Shaping Humanity: How Science, Art, and Imagination Help Us Understand Our Origins
368
Shaping Humanity: How Science, Art, and Imagination Help Us Understand Our Origins
368Paperback(Reprint)
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Overview
Inspired by a lifelong fascination with all things prehistoric, and gifted with a unique artistic vision, Gurche has studied fossil remains, comparative ape and human anatomy, and forensic reconstruction for over three decades. His artworks appear in world-class museums and publications ranging from National Geographic to the journal Science, and he is widely known for his contributions to Steven Spielberg’s Jurassic Park and a number of acclaimed television specials. For the Smithsonian Institution’s groundbreaking David H. Koch Hall of Human Origins, opened in 2010, Gurche created fifteen sculptures representing six million years of human history. In Shaping Humanity he relates how he worked with a team of scientists to depict human evolution in sculpture for the new hall. He reveals the debates and brainstorming that surround these often controversial depictions, and along the way he enriches our awareness of the various paths of human evolution and humanity’s stunning uniqueness in the history of life on Earth.
Product Details
| ISBN-13: | 9780300216844 |
|---|---|
| Publisher: | Yale University Press |
| Publication date: | 08/05/2015 |
| Edition description: | Reprint |
| Pages: | 368 |
| Product dimensions: | 8.30(w) x 9.70(h) x 1.00(d) |
About the Author
Read an Excerpt
Shaping Humanity
How Science, Art, and Imagination Help Us Understand Our Origins
By John Gurche
Yale UNIVERSITY PRESS
Copyright © 2013 John GurcheAll rights reserved.
ISBN: 978-0-300-18202-6
CHAPTER 1
BEGINNINGS
Sahelanthropus tchadensis (6 to 7 million years ago)
Discovery
There is a windy desert in central Africa where blowing sand sometimes reduces visibility to almost nothing. People, if they can be seen at all, appear only as dim shapes at such times. With increasing distance, the shapes become fainter, until at last they disappear entirely. In July 2001, this inhospitable desert yielded a secret. It was a primate skull, about the size of a chimpanzee's, capped by black manganese–stained sediment like a bizarre mop of hair. It was found in what is now northern Chad by members of a French/Chadian team, among fossils from animal species known from other sites to have lived between six and seven million years ago, and so was older than any known human fossil.
The skull seemed to be unique, exhibiting an odd mix of features not seen in any previously known specimen. It appeared apelike in many ways, possessing a small brain, an inclined plane on the base of the skull where neck muscles once attached, and a large brow ridge like a gorilla's. Two of its features in particular caught the attention of the discovery team, as they were unlike those of apes. The foramen magnum, the hole where the spinal cord emerges from the base of the skull, appeared to be farther forward than its position in apes, which is toward the back of the skull. This feature suggested to the team that the head was balanced on a more vertical neck, as it is in upright-walking humans. Secondly, the canine teeth were small; they were of a reasonable size perhaps for a female ape, but the massive brow ridge suggested that the skull was from a male. A reduction in the size of the canine teeth in males is a humanlike characteristic. If this were a male, the team reasoned, such a reduced canine tooth pointed to a close relationship with humans. The discovery team saw indications that the canine teeth were functionally different than those of apes. The canine teeth of this skull are worn blunt at the tips as in humans, unlike the sharpening wear produced on the upper canine teeth of chimpanzees and gorillas as they hone against the lower premolars.
These features are characteristic of hominins (animals that are more closely related to humans than to our closest living relatives, the chimpanzees). Could this new skull be from a human ancestor, so far back in time? The age of the find is very close to the time when the human and chimpanzee lineages diverged, as indicated by genetic studies. It is twice as old as the celebrated fossil "Lucy," and more than two million years older than the oldest known australopith (an informal name for a group consisting of members of the genus Australopithecus and those of Paranthropus). Even a casual glance at the skull confirms that it lacks the large jaws and chewing teeth which characterize australopiths. It was apparent to the team that this skull represents something else entirely. Based on the small canine teeth, the forward position of the foramen magnum, and a number of traits resembling later hominins, a thirty-eight-member international team, led by the Université de Poitiers' Michael Brunet, proposed in a 2002 article in Nature that it represents a new species of human ancestor. They called it Sahelanthropus tchadensis, which translates as "Man from the Sahel Region of Chad."
