john hawks weblog

paleoanthropology, genetics and evolution

Australopithecus

  • The pelvis of Australopithecus

    Sun, 2012-09-02 23:29 -- John Hawks
    Synopsis: 
    Early hominins had a pelvic form adapted to bipedality
    The hominid pelvis is much shorter than ape pelves, with muscle attachments reoriented for effective walking.

    The most dramatic evolutionary change underlying human bipedality is the change in shape of the pelvis. The pelvis is rarely preserved as a fossil, but several partial pelves are available from australopithecines, including the "Lucy" skeleton and several partial pelves from later South African sites. The pelvis consists of three bones — the sacrum, which lies at the bottom of the spine and is composed of several fused vertebra-like elements, and the two os coxae, or hip bones. In early hominids, both the sacrum and hip bones are relatively short compared to apes. The upper portion of each hip bone, called the ilium, is short and curved compared to the long, flattened ilium of chimpanzees and other apes. The curvature places the attachment of the quadriceps muscle closer to the front of the body, allowing the muscle greater leverage in pulling the femur forward in an upright posture.

    Lucy (AL 288-1) skeleton

    Lucy (AL 288-1) skeleton.

    Although the ilia of Australopithecus were short from top to bottom compared to a chimpanzee, they extend more broadly to the side, resulting in a pelvis that is very broad overall. Lucy’s pelvic width was within the range of today’s women, despite her very small body size. As a result, her body was differently shaped from recent people — very broad for its short height.

    The width of the pelvis affects the muscular requirements of walking. Whenever one leg supports the body, gravity tends to tilt the upper body away from the supporting leg. The muscles on the opposite side must counteract this force to prevent the body from falling over. These muscles attach to the lateral part of the ilium and to the femur, pulling the trunk upward around the hip joint. A wide ilium tends to make these muscles more effective, by positioning the point of force further from the joint. A long femur neck also helps, just as long handles on a pair of scissors greatly increase the force with which they can cut. The configuration of these muscles in australopithecines is more extreme than the condition found in living people.

    A wide pelvis and long femur neck may have helped australopithecines to maintain a long stride with short legs. Two things add up to determine the length of a step: how much the leg swings, and how much the pelvis rotates. A wider pelvis rotates farther and thereby increases the length of a step. Another explanation is that widely spaced legs may allow a greater mechanical advantage for the muscles that draw the legs toward the midline. This configuration might help the style of climbing that requires the legs to clamp around a branch or trunk. This kind of climbing would be more necessary to bipeds who lacked the prehensile feet of living apes.

    Study questions: 
    1. Take a moment to walk around. Can you feel which muscles are active as you take a step?
    2. Could you imagine a different way to alter the mechanics of an ape pelvis to make it more effective for bipedality?
    Study terms: 
    pelvis
    os coxa
    sacrum
    bipedality
    Australopithecus
    Australopithecus afarensis
    Tags: 
    Australopithecus
    A. afarensis
    pelvis
    anatomy
    bipedality

  • Bipedality and the pelvis

    Mon, 2011-10-31 23:02 -- John Hawks
    Synopsis: 
    Laboratory exercise introducing the features of the pelvis related to bipedality in hominins.

    Humans are bipeds. The pelvis in humans has undergone radical changes in orientation and shape compared to other anthropoid primates. Many of these changes serve to adapt our muscle orientations to the requirements of upright stance and bipedal locomotion.

    The most significant changes to the pelvis in humans compared to other apes are:

    Ilium
    The ilium (top portion of the innominate bone) in humans is shorter and broader. It curves around the trunk, whereas in apes it is flat against the back of the trunk.
    Greater sciatic notch
    This is very wide in apes, a function of their long, tall ilium. In humans, the notch is actually a notch.
    Anterior inferior iliac spine
    This feature is prominent in the hominin pelvis, absent or small in apes.
    Sacrum
    In humans, the sacrum is broad and short, in apes it is narrow and long, usually incorporating 6 or more sacral vertebral bodies.

