The Leap to Two Feet: The Sudden Appearance of Bipedalism
Did man crawl his way into existence over millions of years? Or did he leap to two feet by supernatural design? Did humans emerge from amoebas or did a Creator intend for life to possess purpose, value, and meaning? Answers to such questions mightily impact how human societies respond to their most pressing problems. A divinely-designed, sentient, spiritual creature deserves greater care and consideration than does a random fluke of nature.
Some scientists say that human beings are a mere quirk of fate—intelligent apes produced by chance events taking place over the last 5 million years. They claim natural selection played a role in the process that led to modern humans. Competitive, predatory, and environmental pressures gradually selected inheritable changes, they say. These changes supposedly imparted increased survivability and reproductive success. Thus, natural selection would have operated on random variation again and again, producing a succession of new species until finally by chance modern humans came to be.
Along with large brain size (actually, brain size-to-body mass ratio), manual dexterity, and advanced culture, bipedalism constitutes one of the most important defining characteristics of humans. For evolutionary anthropologists, understanding the emergence and development of bipedalism equates with understanding the origin of humanity.
In sharp contrast to evolutionary thinking, the Bible reveals human beings as the pinnacle of God’s creative activity, made in His image and distinct from all other creatures.1 Biblical accounts of man’s beginnings leave no room for God’s using an ape-to-human evolutionary transformation process to create man. Scripture describes God’s direct involvement in creating the first humans, physical and spiritual creatures of immense worth from the time of their inception.
With its focus on testability,2 a powerful new approach helps discriminate between the biblical and evolutionary explanations for the origin of humanity (see sidebars). Predictions made by these origin models can be subjected to the rigors of scientific testing. The one with the greatest support from the scientific record and with predictions that best accommodate new discoveries exemplifies the most accurate scenario.
Recent advances in paleoanthropology (the study of the bipedal primate fossil record) and the paleoecology (study of ancient ecologies) associated with bipedalism present an unusual opportunity to make data comparisons. New discoveries in these, as well as other disciplines, argue against a naturalistic origin to bipedalism and provide substantial affirmation for the biblical record.
According to the evolutionary paradigm, since an ape-like ancestor gave rise to both the ape and the human lineages, bipedal primates must have evolved from knuckle-walking quadrupeds. Knuckle-walking exists as a special type of terrestrial quadrupedalism (ground-based locomotion employing all four limbs) possessed by chimpanzees and gorillas.3 Knuckle-walkers rest their weight on their knuckles, not on their palms or fingers. This design allows chimpanzees and gorillas to walk on all four limbs while still having the dexterity of long curved fingers for climbing and swinging through trees.
Paleoanthropologists propose a myriad of hypotheses to explain how bipedalism could arise from natural processes. One early explanation suggests bipedalism emerged to free the hands for tool use. Since the fossil record contradicts this notion, evolutionary biologists have rejected this idea. The archeological record clearly shows the existence of bipedalism at least 2 million years before tool use appeared.4
Most hypotheses seeking to account for bipedalism’s emergence depend on East Africa’s transformation from a woodland and forest environment to an arid, open savanna.5 With such changes, terrestrial quadrupeds faced reduced food supplies, increased risk of falling prey to predators, and the inability to avoid direct sunlight.6
Bipedalism offers a way to address these challenges. Walking erect served as a more energy efficient means of locomotion at slow speeds.7 This allowed bipedal primates to traverse long distances foraging for food. Once having found food, bipedal primates could carry the foraged food long distances as they returned to their “home-base” to provide for their young.8
By the height of their heads, bipedal primates are more effective at avoiding predation in an open savanna than quadrupedal apes. Standing erect would allow these animals to detect predators sooner and from greater distances.
Bipedalism also offers a thermoregulatory advantage.9 A bipedal primate standing upright absorbs 60 percent less heat than does an ape walking on all four limbs. A quadrupedal stance exposes the entire back to direct sunlight, whereas standing erect exposes only the head and shoulders.
Evolutionary biologists have yet to reach a consensus on the selective pressures that could have produced bipedalism in primates, nor have they demonstrated a mechanism that can bring about such dramatic changes in the time permitted (see the following section). To date, the only reasonable source of evolutionary pressure behind the four-to-two transformation remains the loss of a woodland habitat throughout East Africa.