The Debate
There was bound to be controversy. As we pursue the shapes of our ancestors back through time, they become less distinct. At present we see only dim shapes of those that might be our earliest ancestors and, peering further back into the time of our last common ancestor with the chimpanzee lineage, the shapes disappear entirely; visibility has been reduced to nothing. Inferences have been made about this last common ancestor from the study of later species, but it has yet to be discovered.
As we travel back in time, approaching that common ancestor, the hominins we meet will have ever greater resemblances to it. If we could trace the chimpanzee line back similarly, the same would apply, so that members of the two lineages would increasingly resemble each other the further back we go until, as we reach the common ancestor, they leap into one shape. This means that the early members of both lineages will be difficult to tell apart. Anatomical signals that indicate that an ancestor was on the hominin side of the split, or on the chimpanzee side, will be fainter and more debatable.
In the case of Sahelanthropus, the announcement suggesting hominin status for the find met with immediate controversy. Was the foramen magnum in this cranium (meaning the skull minus the lower jawbone), major pieces of which had been moved out of alignment during the fossilization process, really far enough forward to indicate a more vertically held neck? Were the canine teeth small enough to qualify Sahelanthropus as a hominin? Three months after the announcement of Sahelanthropus, a team of four authors published a brief letter to Nature titled "Sahelanthropus or Sahelpithecus?" which roughly translates as "Man of the Sahel or Ape of the Sahel?"
These authors pointed out a number of primitive features in which Sahelanthropus resembles apes and not hominins. They questioned, among other things, the original team's methods of assessing the foramen magnum position in the Sahelanthropus skull. They also judged the plane where neck muscles once attached to be angled toward the rear of the skull like that of an ape, instead of more horizontal like that of most hominins. To them, the pattern of crests indicated in the initial description and photographs of the skull appeared to be more like the pattern of a small, quadrupedal ape with powerful chewing muscles.
The critics noted that the one measurement of the upper canine tooth given in the paper announcing Sahelanthropus fits within the ranges for both chimpanzees and female gorillas, and is near the chimpanzee mean. To these authors, part of the wear pattern originally described for the canine of Sahelanthropus sounded like honing wear typical of apes. They suggested that, in any case, if the tooth is small in some dimensions it is more likely to be because the skull is that of a female ape. Although male apes and humans have, on average, larger brow ridges than females (and those of the Sahelanthropus skull were reported to be thicker than the entire range for gorillas of much larger body size), the critics considered canine size to be a more reliable indicator of sex than brow ridge size, and concluded that female status for the skull cannot be ruled out.
It's worth noting that two of the four authors of this letter disputing hominin status for Sahelanthropus are members of a team that had found, a year before the Sahelanthropus skull was discovered, a set of six-million-year-old remains that they named Orrorin tugenensis and proposed as the oldest known hominin. Assuming both finds are hominins, theirs could claim the title of "oldest hominin" for a period of exactly sixteen months before it was unseated by Sahelanthropus. This doesn't invalidate their claims against Sahelanthropus's proposed hominin status, but it does identify a potential source of vested interest.
The other two authors of the paper were the University of Michigan's Milford Wolpoff, and his former student John Hawks, now of the University of Wisconsin at Madison. Their involvement in the discussion brings up a debate that has had a strong effect on the field of human origins. Should the fossil record of human ancestors be divided into fewer species, representing a simple family tree with only a few branches, or is it more appropriate to divide it into many species, representing a bushier tree with many branches? Milford and some of his students see the human tree as a simpler one. Since they tend to "lump" fossils into fewer species, they are often referred to as "lumpers." Each species they recognize incorporates a significant amount of physical variation, some of which the other camp sees as too great to fit within the range of one species. Since this latter group tends to split the fossil record into a greater number of species, this school is often referred to as "splitters."
