth
Scientists are people.
People tend to be unscientific.
Scientists can be unscientific.
However, if you know what science is
you should be able to tell whether or not
hypotheses are disconfirmed, so
science should stand on its own.
What do paleoanthropologists do?
They seek to test hypotheses like the
ones we just considered.
Human geneticists look at modern
genes in primates.
Paleoanthropologists study the fossil
What happens when fossils are found?
The paleoanthropologist must be careful about…
Recording information on the sediments in which the
fossils were found.
Using PoA, PoS,& PoSIF
Comparative method must be used to answer…
How does a new fossil fit in with modern human skeletons
and with other fossils that have been found?
Modern humans are…
Fully bipedal & big brained.
So two very important questions are…
How big is the fossil specimen’s brain?
 Do the legs & pelvis suggest bipedalism?
How might taphonomy matter here?
What’s in a name?
The most unscientific part of
paleoanthropology is species naming.
What is a species?
A ring species
A new hominin skull is found…
Say it dates to 3.0 million years ago.
What species does it belong to?
What should we call it?
What is a fossil species?
These “species” may not be real groups, but
we use the names to communicate.
Pretty sloppy & unscientific, but the fossils still
exist, and can be used to test our hypotheses.
Why Africa?
The African Rift Valley is splitting along
most of East Africa.
Results in long canyons & valleys where
strata are exposed.
The Rift Valley
Science is about falsification
Scientific theories have not yet been falsified,
which is why they work.
There are testable hypotheses about human
Scientists are human & can be unscientific, like
in hominin naming.
The African Rift Valley exposes sediments of the
correct time period.
The Fossil Record
Humans are primates
Primates are mammals that share
several physical characteristics.
 3-D color, binocular, & peripheral vision
 Prehensile, precise, & powerful gripping
 Flexible limbs
 Similar genes
Humans are primates,cont’d
We share behaviors with other primates.
Live in family groups
 Use tools
 Commit acts of violence
 Intelligent problem solvers
Our lineage
Because of these similarities, it is
hypothesized that apes & humans share
a common ancestor.
This can be tested with the fossil record.
As the paleoanthropological record gets
older, fossil hominids are more ape-like.
Hominin = a member of subfamily
homininae, which includes humans
& extinct close relatives
Hominid = a member of family
hominidae, which includes apes
& all hominins.
Sahelanthropus tchadensis
The earliest know Hominin.
Date 6 to 7 mya in Chad.
Foramen magnum points down.
Orrorin tugenesis
The next oldest, dates between 5
& 6 mya in Kenya (Tugen Hills).
Most of the early hominin fossils
have been found in East Africa.
The Savanna Hypothesis: Why did
Hominins become bipedal?
It is known that between roughly 7 & 4 mya grasslands
expanded in East Africa (paleoenvironmental data)
This hypothesis states that this opening of grassland
produced environments in which bipedalism evolved.
Literally, hominins came down out of the trees, and
bipedalism was selected for.
Eventually these conditions, through natural selection cause the
“ascent of man” into a bipedal walker.
Test this hypothesis using Sahelanthropus and Orrorin
Why is this cartoon funny?
Because the savanna hypothesis has
been falsified.
Scientists, being people, really believed
in this hypothesis.
Alas, being a scientist means you must
set your beliefs aside when your favorite
hypothesis is disconfirmed.
The Orrorin and Sahelanthropus fossils
disconfirm the savanna hypothesis.
This is the case because both forms were
fully bipedal prior to the opening of
Moving forward in time…
The fossil record is patchy & earlier periods
have limited evidence, yet it does offer
evidence of bipedalism & human-like teeth.
The record dating to later than 4 mya,
however, is more complete.
How does site discovery & the preservation
equation matter here?
Survey, excavation, recovery
Diversity in the fossil humans at 4 mya
What do paleoanthropologists mean by
They are not speaking only of modern humans.
They mean fossil-human ancestors & their
cousins that became extinct.
So the Orrorin fossils are human.
Remember, evolution is a tree, some branches of
which go extinct, some which continue to evolve.
The Australopithecines
Members of the genus Australopithecus
The only fossils known between Orrorin &
Australopithecus are:
Ardipithecus ramidus (5.6 – 4.3 mya)
Poorly known from Ethiopia.
 Only fragments, but some are mandibles w/ teeth.
 Estimated brain size is 400 – 450 cc.
Lucy is the most complete skeleton of an early
Found in Hadar, Ethiopia (1974).
Australopithecus afarensis
Found in many areas of East Africa.
4.0 – 3.0 mya.
Lucy was a small-brained biped.
Small brain 380 – 500 cc
Lucy was 3’6” tall 50 lb
Dates to 3.2 mya
The Australopithecines, cont’d
There appear to have been several
species of Australopithecus.
Some of these are likely to have been in
the human line.
There are a few types, however…
The robust australopithecines 3
– 1.5 mya
Australopithecus aethiopicus
Australopithecus boisei
Australopithecus robustus
Brain size around 500 cc
Sometimes “Australopithecus” is
replaced with “Paranthropus.”
1.5 mya
Robust Australopithicus
Homo habilis
Australopithecus africanus
2 mya
Australopithecus afarensis
4 mya
What’s different about Homo habilis?
Homo habilis has a bigger brain
600 – 800 cc
Is missing a sagittal crest
Has a face that is less prognathic
Is missing a diastema
A smaller zygomatic arch
Used stone tools?
PoA matters here, why?
Modern human
Robust Australopithecus
Early hominins: summary
There are fossils dating back to roughly 7mya
that are hominin.
They are bipedal
They are intermediate between apes & humans in
shape & size
By 2 mya brains in hominins are larger and
stone tools appear.
Archaeology really begins with stone tools.
The Earliest Hominins
A picture review
A. afarensis
4 – 3 mya
East Africa
Laetoli footprints, Tanzania: A. afarensis
A. robustus/boisei/
3 – 1.5 mya
East & South Africa