    What to do: This station has four pelvic bones from the species Australopithecus africanus, which existed around 2.6 million years ago in South Africa. Assess the anatomy of these bones in comparison to humans, chimpanzees and gorillas. Are these the pelvic bones of a biped? What features point to your conclusion?

    In addition, the plaques at the front of the room have the near-complete skeleton of a fossil species, Oreopithecus bambolii, found in Tuscany and Sardinia around 8 million years ago. Look carefully at the pelvis of this skeleton. Does it resemble the living apes or humans? Does it look like Australopithecus?

    Study terms: 
    Oreopithecus
    Australopithecus
    Australopithecus africanus
    pelvis
    bipedality
    Tags: 
    Anthropology 105
    explainer
    laboratory
    pelvis
    bipedality
    Australopithecus
    Oreopithecus

  • Meet Australopithecus boisei

    Tue, 2011-10-11 08:25 -- John Hawks
    Synopsis: 
    Compare and contrast A. boisei and A. robustus, with a discussion of their ages and locations.

    The robust australopithecines existed between 2.5 and 1.5 million years ago. At this station are skeletal remains from two kinds of robust australopithecine. You have already met Australopithecus robustus earlier in the semester. The new species for you here is Australopithecus boisei. This species had the largest molar and premolar teeth of any hominin ever to have existed.

    A. boisei comes from East Africa, with remains found in Ethiopia, Kenya, and Tanzania. The most famous fossil is OH 5, from Olduvai Gorge, Tanzania, around 1.7 million years old. Other significant specimens here include KNM-ER 739, KNM-ER 732 and KNM-ER 406, from Koobi Fora, Kenya, around the same age.

    The specimens of Australopithecus robustus here will be familiar to you. All are from South Africa, and they include SK 48 and SK 12, from Swartkrans, South Africa, around 1.7 million years old, and TM 1517 from Kromdraai, South Africa, around 1.8 million years old.

    These species may be closely related, but there are some differences between them. Examine them closely with the following questions:

    1. The defining features of the robust australopithecines are the large postcanine dentition and large jaw musculature. How do these two groups of fossils compare on those features?

    2. Robust australopithecines also have a very reduced anterior dentition (incisors and canines). Which fossils show that morphology?

    3. The premolars in these species have enlarged, at the extreme they become more like molars in their morphology. Which fossils have the most molar-like premolars? Is the trend the same in the upper and lower dentitions?

    3. With such great robusticity of the jaws and teeth, there are potentially great differences between males and females. Are the differences here consistent with sexual dimorphism? Which fossils are male, and which are female?

    Study terms: 
    Australopithecus
    Australopithecus boisei
    Australopithecus robustus
    robust australopithecine
    Tags: 
    Anthropology 105
    laboratory
    anatomy
    A. boisei
    A. robustus

  • The Laetoli footprints

    Fri, 2011-09-02 00:53 -- John Hawks
    Synopsis: 
    A lab exercise in making footprints to compare to the Laetoli G footprint track.

    The most striking piece of evidence for bipedality in our earliest hominin relatives is a series of footprint trails at Laetoli, a fossil-bearing site in Tanzania. The longest trail, known as trail G, was made by at least two individuals, one much larger than the other. These individuals were probably members of a species called Australopithecus afarensis, with fossil remains that have been found in other parts of the Laetoli area from nearly the same time, 3.5 million years ago. This species lived long before any that scientists call humans, they are different from us in many, many respects. But the evidence shows that they walked bipedally in a very humanlike way.

    Studying these footprints poses many challenges to scientists. Their shape should give us clues about the shape of the feet, the way they struck the ground, the length and pattern of steps. Probably the most obvious aspect of these footprints are the big toes, which were aligned more or less with the other toes. This is a very different shape than a chimpanzee or gorilla foot, in which the big toe is relatively short and diverges from the foot, and the other toes are long and curving. Nevertheless, the toes of A. afarensis were not quite the same as ours, as you can compare as you make your own footprints.

    A comparison of one of the Laetoli footprints (bottom) with a footprint from a later site attributed two modern humans (top and middle). The human (middle) is walking with a bent-knee, bent-hip (BKBH) gait, not a normal gait for a person. The image shows the depth of different parts of the print. From a research paper by David Raichlen and colleagues [1].