This difference in ways of seeing the fossil record has had a large influence on many of the debates in paleoanthropology. When a new species has been named for a fossil, the lumpers are more likely to view it as a representative of an existing species. For them the differences others are seeing between the new fossil and previously known forms may be taxonomically insignificant, being on the order of individual variation within a living primate species. Or the differences may be the result of a pathology or distortion during fossilization. Most of the issues debated for the species represented in this book are touched in one way or another by this difference of opinion.
In the case of Sahelanthropus, removing it from the hominins has the effect of reducing the number of species that are candidates for human ancestry. Others have suggested that all three genera proposed as earliest hominins may in fact represent a single genus. Either possibility would simplify the family tree of humans and their relatives.
The debate began to heat up. In a reply to their critics, Brunet and colleagues chided them for using primitive characters to elucidate relationships, as it is now standard practice to use derived (newly evolved) characters which are uniquely shared instead. They pointed out that some primitive characters would be expected in a hominin of this antiquity. The critics were mistaken in their assessment of an angled plane for the neck muscles; when distortion is removed the angle is outside the range of chimpanzees and within the range of early hominins. The reply restated the original team's claim that in relative size, morphology (form), and wear, the canine tooth is evolved in a hominin direction.
A stronger case was made in 2005 for Sahelanthropus having the kind of vertically positioned neck associated with upright walking. A Swiss team, Christoff Zollikofer and Marcia Ponce de León of the University of Zurich, made high resolution scans of the skull and digitally separated the various pieces that had been pushed out of alignment during fossilization, then restored them to their original position. The virtual skull that resulted has a foramen magnum that is clearly at the base of the skull, facing downward, rather than one farther back which faces more rearward as in chimps, gorillas, and other apes. The reconstructed skull also features a plane for the attachment of neck muscles that is less angled than in the original fossil, so that it is more similar to hominins than to living African apes. And, as if all of this was not already confusing enough, the reconstruction revised the brow ridge thickness by subtracting two millimeters. This put it within the size range of both male and female gorillas, making sexual attribution for the skull much less certain. If the skull is that of a female, canine size can no longer be used as a hominin character for it.
Papers from both teams came out during the next two years. A geometric shape analysis performed by the supporters of Sahelanthropus's hominin status found that the skull sorted with those of hominins and not with recent African apes. The critics pointed out the extensive overlap in the size of the brow ridges in male and female apes, and again questioned the proposed male status for the skull. They argued that females of several species of Miocene ape have canine teeth with size and wear similar to those described for Sahelanthropus, including wear at the tips and poorly developed canine/premolar honing (sharpening).
They also maintained that, although primitive features of the neck muscles' plane are of limited use in determining relationships, they are informative about posture and locomotion. In their view, features of the long, somewhat angled plane reflect unusually powerful neck muscles, used in a way that differs from known hominins. They saw no clear indication of bipedal locomotion (walking on two legs).
As I began the reconstruction of Sahelanthropus, these issues were not resolved. I did not necessarily need them to be resolved, as my job had less to do with Sahelanthropus's relationships than it did with interpretation of the skull regarding implications for soft tissue anatomy. I base my estimation of soft tissue anatomy on anatomical clues whenever possible and try to avoid using the weaker basis of taxonomic classification or phylogenetic relationships, so that if Sahelanthropus is thrown out of the hominins tomorrow, the reconstruction will remain the same. The Smithsonian team and I decided to accept the skull as reconstructed by the Swiss team as the basis for my reconstruction.