Anatomy of Bipedalism
To transition from a knuckle-walking quadruped to an upright biped involves extensive anatomical changes.10 These changes include the following:
- Relocation of the spinal cord opening
The foramen magnum (the opening in the base of the skull that receives the spinal cord) must be relocated from the back to the center of the skull base. In this position the vertebral column effectively balances the head, eliminating the need for powerful neck muscles.
- Restructuring of the inner ear bones
The inner ear bones, which play a role in balance, must be altered to support bipedalism.
- Introduction of spinal curvature
The lower and upper vertebral column must possess forward curvature to maintain bipedalism. This forward curvature coupled with the backward curvature in the middle of the spinal column allows the backbone to function as a spring, facilitating movement.
- Restructuring of the rib cage
Apes’ inverted funnel-shaped rib cage accommodates arm use for locomotion. The barrel-shaped rib cage of bipeds permits effective use of the arms for nonlocomotory functions.
- Reshaping the pelvis
To accommodate the hip joints and muscles necessary for bipedalism, the pelvis of bipedal primates must be lower and broader than that of knuckle-walking apes.
- Altered lower limbs
Bipedal primates not only have longer lower limbs than quadrupeds, the valgus angle (the angle that the femur makes with the midline of the body) is also different. Longer lower limbs shift the center of mass towards the lower body. Angling the femurs inward moves the center of mass closer to the midline of the body. The altered center of mass allows stable bipedal locomotion.
- Enlarged joint surfaces
Not only does the knee need to be restructured to accommodate the changed valgus angle, but joint surfaces must also be enlarged. This enlargement increases the contact area, helping the knee and other joints withstand the stress of standing or walking upright.
- Restructured foot
Even the feet must be structured differently to support bipedalism. A platform foot with an arch allows for a greater surface area, one that can better withstand shock. In bipedal primates, the big toe is more elongated and aligned with the other toes and, thus, needs a different location. This new placement allows the toe to make the last point of contact with the ground as the leg swings forward during a bipedal stride.
- Restructuring of the body’s musculature
In order to accommodate the extensive skeletal changes required by the transition from a quadruped to a biped, much of the musculature must also be altered.
The dramatic anatomical changes that must occur to transform knuckle-walking quadrupeds to bipedal primates thwart efforts to envision how this transformation could take place. Nevertheless, if bipedalism did emerge through natural-process biological evolution, it should occur gradually and appear well after the time that apes and humans are supposed to have diverged. Moreover, the first form of bipedalism to appear should be crude and inefficient. Once appearing, it should gradually transition to the more efficient obligatory bipedalism of modern humans. Lastly, significant evolutionary pressure would be necessary to force knuckle-walking apes, perfectly suited for their environment and lifestyle, to change into upright walking primates, if such change actually could occur.
Recent Scientific Advances
Several recent discoveries from the fossil and geological records have radically transformed paleoanthropologists’ view of the origin and natural history of bipedalism. These new scientific advances sharply contradict predictions stemming from evolutionary scenarios.
Bipedalism’s First Appearance
In 1994 and 1995 paleoanthropologists reported two sets of discoveries that described the fossil remains of two species of australopithecines. One research team uncovered the remains of a hominid in Ethiopia dated at 4.4 million years in age.11 This specimen they named Australopithecus ramidus, though it was later reassigned to a new genus, Ardipithecus.12
Meanwhile, another team of researchers discovered a set of hominid fossils in Kenya determined to be between 3.9 and 4.2 million years in age.13 These specimens were attributed to a newly recognized australopithecine species, Australopithecus anamensis. A follow-up discovery confirmed the date for this species at 4.07 million years ago.14 Analysis of an A. anamensis tibia clearly established its bipedal capacity, pushing the appearance of bipedalism back by at least a half a million years. Prior to this discovery the oldest primate with bipedal capabilities was believed to be Australopithecus afarensis (~3.9 million years ago).
It is still not clear if Ardipithecus ramidus possessed bipedal capabilities. If so, bipedalism’s first appearance occurs very close to the time that the ape and human lineages supposedly split. This allows the forces of natural selection only a few hundred thousand years to generate bipedalism—a time period far too short, according to evolutionary biologists, given the extensive anatomical changes necessary for a quadrupedalism-to-bipedalism transition.
If A. ramidus lacked bipedal capabilities, this too creates problems for the evolutionary paradigm. Evolutionary biologists view A. ramidus as the ancestral species that gave rise to A. anamensis. In this scenario, bipedalism must have emerged in less than two hundred thousand years—an even shorter (hence less feasible) time period for the enormous species’ differentiation to occur.
