What Makes a Fossil a Hominin? Major hominin fossil discoveries are important and only happen occasionally.  But how do scholars even know that what they have found is a hominin at all?  In Module 4 we saw that after discovery, determining the age of a fossil is important because it places it in temporal context.  Paleoanthropologists and archaeologists excavate fossil discoveries very carefully in order to avoid contaminating samples that must be pristine for dating to be accurate.  As you learned, dating is sometimes done indirectly , meaning that the surrounding matrix (strata) or an associated object might be what is dated in order to derive the date of the fossil.  It is normal for many samples to be repeatedly dated in order to verify that a result is accurate.  It is also normal to use as many techniques as possible to verify a fossil's date. Thus, when Orrorin tugenensis (image above left) was discovered and excavated by Martin Pickford and Brigitte Senut  in Kenya in 2000, it was chronometrically dated using K-Ar ( Potassium-Argon ) dating of the trachyte volcanic layer it was found below, but additional techniques like biostratigraphy , paleomagnetism , and comparative analysis of fossilized algae were all used to confirm and refine the date as well as verify that the fossil hadn't been displaced into the strata it was found in.  The application and combination of several research methods in the study of the same phenomenon is known as triangulation and it is critical in scientific research because it helps strengthen and support claims. Situating a new discovery in the existing fossil record requires knowing its age and scientists use every means possible to confirm their claims. Dating the fossil is critical but the real work only starts there.   Pickford and Senut claimed the fossilized bones in the image above were a hominin, but how did they know this?  Determining taxonomic placement of fossils is done by comparing morphological traits .  As you know, the single defining derived trait shared by hominins is upright walking or bipedalism , so determining that a fossil is a hominin requires showing it has the skeletal traits of a biped.  When walking on two legs on the ground is the normal form of locomotion, it is called obligate bipedalism . The evolution of bipedalism as a form of locomotion brought significant anatomical modifications, as can be seen when we compare the skeletons of a gorilla with a human: Some differences between the skeletons of a gorilla and a human. The anatomical differences between a gorilla and a human show how different the skeleton of a biped is from a quadruped.  Differences include changes to the pelvis, the spine, the lower limbs and even the skull (the position of the foramen magnum discussed in the module Introduction). When assessing bipedalism, one of the best parts of the skeleton to have is the pelvis . The human pelvis is bowl-shaped to support the bodies inner organs wh.

1. What Makes a Fossil a Hominin?
Major hominin fossil discoveries are important and only happen
occasionally. But how do scholars even know that what they
have found is a hominin at all? In Module 4 we saw that after
discovery, determining the age of a fossil is important because
it places it in temporal context. Paleoanthropologists and
archaeologists excavate fossil discoveries very carefully in
order to avoid contaminating samples that must be pristine for
dating to be accurate. As you learned,
dating is sometimes done indirectly
, meaning that the surrounding matrix (strata) or an associated
object might be what is dated in order to derive the date of the
fossil. It is normal for many samples to be repeatedly dated in
order to verify that a result is accurate. It is also normal to use
as many techniques as possible to verify a fossil's date. Thus,
when
Orrorin tugenensis
(image above left) was discovered and excavated by Martin
Pickford and Brigitte Senut in Kenya in 2000, it was
chronometrically dated using K-Ar (
Potassium-Argon
) dating of the trachyte volcanic layer it was found below, but
additional techniques like
biostratigraphy
,
paleomagnetism
, and comparative analysis of fossilized algae were all used to
confirm and refine the date as well as verify that the fossil
hadn't been displaced into the strata it was found in. The
application and combination of several research methods in the
study of the same phenomenon is known as

2. triangulation
and it is critical in scientific research because it helps
strengthen and support claims. Situating a new discovery in the
existing fossil record requires knowing its age and scientists use
every means possible to confirm their claims.
Dating the fossil is critical but the real work only starts there.
Pickford and Senut claimed the fossilized bones in the image
above were a hominin, but how did they know this?
Determining taxonomic placement of fossils is done by
comparing morphological traits
. As you know, the single defining derived trait shared by
hominins is upright walking or
bipedalism
, so determining that a fossil is a hominin requires showing it
has the skeletal traits of a biped. When walking on two legs on
the ground is the normal form of locomotion, it is called
obligate bipedalism
.
The evolution of bipedalism as a form of locomotion brought
significant anatomical modifications, as can be seen when we
compare the skeletons of a gorilla with a human:
Some differences between the skeletons of a gorilla and a
human.
The anatomical differences between a gorilla and a human show
how different the skeleton of a biped is from a quadruped.
Differences include changes to the pelvis, the spine, the lower
limbs and even the skull (the position of the
foramen magnum
discussed in the module Introduction). When assessing
bipedalism, one of the best parts of the skeleton to have is the
pelvis

