The Gap

by Thomas Suddendorf

  Australopithecus afarensis may have used stone tools. In 2010 researchers reported 3.39-million-year-old bones with cut marks for the removal of flesh and percussion marks for access to marrow. Their conclusion has been disputed. However, some hominins certainly discovered the power of rocks a few hundred thousand years later. The oldest stone tools are currently about 2.5 million years old and are associated with fossils of yet another species, Australopithecus garhi. This Australopithecine had a cranial capacity of 450 cubic centimeters and was found in the rich Afar Triangle. Cut marks on bones of various ungulates indicate the tools were used to butcher carcasses.

  A variety of other hominins appear to have walked the Earth during the same period. Fossils from the Chad, west of the Great Rift Valley, although resembling afarensis, have been proposed to belong to a different species (Australopithecus bahrelghazali). Meave Leakey and colleagues reported a 3.5-million-year-old fossil with such an unusually flat face that they proposed a new genus: Kenyanthropus platyops (although lumpers reckon it may merely be a squashed Australopithecine). In 2010 two partial skeletons of yet another distinct Australopithecine were found in South Africa. This species has been called Australopithecus sediba and lived as recently as 1.8 million years ago. While there is debate about the exact number of Australopithecine species, it is clear that quite a few existed, all with diverse features. Given the frequency of recent discoveries, several more species likely remain to be found. With overlapping ecological niches, there would have been some competition for resources. Closely related species would have evolved along different lines to exploit different resources (like chimpanzees and gorillas do today).

  Australopithecines eventually diverged along a minimum of two distinct evolutionary paths. Some became increasingly stocky and developed enormous jaws with which they could chew hard foods such as nuts and fibrous vegetation. Once known as “robust” Australopithecines, they are today more commonly classified as the distinct genus Paranthropus. They had large chewing muscles, attached to the top of the skull on a distinctive sagittal crest. Such a crest is absent in humans but prominent in extensive chewers such as gorillas.10 The teeth are large and ideal for grinding. During times of plenty, they probably ate a variety of foods, including animal protein. During times of scarcity they could fall back on hard foods others could not eat.

  FIGURE 11.4.

  Paranthropus boisei (KNM OH 5), “Nutcracker Man,” 1.8 million years old.

  At least three different species of Paranthropus are widely recognized. The earliest one is P. aethiopicus, followed by P. robustus and P. boisei. Paranthropus cranial capacities were similar or slightly larger than those of earlier Australopithecines: P. boisei had an average of 481 cubic centimeters and P. robustus of 563 cubic centimeters. They used their smarts to employ stone and bone tools to dig up tubers. Wear patterns on these tools indicate that they also used them to access foods such as termites. Unlike chimpanzees, who probe for termites with twigs, Paranthropus used bones to break open the mounds. Although they used tools to get food, there is so far no evidence that they changed the shape of these implements.

  P. boisei is one of the greatest success stories of all hominins that ever graced the Earth, surviving for over a million years. It is unclear why they eventually went extinct. One possibility is that their feeding adaptation became increasingly specialized, such that when a radical environmental change affected their main food sources, they were unable to survive. Cyclic glaciations that began some three million years ago increasingly intensified. In such times of instability, rapid adaptability would have been crucial. It is also possible that other hominins contributed to the demise of their cousins. For hundreds of thousands of years Paranthropus lived side-by-side with the earliest creatures science calls humans.

  SOME AUSTRALOPITHECINES BECAME MORE GRACILE and evolved into Homo. The genus Homo has a smaller, less projecting face; a smaller gut; more efficient bipedalism; smaller teeth; and a bigger brain than Australopithecus and Paranthropus. A recent study suggests that a mutation in a gene crucial for masticatory muscles appeared around 2.4 million years ago, leading to a reduction of chewing strength and removing, according to the authors, an evolutionary constraint on the expansion of the cranium and brain.

