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human evolution

Human Evolution

The Questions & the Best Current Answers

the rules of science
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
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
    • Mutation
    • Gene flow
    • Extinction
what are the questions
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
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
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
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
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 ape-like.
  • 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
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
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
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 record.
what happens when fossils are found
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
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
What’s in a name?
  • The most unscientific part of paleoanthropology is species naming.
  • Why?
  • What is a species?

A ring species

a new hominin skull is found
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
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 evolution.
  • Scientists are human & can be unscientific, like in hominin naming.
  • The African Rift Valley exposes sediments of the correct time period.
the fossil record

The Fossil Record

humans are primates
Humans are primates
  • Primates are mammals that share several physical characteristics.
    • Pentadactylism
    • 3-D color, binocular, & peripheral vision
    • Prehensile, precise, & powerful gripping
    • Flexible limbs
    • Similar genes
humans are primates cont d
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
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.

Hominid = a member of family hominidae, which includes apes & all hominins.

Hominin = a member of subfamily homininae, which includes humans & extinct close relatives

sahelanthropus tchadensis
Sahelanthropus tchadensis

  • The earliest know Hominin.
  • Date 6 to 7 mya in Chad.
  • Foramen magnum points down.

orrorin tugenesis
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
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 fossils…

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 savanna.

moving forward in time
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?
    • P = MCDST
    • Survey, excavation, recovery
diversity in the fossil humans at 4 mya
Diversity in the fossil humans at 4 mya
  • What do paleoanthropologists mean by “human?”
    • 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
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 hominid.
    • 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
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.”


Homo habilis

1.5 mya

Robust Australopithicus


Australopithecus africanus

2 mya

Australopithecus afarensis

4 mya

what s different about homo habilis
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
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

The Earliest Hominins

A picture review


A. afarensis

4 – 3 mya

East Africa


A. robustus/boisei/


3 – 1.5 mya

East & South Africa