3. . The human pelvis is bowl-shaped to support the bodies inner
organs when standing and moving upright. Its shape also
stabilizes the weight transmission necessary while walking or
running. The human pelvis is shorter top-to-bottom and wider
side to side when compared to a gorilla's pelvis, which is both
flatter when laid on top of a table for example, and longer top-
to-bottom and narrower side-to-side. The
human spine
has an S-shaped complex curvature, with
lumbar lordosis
(lower curve) as well as a thoracic upper curve. The need for
lumbar strength makes the lower vertebrae more robust. This
helps support upper body weight and keep the upper body
centered above the pelvis. Unfortunately since the spine is
segmented into vertebrae, it is not common to recover a fossil
specimen with a complete spine.
The lower limbs are also modified and their features can be
diagnostic of bipedalism if they are present in fossils. Note in
the image above that humans have longer legs and shorter arms,
whereas for apes like gorillas, this is reversed. This ratio is
called the
intermembral index
, which is calculated by adding the length of the upper and
lower arm bones (the
humerus
and
radius
) and dividing it by the length of the upper and lower leg bones
(the
femur
and
tibia)
before multiplying by 100. Humans have an intermembral
index that ranges between 68-70, meaning that people's arms are
about 70% as long as their legs. Gorillas on the other hand,

4. have an average intermembral index of 116. This comparison
suggests that as bidpedalism evolved, the intermembral index
was reduced with arm length decreasing and leg length
increasing. Determining a fossil's intermembral index requires
that multiple complete leg bones are found-- possible but not
common. What if not all of the limb bones are discovered?
The
femur
is one of the largest bone masses in the body, so there is a
relatively better chance of a fossil discovery including it. The
femur
of bipeds angles inward so the lower legs are directly under the
body for weight support. This creates an effect known and
valgus knee,
where because the femur is angled, the knees of both legs angle
in and almost touch each other when a biped is standing. The
human knee joint also permits full extension the lower joint for
walking too-- apes aren't capable of that kind of extension. The
human foot shoes several major adaptations-- the big toe (the
hallux
) has moved over in line with the other toes. For apes, the big
toe is opposable so they can hold food in their feet while
hanging from their arms. Humans also have a longitudinal arch,
to help absorb shock and push off as they stride. These
illustrations provide additional visual comparisons:
Comparing the anatomy of humans and apes. Left to right:
pelvis, lower limbs and feet.
Additional information on some skeletal differences between
obligate and non-obligate bipeds can be seen in figure 9.6 of
chapter 9 of your textbook. Modern humans (
Homo sapiens
) exhibit some key derived morphological traits that distinguish

5. us from our early primate ancestors. Hominin evolution from
the earliest bipeds to the present day exhibits the general trends
shown in this image:
As the image makes clear, not all of these evolutionary changes
happen at the same time or rate, a condition referred to as
mosaic evolution
.
Lucy taught us that many of the anatomical changes associated
with bipedalism occurred far earlier than those associated with
encephalization. Here we are concerned with early hominin
evolution. To be more specific, determining whether a fossil is
that of a biped can be done through close study of a variety of
morphological features. These include but are not limited to:
Various skeletal modifications of pelvis, lower limbs, and feet
for bipedalism discussed above
Decrease in
intermembral index
as legs become proportionately longer (see below)
Movement of the
foramen magnum
forward from its position in quadrupeds
Modifications to the hand as it shifts from a means of arboreal
locomotion to manual dexterity, including the
lengthening of the thumb
Trend toward
microdonture
(smaller teeth) including
reduction of the canines

6. Absence of the
C/P3 honing complex
(the combination of canine and first premolar teeth that self-
sharpens the upper canine in apes)
Reduction of the facial
prognathism
, (the face becomes flatter, more
orthognathic
)
Ancient fossil discoveries are normally only partial, so which of
these traits is examined is determined by the nature of the fossil
sample. As we saw previously, the first hominin fossil
discovered in Africa, Raymond Dart's Taung Child (
A. africanus
), was known from only the skull, including a brain endocast
and the
mandible
(lower jaw). Dart asserted he could tell
A. africanus
walked upright due to the position of the
foramen magnum,
hole through which the spinal chord connects to the brain.
In 1974, Donald Johanson and his graduate student Tom Gray
discovered the famous fossil known as
Lucy
in the Afar region of Ethiopia. Johanson and his team knew
what they had found was very ancient because the geological
formations there had been dated to the Pliocene. Lucy (AL-288-
1) was even older than the Taung Child and is dated to 3.2
mya. Johanson later placed her into a new species
Australopithecus afarensis