Paleoecology of Bipedalism
Recent work characterizing the environment in which the oldest bipedal primates lived yielded unexpected results. A. ramidus and A. anamensis did not live in open savannas, but rather in woodlands and forests.15 Moreover, recent studies indicate that A. afarensis lived in a mix of woodland and open savanna environments.16
A newly discovered australopithecine species, Australopithecus bahrelghazali, recovered in Chad and dated to be between 3.0 and 3.5 million years in age also lived in a mixed habitat.17 And the newly discovered hominid specimen, Kenyanthropus platyops, dated at 3.5 million years in age, lived in a predominantly woodland and forest environment that included open grasslands.18
In the words of anthropologist and science writer Roger Lewin, “The popular notion of our forebears striding out of dense forest onto grassland savanna is likely to be more fiction than fact.”19 This new recognition, expressed by Lewin, creates profound trouble for the evolutionary paradigm, eliminating the evolutionary driving force long predicted to have generated bipedalism.
A recent geological study conducted to understand the aridification of East Africa—the event that caused a transformation of its woodlands into an open savanna—provides further evidence that the loss of a woodland environment could not have been the driving force in the emergence of bipedalism. This study indicates that the closure of the Indonesian seaway 3-4 million years ago led to reduced rainfall in East Africa and eventually to the transition from woodlands to grasslands.20 By the time East Africa became arid, bipedalism had already appeared.
A recent mathematical and statistical analysis of over two hundred pelvic bone specimens from apes, extinct hominids, and modern humans uncovered a historical pattern that challenges the evolutionary explanation of bipedalism at its core.21 Instead of gradually changing over time, bipedalism appeared suddenly, remained static (unchanged) for a long period of time, then underwent rapid transformation before again remaining static and undergoing another rapid change.
Australopithecines, the first bipedal primates, possessed a form of bipedalism distinct from that of the Homo primates. Australopithecines displayed facultative (optional) bipedalism whereas the Homo genus possessed and continues to possess obligatory bipedalism. Though australopithecines existed for nearly 3 million years, their bipedalism did not gradually change into the obligatory bipedalism of the Homo primates. Rather, it remained static throughout the duration of the australopithecine’s existence.
With the appearance of the Homo genus, obligatory bipedalism suddenly appeared in the fossil record. From an evolutionary perspective, this quick change demanded a rapid transition process from facultative to obligatory bipedalism. Obligatory bipedalism in the Homo genus has remained static for nearly 2 million years. Interestingly, Homo erectus and Neandertals possessed an identical form of bipedalism, but a form distinct from that seen in modern humans. With the appearance of modern humans, yet another form of bipedalism suddenly appeared and has continued since its introduction.
While the pattern of stasis punctuated by sudden change seen in the fossil record runs counter to evolutionary expectations, it serves as a clear indicator of God’s creative activity. If God created the australopithecines, the Homo bipedal primates, and other similar genera—a prediction can be made that the bipedalism possessed by each genus should be optimal within the context of its respective environment and lifestyle. Once created, natural selection would be expected to keep each type of bipedalism static, since any change would result in a nonoptimal form of bipedalism, compromising fitness.22 Moreover, given the differences in lifestyle and environment, it readily follows from a creation model perspective that God would create the australopithecines and Homo primates with different forms of bipedalism, as observed in the fossil record.
Recent scientific advances in the natural history of bipedalism provide a useful collection of observations that allow evaluation of both evolutionary and biblical scenarios for the origin of humanity. The sudden and early appearance of bipedalism in the fossil records allows insufficient time for bipedalism to emerge through natural process biological evolution. The fossil record also fails to reveal a pattern of gradual transformation from rudimentary bipedalism to a more sophisticated, efficient form. The absence of any significant evolutionary pressure to force these changes makes them even more remarkable.
A sudden and early appearance with two periods of stasis interspersed by rapid change defines bipedalism’s natural history. These characteristics perfectly match the pattern special creation would predict.
A biblical creation model, in which God creates large bipedal primates, predicts long periods of stasis; a perfect Creator could be expected to bring about a form of bipedalism ideally suited for His creatures’ environmental, predatory, and competitive challenges. The recent scientific discoveries provide explicit evidence that one of the most important defining features of humanity—bipedalism—came about through God’s direct creative activity. Though not human, bipedal primates were designed for a specific purpose and function. They were the handiwork of a Creator.