Later, when the head reconstruction was in progress, a 2011 article in Nature blew the whole argument into the stratosphere. George Washington University's Bernard Wood and New York University's Terry Harrison suggested that all bets were off in determining hominin status for Sahelanthropus and for three other early hominin candidates. They emphasized that a number of late Miocene apes also developed small canine teeth with reduced canine/premolar honing, which they suggested is related to diet. They also named several nonbipedal species of living primates which have a forward-positioned foramen magnum and suggested that this feature is related to differences in facial length and carriage of the head and can't be used as an indicator of bipedal walking. In addition, they questioned the postcranial (below the head) evidence for bipedalism in the three other species proposed to be among the earliest hominins.
Can there be any resolution to these debates? I have included details of this discussion to show how difficult it can be, even for experts, to decide on the hominin or ape status of a fossil this close in time to the splitting of human and chimpanzee lineages. There may not be consensus either on this or on the issue of upright walking for these species until more postcranial bones are found and analyzed in scientific publications. Many researchers have provisionally accepted Sahelanthropus as a human ancestor that lived between six and seven million years ago, not long after the last common ancestor of chimpanzees and humans. Most are awaiting the discovery of more remains before committing more solidly.
The Plateau
There are now three candidates for oldest hominin: Ardipithecus kadabba (which has been dated to 5.8 million years old), Orrorin tugenensis (6 million years old), and Sahelanthropus tchadensis (between 6 and 7 million years old). The first two of these species are known only from teeth, jaws, and postcranial bones. In both cases they are argued to be upright walkers based on features of the hind limb bones. All three feature canine teeth that are reduced, at least in comparison to those of male apes, and show reduced honing function.
Some anthropologists believe that these three represent a common adaptive plateau (a broad grouping of related organisms with similar adaptations), the first for hominins after their last common ancestor with chimpanzees. Inferences about this plateau have been made by Berkeley's Tim White and colleagues based on a 2009 study of the fairly complete skeleton of a later hominin: Ardipithecus ramidus—"Ardi"—dated to 4.4 million years ago. If its discoverers are correct, this adaptive plateau included the first changes in the evolutionary development of a biped for a creature that was still a quadruped in the trees. The reconstructed pelvis suggests that these changes included the evolution of a vertically short pelvis with blades that faced somewhat to the side. In human locomotion, this orientation allows muscles of the hip to stabilize the trunk over the standing leg while the other leg leaves the ground to swing forward.
Although no vertebrae from Ardi's lower spine were found, the team inferred that the skeleton had a long lumbar (lower back) section of the spine with a humanlike backward curve, allowing the positioning of the upper body's center of gravity over the hips, farther back than is possible with a great ape's spine, which is short, strait, and stiff in this area. Other areas of the Ardi skeleton, especially the foot, are still very apelike, and some have questioned the anatomical basis for Ardipithecus's proposed status as a biped.
(Continues...)
Excerpted from Shaping Humanity by John Gurche. Copyright © 2013 John Gurche. Excerpted by permission of Yale UNIVERSITY PRESS.
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Table of Contents
Contents
Preface, xi,Acknowledgments, xv,
Prologue, 1,
Chapter 1. Beginnings: Sahelanthropus tchadensis (6 to 7 million years ago), 7,
Chapter 2. Walkers and Climbers: Australopithecus afarensis (3.6 to 2.9 million years ago), 31,
Chapter 3. The Impossible Discovery: Australopithecus africanus (3.3 to 2.1 million years ago), 69,
Chapter 4. The Paradoxical Specialist: Paranthropus boisei (2.3 to 1.0 million years ago), 87,
Chapter 5. The Traveler: Homo erectus (1.8 to 0.1 million years ago), 147,
Chapter 6. A Symbolic Animal: Homo heidelbergensis (0.7 to 0.2 million years ago), 193,
Chapter 7. The Other: Homo neanderthalensis (0.25 to 0.027 million years ago), 219,
Chapter 8. The Unlikely Survivor: Homo floresiensis (0.095 to 0.017 million years ago), 265,
Chapter 9. Linked: Homo sapiens (0.2 million years ago–?), 297,
Chapter 10. Endings, 309,
Notes, 319,
Glossary, 323,
Bibliography, 327,
Index, 339,