    This lab station has you making footprints, to see how you might study the shape and conditions under which the Laetoli footprints were made. As you make footprints, try to use different styles of gait. Move fast or slow, maybe try to simulate a running step. Can you rule out some patterns of movement for the makers of the Laetoli footprint trail?

    References

    1. Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, Haas WR. Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics Rosenberg K. PLoS ONE. 2010;5(3):e9769.
    Study questions: 
    1. What kind of locomotion can you imagine would be intermediate between human-like bipedality and ape-like quadrupedality?
    2. One of the main points of contention about the Laetoli footprints is whether they preserve human-like arches in the midfoot. What do your comparisons indicate?
    Study terms: 
    bipedality
    trace fossil
    Australopithecus
    Australopithecus afarensis
    Tags: 
    bipedality
    A. afarensis
    Laetoli
    Tanzania
    East Africa
    laboratory
    Anthropology 105

  • Meet Australopithecus robustus

    Thu, 2011-09-01 21:39 -- John Hawks
    Synopsis: 
    This lab station gives an opportunity to examine fossil casts of A. robustus in comparison to humans and apes.

    The region just north of Johannesburg, South Africa, is a formation of ancient limestone in which groundwater has formed numerous caves and sinkholes. Some of these caves are used by animals for cool shade, water, and minerals; some are used by leopards, or in ancient times, sabretooths. By accident and predation, the skeletons of animals fall or are dragged into these caves, including our relatives the hominins. After around 2 million years ago, the most common kind of hominin in these caves was a species we call Australopithecus robustus.

    The word "robust" refers to size and strength. A. robustus was not very large in body size, but it had exceptionally large molar and premolar teeth, and a very large and thick mandible, or jawbone. The main muscles of the jaw, the temporalis muscles, were so large that they ran up the complete height of the skull to meet at the midline. The high ridge of bone where these muscles attached to the top of the skull is called the sagittal crest.

    A. robustus is one of the best-represented species of early hominins. The first specimen to be found was TM 1517, a partial skeleton with cranial remains from Kromdraai, presently in the Cradle of Humankind World Heritage Site. The largest sample of A. robustus fossils come from Swartkrans, less than 3 km from Kromdraai. The iconic skull, SK 48, provides a good illustration of the anatomy of the cranium of A. robustus with its sagittal crest, large, thick cheekbones, and relatively large molar teeth.

    The most obvious features that A. robustus shares with living people are related to locomotion. Human bipedality, or upright walking, caused many changes to the skeleton. A simple comparison of the distal end of the femur, the end nearest the knee, is enough to tell that A. robustus was bipedal like humans. Quadrupedal animals, who go on all fours, very rarely support their weight on one leg and do not have to balance their centers of mass over a single point. Their legs are typically oriented straight from the hip joint to the ground. Humans, in contrast, have to support their weight on one leg every time they take a step. To accomplish this, their legs must angle from the hip joint under the body's center of mass. The human knee angles very obviously at the distal femur, so that when the condyles of the femur rest flat on the tibia (or a table), the shaft of the bone angles markedly from vertical.

    This angle is called the valgus angle, and is one of the easiest-to-see traces of bipedality in fossil hominins.

    Study questions: 
    1. Explore the fossil skulls of A. robustus in comparison to the human and ape skulls at this station.
    2. Which features are more humanlike?
    3. Which features are more like the ape skulls?
    4. What kinds of foods do you think A. robustus would have eaten?
    5. The femur provides key evidence of locomotion. Examine the valgus angle on the distal femur from Swartkrans. Is it more like a human or an ape femur?
    6. Looking at the femur of A. robustus from Swartkrans, how big do you think these creatures were?
    Study terms: 
    sagittal crest
    femur
    tibia
    valgus angle
    fossil
    species
    hominin
    cranium
    Australopithecus robustus
    Australopithecus
    Swartkrans
    Kromdraai
    Tags: 
    A. robustus
    South Africa
    explainer
    teaching
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