This is the second part of a series by James Kidder on the human fossil record. The first part can be found here.
One of the most fruitful and exciting areas of research in palaeoanthropology is the search for the last common ancestor to the higher apes and humans. This question is inextricably tied to concepts of what separates humanity from the animals around us. This is a question that has spiritual as well as physical ramifications. In this set of posts, we are dealing with what makes us human from a biophysical perspective.
Traditionally, paleoanthropologists have considered the hallmark of humanity to be habitual bipedalism. While we share many characteristics in common with the higher apes, this trait alone is practiced by no other animal. Some animals practice facultative bipedalism, allowing them to go short distances on two legs when necessary, but only humans use it as their only form of locomotion. Put a man on all fours and even a squirrel can outrun him. Bipedalism is a skill that we learn early in life, before we are sentient and even understand what makes us different from the animals around us. It is programmed into us.
Bipedalism is marked by a number of anatomical modifications to the standard primate body. These center on the pelvis and involve changes in the head (cranium) and the rest of the body (postcranium), reflecting a shifting of the center of balance from the abdominal cavity to the hip. In mammals, the hip is composed of three mirrored sets of bones: the ilium, the ischium and the pubis (Figure 1). The top part of the leg fits into the bottom-rear portion of the ilium, into a round socket called the acetabulum. It is one of two ball-and-socket joints in the body, the other being where the arm fits into the scapula at the top of your shoulder.
Where the two pubis bones fit together in the front and the two ilia meet in the back with the sacrum forms the birth canal. In chimpanzees and gorillas, the ilium is narrow and tall (Figure 2). Consequently, the connection to the upper leg bone, the femur, is straight up and down. In humans, the ilium is flat and flared, creating an outward bowing of the top of the femur, which allows for the balance necessary for walking upright (see Figure 1). This, in turn, creates what is known as the valgus knee, where the bottom of the femur meets the top of the large lower leg bone, the tibia, at an angle. The fact that the two bones meet at an angle provides for a better balance of the body mass for upright walking. In contrast, when higher apes such as gorillas and chimpanzees stand, the femur and the tibia are both perpendicular to the ground, resulting in a straight knee joint. Consequently, when chimpanzees walk upright, they swing side to side in an ungainly fashion to simulate the balance that is inherent in human walking.
Other changes are present above the hip, as well. Because they are quadrupedal, chimpanzees, gorillas and orangutans have a straight backbone, or vertebral column (Figure 3). In humans, the vertebral column resembles a double “s” shape, which balances the torso above the hip (and creates the back problems we suffer later in life). At the top of the spinal column, the top vertebra, the atlas, has facets that balance the head and the second vertebra, the axis, has a prong that fits directly into a hole in the skull. This hole, which is called the foramen magnum, is at the back of the skull in higher apes, as in all quadrupedal animals. This allows the animal to keep its head up while it is trotting along the ground. In humans, the foramen magnum is at the base of the skull, allowing us to look forward as we walk. It also makes it hard to look forward when we crawl on all fours. Each of these modifications is diagnostic of humans and easily recognizable in the fossil record in specimens for which these anatomical areas are present.
Ardipithecus ramidus and the Origins of Bipedality
The origins of bipedality have traditionally been understood as having evolved at the end of the Miocene epoch, around 6 to 8 million years ago (Crompton, Sellers and Thorpe 2010) when the climate began to dry out and cool. Unfortunately, there are only scattered presumed hominin remains from this time period, all of which are taxonomically controversial and fragmentary and none of which have diagnostic postcranial remains. It has also been thought that the transition to bipedality likely did not happen all at once but in mosaic fashion (as evolution often proceeds) and this has recently supported by the fossil record. Up until a few years ago, the most widely-accepted model was that bipedality originated among a group of large-bodied hominoids that had adapted to the savannah-jungle fringe. The jungle, itself, was ceded to the precursors of the modern chimpanzees and gorillas and the savannah to the precursors of the modern baboons and other terrestrial monkeys. As a result, some workers (Crompton et al. 2010) suggested that both arboreal (tree-dwelling) and terrestrial locomotion might have been present in our earliest ancestors. Recent evidence has corroborated some parts of this model, but not others.
In 1994, the remains of a remarkable hominin, dated to 4.4 million years ago, were unearthed in the Afar Triangle of Northeastern Ethiopia. Examination of the surrounding deposits, however, yielded a conclusion that this hominin lived in a woodland environment, rather than a savannah/forest fringe environment (White et al. 2009a). Requiring over ten years of extrication from the surrounding rock and painstaking reconstruction, this fossil form, Ardipithecus ramidus (now represented by 110 individuals) yields diagnostic parts of the pelvis (Figure 4), as well as sections of the arms and skull (Figure 5) (White et al. 2009b). Although the base of the skull is not preserved, one striking aspect of humanity is present in the teeth. The canine (eye tooth) does not extend beyond the tooth row. Humans are the only hominins for which this is the case. In all other ape species, fossil and extant, the canine projects well beyond the tooth row.
Biomechanics specialist Owen Lovejoy and colleagues (Lovejoy et al. 2009) write about this species:
“The gluteal muscles had been repositioned so that Ar. Ramidus could walk without shifting its center of mass from side to side. This is made clear not only by the shape of its ilium, but by the appearance of a special growth site unique to hominids among all primates (the anterior inferior iliac spine). However, its lower pelvis was still almost entirely ape-like, presumably because it still had massive hindlimb muscles for active climbing.”
Figure 6 shows the intermediate nature of the pelvis of Ardipithecus ramidus compared to later hominins (Homo sapiens, Au. Afarensis) and chimpanzees (P. troglodytes).
Ardipithecus, then, represents a shift away from the primitive locomotion employed by the last common ancestor of our line and that of modern chimpanzees. Here is a hominin that maintained a link with its tree-dwelling past and yet had progressed toward the bipedal future. This evidence is striking because it firmly demonstrates that a species had arisen that was advanced in the human direction. Whether or not it led to the hominin forms that followed is not known but it clearly represents a phenomenal example of a transitional form in the human fossil record.
From this point on, the forms become noticeably more human in appearance, leading eventually our own species some four million years later. In his infinite wisdom, God had set us on a path toward our eventual communion with Him. That this path took such a long period of time and through so many varied forms of humanity is a testament to His creative power and patience.
Next, the successors to Ardipithecus and true human walking.