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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 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, 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 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
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, 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 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 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 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, 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 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).