  The earliest member of the genus Homo is a species Louis Leakey and his colleagues called “handy man” or Homo habilis, because of their association with what were at the time the oldest known stone tools. The oldest fossils of this species are 2.4 million years old; their cranial capacity averaged around 600 cubic centimeters. The stone tools Homo habilis manufactured, called Oldowan tools because they were initially found in Tanzania’s Olduvai Gorge, consisted of modified river cobbles. By removing flakes of a cobble, H. habilis produced sharp-edged tools, effective for cutting hides and removing meat from bone. Bigger stones were used as hammers to crack bones. In this way Homo habilis were able to consume large animals such as antelopes, rhinoceroses, and hippopotamuses.

  Although meat was part of the diet of earlier hominins, just as it is part of the diet of modern chimpanzees, these stone tools would have significantly increased access to this high-energy food source. This is not to say that Homo habilis were efficient hunters. A large part of their diet included plant materials, and much of the meat was probably scavenged. On at least some of the fossil bones other carnivores’ bite marks precede the marks of stone tools. Homo habilis might have competed with scavengers for what predators left behind. They repeatedly moved carcasses to special sites, where they used their stone tools to butcher them. A recent find from a 1.95-million-year-old butchering site from Turkana includes bones from aquatic animals such as turtles, fish, and crocodiles. These animals are rich in nutrients critical for brain growth, leading to speculation that a changing diet played a key role in allowing early Homo to grow bigger brains. Increased cognitive powers, in turn, may have been used to safely and effectively secure high-quality foods.

  Butchering sites show that Homo habilis transported objects sometimes several kilometers from their source; it is possible they also carried tools on their person to be ready for future use. Clubs and stone tools could have been used to strike approaching predators or poachers. It has been argued that early stone tools were of good throwing size and that the hand of early Homo was able to perform the power and precision grips necessary for clubbing and aimed throwing. In other words, these hominins may have been armed.

  Eventually, Homo used weapons to hunt, but it remains unclear when the practice started. Weapons are likely to have played a key role in hominins’ evolution from prey and scavenger into a top predator, and they were undoubtedly used in conflicts within and between hominin groups. Yet while a capacity for precise striking and clubbing is self-evident in Homo habilis—after all, that is how the stone tools were made—it is not clear when hominins began to use aimed throwing in any systematic way. (Incontrovertible evidence for projectile weapons appears much later in the archaeological record.)

  FIGURE 11.5.

  Homo habilis (OH-24), 1.8 million years old.

  One particular fossil dated 1.8 million years ago already had a much larger cranial capacity of 775 cubic centimeters. Some scientists have hence argued that this find deserves a separate species name, Homo rudolfensis, but it remains possible that this is part of normal variation or even sexual dimorphism.

  Homo habilis existed from South Africa to Ethiopia and survived until 1.4 million years ago. By the eastern shores of Lake Turkana they lived side-by-side for several hundred thousand years with another hominin species, one that was ultimately even more successful: Homo erectus.

  SOME SCHOLARS ARGUE THAT THE genus Homo really began with Homo erectus rather than habilis. As adults they stood up to 1.8 meters tall and had large cranial capacities with a mean of 1,000 cubic centimeters—more than twice that of Australopithecines and modern apes. In contrast to Australopithecines, erectus females were not much smaller than their male counterparts. Although they sported a low forehe
ad with massive brows and lacked a chin, Homo erectus looked and walked a lot like modern humans. Their footprints are essentially indistinguishable from ours.

  In 1984 Richard Leakey and colleagues discovered a nearly complete 1.6 million-year-old skeleton of a Homo erectus near Lake Turkana, known as Nariokotome Boy. He stood 1.6 meters tall and had a cranial capacity of some 880 cubic centimeters and body proportions much like those of a modern human. It was initially thought that the boy died around the age of twelve, based on the denture and the overall size of the body. However, detailed study of the growth patterns point to an age of only eight years, implying Homo erectus grew up a lot more quickly than modern humans do.