7. , named for the region where she was found. After determining
that she was probably female based on the size of her bones, she
was nicknamed "Lucy" because the Beatle's song "Lucy in the
Sky with Diamonds" was playing in the excavator's camp that
day. What was most remarkable about Lucy was how complete
her skeleton was-- Johanson and his team were able to recover
40% of her skeleton. As the
Explorations
textbook makes clear, even more-complete ancient skeletons
have been discovered since Lucy was found, including Ardi
(ARA-VP-6/500), "Little Foot" (StW 573), and Selam (aka
"Lucy's child"). At the time of her discovery, however, Lucy
was the most complete ancient hominin ever discovered.
In the image below, Lucy's recovered fossil is shown on the
left, with a comparison with an adult human female (center,
with recovered human bones shown in red) and Johanson with a
plastic cast of Lucy's skull on the right. Note that the skull cast
is a reconstruction, with the fossilized bones that were found
shown in a darker color and the reconstructed (missing) parts
shown in white:
The Lucy fossil, discovered 1974
Lucy's relatively complete skeleton gave paleoanthropologists a
great deal of information. Her teeth indicated that she was a
young adult and her small body size meant she was almost
certainly a female. To this day, Lucy remains the smallest adult
member of her species ever found at an estimated 3.7 feet tall.
Two bones in particular showed immediately that
Lucy was a biped
. Her pelvis, while only half present, included her
os coxae
(hip bone) and

8. sacrum
. The shape of the pelvis is much closer to the the biped pelvis
described above-- bowl-shaped and broad side-to-side and short
top-to-bottom. Lucy's femur was also angled inward, exhibiting
a highly valgus knee. The fact that Lucy's cranial features were
much less derived than her post-cranial (below the skull)
features showed conclusively that
bipedalism preceded encephalization
(the evolution of a larger brain) by a significant amount of time
in the hominin lineage. This was an important discovery and
one that has been supported by many fossil discoveries since.
Lucy's fossil was so complete that a reasonably accurate
intermembral index for the specimen could be estimated.
Her intermembral index was estimated at 88
, putting her between the great apes and humans. The change
from the higher index of apes resulted primarily from a
relatively shortened
humerus
(the long bone of the upper arm). Since the evolutionary trend
in hominins is toward longer hindlimbs than forelimbs, as in
modern humans, Lucy’s skeleton suggests that arm length
reduced before thigh elongation evolved later. Needless-to-say,
all of this is very important information for understanding how
humans evolved.
The completeness of Lucy's skeleton made her bipedalism a
foregone conclusion. But what about less complete fossil finds
where things aren't so easy? When Lucy was discovered in
1974, the fossil's ancient age shocked the world. In the decades
since Lucy was found, several other major discoveries of even
more ancient hominins have been found. You read about these
in this week's
Explorations
textbook chapter. Each of them was placed by their
discoverers into a distinct genus and species, so that it is now
claimed that hominins include three pre-australopithecus

9. genera:
Sahelanthropus, Orrorin,
and
Ardipithecus
. As a group, they are sometimes referred to as
basal hominins
because they are dated very close to the time that the hominin
lineage split off from the apes. All three genera actually date
back to the late
Miocene epoch
(23-5.3 mya). All of them were discovered and announced to
the world in scientific publications after the year 2000. They
are shown with the location of their discoveries on this map:
Source:
Nature Education (Links to an external site.)
(2013), with the specimen images taken from publications
found in the assignment below
Critical to the importance of all of the basal hominins is the
claim that they are bipedal, like Lucy. In each case, the
discoverer has to lay out the case for bipedality using what
morphological features are available. An interesting example
of the challenges of such interpretation can be seen with
Orrorin tugenensis
(shown in the image at the top of this page). The original
Orrorin
fossil discovery included 13 specimens from what were
determined to be from 5 different individuals. After several
years of continued excavation, there are now 20 specimens
total. The recovered bones included two jaw fragments,
isolated teeth, the upper part of a femur and a humerus. How
did Pickford and Senut determine it was a biped? Without a full