With the evidence of such care toward bipedal primates, the prestige of human beings, uniquely created in the image of God, takes on tremendous significance. Being purposefully created human by a God who cares makes a person’s life worth living. A society that understands such implications can extend value, meaning, and purpose to its people. And that understanding makes the discoveries related to a leap to two feet priceless.
- Genesis 1:26-27; Genesis 2:7; Genesis 2:22; Mark 10:6; Matthew 19:4; Psalm 8:4-5.
- Hugh Ross, “Can Science Test a ‘God-Created-It’ Model? Yes!” Facts for Faith (Q2 2000), 40-47; 55-58.
- John G. Fleagle, “Primate Locomotion and Posture,” in The Cambridge Encyclopedia of Human Evolution, paperback edition, ed. Steve Jones, Robert Martin, and David Pilbeam (New York: Cambridge University Press, 1994), 75-85.
- Eric Delson et al., eds., Encyclopedia of Human Evolution and Prehistory, 2d ed. (New York: Garland Publishing, 2000), 394-95; B. Bower, “African Fossils Flesh Out Humanity’s Past,” Science News 155 (1999), 262; Elizabeth Culotta, “A New Human Ancestor?” Science 284 (1999), 572-73; Jean de Heinzelin et al., “Environment and Behavior of 2.5 Million-Year-Old Bouri Hominids,” Science 284 (1999), 625-29; Berhane Asfaw et al., “Australopithecus garhi: A New Species of Early Hominid from Ethiopia,” Science 284 (1999), 629-35.
- Roger Lewin, Principles of Human Evolution: A Core Textbook (Malden, MA: Blackwell Science, 1998), 219-22.
- Lewin, 227.
- Lewin, 224-26.
- Fleagle, 75-78.
- Lewin, 227.
- Lewin, 218; Robert Martin, “Walking on Two Legs,” in The Cambridge Encyclopedia of Human Evolution, paperback edition, ed. Steve Jones, Robert Martin, and David Pilbeam (New York: Cambridge University Press, 1994), 78; Fred Spoor et al., “Implications of Early Hominid Labyrinithine Morphology for Evolution of Human Bipedal Locomotion,” Nature 369 (1994), 645-49.
- Tim D. White et al., “Australopithecus ramidus, a New Species of Early Hominid from Aramis, Ethiopia,” Nature 371 (1994), 306-12; Henry Gee, “New Hominid Remains Found in Ethiopia,” Nature 373 (1995), 272.
- Tim D. White et al., “Corrigendum,” Nature 375 (1995), 88.
- Meave G. Leakey et al., “New Four-Million-Year-Old Hominid Species from Kanapoi and Allie Bay, Kenya,” Nature 376 (1995), 565-71.
- Meave G. Leakey et al., “New Specimens and Confirmation of an Early Age for Australopithecus anamensis,” Nature 393 (1998), 62-66; B. Bower, “Early Hominid Rises Again,” Science News 153 (1998), 315.
- Meave Leakey and Alan Walker, “Early Hominid Fossils from Africa,” Scientific American (June 1997), 74-79; Clark Spencer Larsen, Robert M. Matter and Daniel L. Gebo, Human Origins: The Fossil Record, 3d ed. (Prospect Heights, IL: Waveland Press, 1998), 46.
- Lewin, 258; 266-69.
- Michel Brunet et al., “The First Australopithecine 2,500 Kilometers West of the Rift Valley (Chad),” Nature 378 (1995), 273-75.
- Meave G. Leakey et al., “New Hominid Genus from Eastern Africa Shows Diverse Middle Pliocene Lineages,” Nature 410 (2001), 433-40.
- Lewin, 222.
- Mark A. Cane and Peter Molnar, “Closing of the Indonesian Seaway as a Precursor to East Africa Aridification Around 3-4 Million Years Ago,” Nature 411 (2001), 157-62.
- François Marchal, “A New Morphometric Analysis of the Hominid Pelvis Bone,” Journal of Human Evolution 38 (2000): 347-65.
- Niles Eldredge, Reinventing Darwin: The Great Debate at the High Table of Evolutionary Theory (New York: John Wiley, 1995), 78-81.