  FIGURE 11.6.A–B

  Two examples of Homo erectus: on the left is a 1.6-million-year-old skull without jaw from Africa; on the right is a 450,000-year-old skull from China (also called “Peking Man”).11

  Whereas the earlier hominin cranial capacities do not differ statistically from those of modern apes,12 those of early Homo demonstrate a significant increase. A larger brain requires more nutrients to run. Homo erectus, like modern humans, had relatively large bodies, small guts, and small teeth—features that suggest they underwent a fundamental change to a higher-quality diet. The most common explanation for this is that they consumed more meat. The increase in protein-rich food could explain the reduction in gut and might have enabled the increase in brain size.

  The reason for better access to meat may have to do with advances in locomotion. Tendons, ligaments, and bones changed in ways that favor stable and efficient long-distance running. Modern humans are by no means great sprinters compared to other mammals, but we do have extraordinary endurance. Like hyenas and migratory ungulates such as wildebeest (and unlike most other savannah animals, let alone primates), humans can run for extended periods of time, potentially giving them a distinct advantage when it came to hunting and scavenging. Running to wherever the vultures gathered may have improved the chances for Homo erectus to beat other scavengers to the feast. A key constraint for long-distance running is overheating.13 Reduced fur and whole-body sweating allow humans to dissipate heat rapidly. Instead of nasal breathing typical of apes, we increase ventilation during strenuous activity by mouth breathing. It is possible that Homo erectus was already so prepared and may hence have become capable of pursuing prey until they got within close enough range to strike. It may have even allowed them to run down prey to exhaustion. Such hunting requires persistence and perhaps an increased degree of foresight, and it may have selected for better mental scenario building and exchange to support cooperative hunting. Modern human hunters read tracks to determine what prey to pursue. While we do not know when track reading first emerged, it may have been a crucial step toward recognizing that one thing can symbolically stand for another.

  It is likely that Homo erectus made significant strides toward becoming a top predator. It has been suggested that they outhunted other large carnivores, many of which may have gone extinct as a result. Much of this is speculation, and there are alternatives to the idea that improved hunting and scavenging drove the evolution of Homo erectus. For instance, Kristen Hawkes and colleagues argue that the grandmother effect began to take hold and was the crucial advantage. Increase in brain size and adult body weight suggest that Homo erectus lived longer lives (though estimates of the proportion of old to young fossils are still low). Climate-driven changes in female foraging—perhaps the gathering of tubers—and food-sharing practices may have led to changes in life history and ecology. The key foods may have been gathered rather than hunted.

  Richard Wrangham has championed the case that the cooking of food was the crucial step. Perhaps the smartest thing any of our ancestors ever did was to figure out how to control fire.14 Cooking enables us to break down foods that are otherwise not digestible, rids them of common parasites, and stops them from spoiling quickly. Burned earth associated with Homo and animal fossils comes from as early as 1.6 million years ago. However, it is unclear whether the fires were started deliberately or whether they were wild fires that burned through the area. Fire, once brought under control, enabled not only cooking but many other advances in terms of vision and heat, attack and defense. Spending nights around the campfire might have, over many millennia, been a catalyst for a number of milestones in human evolution, fostering traditions, communication, and innovation. Fire gave us immense new powers (remember Prometheus), enabling us to drive away predators and drive out prey from their hideouts. It allowed us to change entire landscapes and forge countless tools.15 However, there is little evidence to support the idea that control of fire was present, let alone widespread enough, to account for the rise of Homo erectus. The most convincing evidence for early fire use is 790,000 years old (though a recent report suggests fire use by 1 million years ago).

  Homo erectus was the first hominin species certain to have dispersed across the old world. The oldest Homo erectus fossil currently comes from East Turkana and is 1.8 million years old. Almost equally old fossils were found in the country of Georgia; by 1.2 million years ago they are evident in Spain. The migration, however, appears to have first gone rapidly eastward.16 By 1.6 million years ago Homo erectus were living in China (“Peking Man”) and Indonesia (“Java Man”). They made these new territories their own and persisted for a very long time. It used to be thought that they died out a few hundred thousand years ago, but new evidence suggests that in pockets, at least, they survived until quite recently. New dating of three Indonesian skulls has yielded estimates of between 40,000 and 70,000 years ago. Another study suggests that the descendants of “Java Man” may have survived until as recently as 27,000 years ago. If these dates are correct, then Homo erectus lived on Java for 1.6 million years.