10. skull, a pelvis, or even a full femur (the knee is missing), it was
difficult to know for certain. The partial femur was still critical
to their case. According the them, the femur head is spherical
and rotated like a biped, with an elongated neck that shows
regular wear from ligament friction consistent with bipedalism.
Finally, Senut and Pickford used a
CT-scan
(computed tomography combining multiple xrays) of the femur
bone to show that the neck of the femur exhibited a bone
density pattern present in bipeds-- density that builds up as a
result of the pressure put on bones from walking bipedally.
The CT scan of Orrorin's femur.
The image at left shows where the cross-section views were
taken when the scans were performed. Those cross sections are
shown center and right. What the scans showed was that the
distribution of the cortical bone on the femoral neck was
asymmetrical, denser on the bottom side (indicated by the blue
arrows). Humans have similar asymmetrical cortical bone
distribution due to bipedal locomotion; apes have a much more
even distribution. Notably, Orrorin's other postcranial features
suggest it was also still climbing trees regularly in addition to
its bipedalism, but its normal locomotion included walking on
two legs on the ground. Naturally, not all scholars agree with
the conclusions reached by Pickford and Senut.
The Assignment
Who exactly is the most ancient hominin ever found? That
depends who you talk to. Along with
Orrorin tugenensis
(described above), the leading contender for the earliest
possible hominin is
Sahelanthropus tchadensis

11. , known also known as
Toumai
or TM 266 (image at the top of the page). There has been
significant debate and assessment of whether these two fossils
are actually hominins. For this assignment you are to use the
scientific literature provided below to argue a position about
which of them has the best case for being the earliest hominin.
Here are some things to address in your paper:
How, where, and by whom were each of the fossils discovered?
How old is each and how were they dated?
On what bases specifically do the discoverers of
Sahelanthropous make the claim that it is a hominin? Explain
in detail.
On what bases are the claims of the discoverers of both
Sahelanthropus
and
Orrorin
disputed or supported and why? Explain in detail using
examples from the literature below.
Your conclusion: which of the two fossil discoveries has the
best case for the claim of earliest possible ancestor? Why?
Some of the information I provided earlier for
Orrorin tugenensis
can help you and act as a model for your assignment. As you
bring information about Orrorin into your assignment, please
draw on the literature below and not my summary above.
Please cite your information using

12. APA references
for the articles provided
. Do not bring in outside sources for the assignment, though
you are free to utilize the internet and the
Explorations
textbook to help you make sense of the information.
The case you make must be evidence-based and not simply a
reflection of your opinion
. To be clear, you certainly do not need to read every word of
the articles below, but you should look over all of them and
make direct use of at least 6 of them
. Some of the articles are by the discoverers, some are critiques,
and some are reassessments by others.
The
paper you submit should be a minimum of 1000 words
.
Almécija, S. et al. The femur of Orrorin tugenensis exhibits
morphometric affinities with both Miocene apes and later
hominins. Nat. Commun. 4:2888 doi: 10.1038/ncomms3888
(2013).
Brunet, M.
et al
. A new hominin from the upper Miocene of Chad, Central
Africa.
Nature

13. 418
,
145-151 (2002).
Brunet, M.
et al.
New material of the earliest hominin from the upper Miocene of
Chad.
Nature
434
,
752-755 (2005).
Pickford, M. & Senut, B. The geological and faunal context of
late Miocene hominin remains from Lukeino, Kenya.
Comptes Rendus Académie de la Terres et des Planètes
332
,
145-152 (2001).
Pickford, M.
et al
. Bipedalism in
Orrorin tugenensis

14. revealed by its femora.
Comptes Rendus Palevol
1,
191-203 (2002).
Richmond, B. G. & Jungers, W. L. Orrorin tugenensis femoral
morphology and the evolution of hominin bipedalism.
Science
319, 1662-1665 (2008).
Senut, B.
et al
. First hominin from the Miocene (Lukeino Formation, Kenya).
Comptes Rendus Académie de la Terres et des Planètes
332
,
137-144 (2001).
White, T. D. Early hominin femora: The inside story.
Comptes Rendus Palevol
5,
99-108 (2006).
Wolpoff, M. H.

15. et al
.
Sahelanthropus
or ‘
Sahelpithecus'
?
Nature
419
,
581-582 (2002).
Wolpoff, M. H.
et al.
An ape or the ape: Is the Toumaï cranium TM 266 a hominin?
PaleoAnthropology
2006
,
36-50 (2006)
.