Sidebar: The Evolutionary Perspective of Human Origins
Fazale R. Rana
Current models for human evolution describe modern humans as gradually emerging from more primitive “hominids” (members of the primate family Hominidae) through descent with modification via natural selection and mutations. Evolutionary biologists think this process began around 5 million years ago when hominids and apes supposedly diverged from a shared ape-like ancestor.1
Australopithecines occur in the fossil record between 4.5 to 1.5 million years ago as the first bipedal primates.2 The genus Australopithecus encompasses a diverse group of hominids with ape-size brains; ape-like cranial, facial, and dental features; an ape-like torso and upper limbs; and limited bipedal capabilities distinct from modern humans.3
Until recently, paleoanthropologists viewed the australopithecines as part of the evolutionary pathway leading to modern humans. Controversy now centers around the role of australopithecines in human origins with the discovery of a new hominid genus, Kenyanthropus, dated at 3.5 million years in age.4 Some paleoanthropologists suggest that Kenyanthropus gave rise to Homo bipedal primates.5
No consensus view exists among paleoanthropologists to describe the evolutionary relationships among (extinct) australopithecines and a closely related genus, Paranthropus (originally considered “robust australopithecines”).6 However, the latter’s relatively large size and other unique, distinguishing features prompted paleoanthropologists to reclassify the robust australopithecines as a separate genus. Paleoanthropologists view Paranthropus as an evolutionary dead end. Given the bewildering array of species, paleoanthropologists are unclear as to which of the australopithecines could have given rise to the Homo genus of bipedal primates.
Homo bipedal primates first appear in the fossil record about 2 million years ago. Traditionally Homo habilis was regarded as the first Homo bipedal primate and the key transitional species linking the australopithecines to the Homo genus. However, newly recognized features (features more closely aligned with those of the australopithecines than with those of other Homo bipedal primates such as Homo erectus), caused H. habilis to be reclassified as an australopithecine.7 This new understanding seriously weakens the position of H. habilis as a transitional species, thus leaving a discontinuity in the hominid phylogeny.
Homo erectus and Homo neandertalensis are the two bipedal primates that have been most closely linked to modern humans. However, recent work has all but severed the link between modern humans and H. erectus, and has completely cut the connection between Neandertals and modern humans.8 Paleoanthropologists increasingly regard H. erectus as representing a side branch that resulted in an evolutionary dead end, since this bipedal primate was confined to Asia, and analysis of DNA isolated from three distinct Neandertal remains all indicate that Neandertals made no contribution to human genetic makeup.
As with the australopithecines, a menagerie of Homo bipedal primates existed for most of the last 2 million years. Paleoanthropologists, unable to reach a consensus on the evolutionary relationships among the members of the Homo genus, have been unable to identify a direct ancestor to modern humans.9 Nevertheless, many evolutionary biologists are convinced these relationships exist and the missing ancestor of modern humans will someday be discovered.
- Richard Morris, The Evolutionists: The Struggle for Darwin’s Soul (New York: W. H. Freeman, 2001), 34-37.
- B. A. Wood, “Evolution of Australopithecines,” in The Cambridge Encyclopedia of Human Evolution, paperback edition, ed. Steve Jones, Robert Martin and David Pilbeam (New York: Cambridge University Press, 1994), 231-240.
- Roger Lewin, Principles of Human Evolution: A Core Textbook (Malden, MA: Blackwell Science, 1998), 241-282.
- Meave G. Leakey et al., “New Hominid Genus from Eastern Africa Shows Diverse Middle Pliocene Lineages,” Nature 410 (2001), 433-40; Daniel E. Lieberman, “Another Face in Our Family Tree,” Nature 410 (2001), 419-20.
- B. Bower, “Fossil Skull Diversifies Family Tree,” Science News 159 (2001), 180.
- Lewin, 297-307.
- Bernard Wood and Mark Collard, “The Human Genus,” Science 284 (1999), 65-71; B. Bower, “Redrawing the Human Line,” Science News 155 (1999), 267.
- J. M. Bermudez de Castro et al., “A Hominid from the Lower Pleistocene of Atapuereca, Spain: Possible Ancestors to Neandertals and Modern Humans,” Science 276 (1997), 1392-95; Ann Gibbons, “A New Face for Human Ancestors,” Science 276 (1997), 1331-33; Fuz Rana, “Up (and Away) from the Apes,” Connections 1, no. 2 (2000), 3-4; Hugh Ross, “Neandertal Takes a One-Eighty,” Facts & Faith 11, no. 3 (1997), 4-5; Fazale R. Rana, “DNA Study Cuts Link With The Past,” Connections 2, no. 3 (2000), 3; Fazale R. Rana, “Neanderthal Genetic Diversity: From Missing Link to Special Creation,” Facts for Faith (Q4 2000), 5.