  Homo erectus must have had some distinct adaptive advantages. They did not sport obvious new biological weapons such as claws, venom, or incisors, but they had a bigger brain. Recent analysis of teeth found that Homo erectus had much more variation in complexity of microwear compared to earlier hominins. This finding suggests not specialization but a new degree of behavioral flexibility. Rather than purely focusing on one type of food, be that meat, cooked tubers, or something else, they appear to have relied on a great diversity of foods. This, in turn, suggests changes in their mental capacities were the primary cause of their success, rather than any specific diet or behavior. They may have made some serious headway in both key aspects of the gap: open-ended, reflective mental scenario building and connecting of minds.

  Some capacity for mental scenario building is evident in the tools they made. The Acheulean stone-tool industry is the most successful tool set ever created. It includes symmetrical hand axes and cleavers, flaked on both sides of the stone rather than just one. Bifacial hand axes are teardrop shaped and can be very flexibly deployed. They were useful for butchering and for working plant materials. They sit comfortably in one’s hand (see Figure 11.7) and allow one to cut effectively for long periods. The creation of these tools implies a capacity to plan, to create a mental scenario of the way to the final product. The stones were carefully selected for suitability in weight and size, and precise blows were used to change them gradually into the desired form. The stone was first worked roughly to create the shape and then with more subtle flaking to produce the sharp, symmetrical tool. The toolmakers needed to understand a thing or two about the properties of the stone to predict how it would break and splinter. Making a bifacial hand ax is a difficult craft—as my fingers had to discover the hard way. Even an experienced knapper takes significant time to make such a tool. This effort was not expended for one-time use. These tools were carried over distances and used repeatedly, implying some foresight of their future utility.

  Perhaps the most curious aspect of these versatile tools is that they were made in a standardized fashion. The design stayed much the same for well over a million years. The earliest example appears some 1.76 million years ago in Africa. The biface I hold in Figure 11.7 was made almost
1.5 million years later. It is the most lasting piece of technology our lineage has ever produced. Although tools became thinner and more trimming flakes were removed, there was little variation over time. These multipurpose tools gave our ancestors a distinct edge—in the literal and metaphorical sense of the word.17 They were often made out of flint; as with modern computer chips, some of our most amazing powers derive from silicon.

  FIGURE 11.7.A–D

  Acheulean bifacial hand ax, approximately two hundred thousand years old, from modern Israel (photo Sally Clark, stone tool courtesy of Ceri Shipton).

  The fact that these tools were made the same way for so long shows that Homo erectus were capable and motivated to learn from each other the skill of shaping them. They were maintaining social traditions with high fidelity. On the other hand, the lack of variation shows little of the cumulative character that marks our modern culture. Why did they not regularly improve on the design? Perhaps their minds were not ready for it. The ratchet effect that Tomasello highlighted may not have been operating yet. Nevertheless, there was some increase in complexity, and other inventions of a more perishable nature may well have been accumulated. Tools made out of wood or hide are less likely to leave traces, and important inventions, such as new means of communication and cooperation, may leave none.

  Our drive to connect our minds is primarily expressed through language. In line with the social learning the Acheulean tool tradition implies, several theorists have suggested that Homo erectus developed a more sophisticated communication system. William Calvin, for instance, made a case based on the claim that Homo erectus increasingly employed aimed throwing to bring down prey, in particular with bifacial hand axes. He has argued that reliance on one-armed, aimed throwing selected for a refinement of motor sequence capacities in the left hemisphere and that these were later adapted for speech. Holloway has claimed endocasts of Homo rudolfensis and Homo erectus already indicate changes in the brain that look like a Broca’s area—an area in the left hemisphere implicated in speech production.