- Lewin, 385-428.
Sidebar: Biblical Perspective on the Hominids
Fazale R. Rana
If humans are made in the image of God through His direct creative activity, then what is the proper biblical perspective on hominids or bipedal primates? The biblical model employed here views bipedal primates as separate species that have gone extinct since their creation. The genera Australopithecus, Kenyanthropus, and Paranthropus—all ape-like creatures—possessed limited intelligence, limited bipedal capability and, in some cases, extremely crude tools. The bipedal primates assigned to the Homo genus, such as Homo erectus and Homo neandertalensis walked upright, used crude tools, possessed intelligence and perhaps even emotional capacity, yet they were devoid of spiritual capacity and, therefore, must be regarded as distinct from modern humans.1
Bipedal primates were not created in the image of God. Paleoanthropologists have no indication from the archeological record that Neandertals, or any bipedal primates, engaged in religious activity.2 Although the Homo bipedal primates used tools, they were crude and qualitatively distinct from the sophisticated tools used by modern humans.3 Neandertals, in all likelihood did not possess language capacity.4 Genesis 1 makes no specific allusion to bipedal primates. Their creation by God on Days 5 and 6 in the group of nephesh or animals endowed with will, emotion, and intelligence can be inferred.
- Hugh Ross, The Genesis Question (Colorado Springs, CO: NavPress, 1998), 54-55; 110.
- Eric Delson et al., eds., Encyclopedia of Human Evolution and Prehistory, 2d ed. (New York: Garland Publishing, 2000), 615-17.
- Tom Clarke, “Relics: Early Modern Humans Won Hand Over Fist,” Nature Science Update, (6 February 2001); Steven E. Churchill, “Hand Morphology, Manipulation, and Tool Use in Neandertals and Early Modern Humans of the Near East,” Proceedings of the National Academy of Sciences, USA 98 (2001): 2953-55; Wesley A. Niewoehner, “Behavioral Inferences from the Skhul/Qafzeh Early Modern Human Hand Remains,” Proceedings of the National Academy of Sciences, USA 98 (2001): 2979-84.
- Christopher Stringer and Robin McKie, African Exodus: The Origin of Modern Humanity (New York: Heary Holt and Company, 1996), 85-114.
By Fazale R. Rana
During the final publication stages of this article, a team of paleontologists from the University of California, Berkeley reported the discovery of hominid remains dated between 5.2 and 5.8 million years ago and described this animal’s environment.1 The results of their work bolster the case for the supernatural appearance of bipedalism.2
Paleoanthropologists making this discovery assigned the fossil remains to Ardipithecus ramidus. Analysis clearly indicates that A. ramidus walked erect. This dramatic discovery not only pushes the hominid fossil record back by nearly one million years but also places the appearance of bipedalism coincidental to the first appearance of hominids. Bipedalism, indeed, appears suddenly in the fossil record.
The paleoanthropologists also determined that A. ramidus lived exclusively in a wet woodland environment. Likewise, the A. ramidus specimen dated at 4.4 million years in age lived in a wet woodland habitat.3 These discoveries fully eliminate the evolutionary driving force for bipedalism’s emergence. As one researcher commented, these discoveries “challenge some long-cherished ideas about the mode and timing of hominid evolution.”4
- Yohannes Haile-Selassie, “Late Miocene Hominids from the Middle Awash, Ethiopia,” Nature 412 (2001), 178-81; Giday WoldeGabriel et al., “Geology and Paleontology of the Late Miocene Middle Awash Valley, Afar Rift, Ethiopia,” Nature 412 (2001), 175-78.
- Henry Gee, “Return to the Planet of the Apes,” Nature 412 (2001), 131-32; Michael Balter and Ann Gibbons, “Human Evolution: Another Emissary from the Dawn of Humanity,” Science 293 (2001), 187-89.
- Giday WoldeGabriel et al., “Ecological and Temporal Placement of Early Pliocene Hominids at Aramis, Ethiopia,” Nature 371 (1994), 330-33.
- Balter and Gibbons, 187-88.