The Gap

by Thomas Suddendorf

  DNA studies have an increasing impact on our understanding of our origins,2 providing an independent source of information beyond what is known from fossils. Fortunately, many of the major findings do appear to agree with what is known through the archaeological record. The oldest anatomically modern human fossils are dated at almost two hundred thousand years old: the Omo fossils from Ethiopia, discovered by a team directed by Richard Leakey.

  The earliest mitochondrial haplogroup (L0) includes the Khoisan of Southwest Africa, famous for click sounds in their languages. From this group three African lineages—L1, L2, and L3—split off. People belonging to the L3 group spread rapidly over the globe between sixty thousand and eighty thousand years ago and gave rise to the two haplogroups of all non-Africans (M and N) that are estimated to have migrated out of Africa around that time. They dispersed into Asia, reaching Australia some ten thousand to twenty thousand years later. They settled in Europe around forty thousand years ago. From East Asia they moved into the Americas and quickly spread across the New World.

  FIGURE 11.1.

  Rough sketch of human migration as currently indicated by DNA and fossil evidence.

  Over millennia our ancestors proceeded to settle most landmasses of the world and profoundly alter the landscapes they found. It was less than a thousand years ago that they finally settled the last major habitable lands they named Aotearoa, what the English-speaking world calls New Zealand. Aotearoa was devoid of humans, even of mammals, other than a couple of bat species, and Maoris arrived on canoes to find a land of plenty. Maori had brought chickens with them, which they called Moa, but stopped breeding them when they discovered the enormous birds available in the bush. The large flightless native Moa were not used to being predated by mammals, and as Maori flourished and hunted them, the birds rapidly went extinct, possibly within a century. As protein became increasingly scarce, Maori came to rely on shellfish and even, at times, on cannibalism. This was not an unusual turn of events. When humans first encountered lands where milk and honey flowed, they enjoyed and exploited them. Mass extinctions of local megafauna frequently followed. In some cases, the ecosystem collapsed, as happened on Easter Island. In other cases, severe periods of scarcity may have occurred before people managed to establish a new sustainable equilibrium.

  Many of our migrating forebears, however, did not find virgin land. Instead, they encountered worlds already occupied by other hominins who had established themselves there much earlier. Homo erectus had lived in Asia for well over a million years before modern humans appeared. Neanderthals were living in Europe and West Asia. Denisovans were living in Siberia. And in Indonesia a small people reminiscent of Tolkien’s hobbits lived on the island of Flores. We do not know how these encounters unfolded, but we do know that these ancient cousins disappeared and that, in the case of Denisovans and Neanderthals at least, some interbreeding with the newcomers occurred. In some circumstances, the new migrants probably established peaceful relations and learned important local knowledge from the experienced natives. In other circumstances, particularly in times of scarce resources, they would have had to compete. Ultimately, our ancestors prevailed. But who were these ancient folk, and where did they come from? What do we know about the evolution of hominins and their minds?

  IN 1925 RAYMOND DART REPORTED the discovery in South Africa of the fossil of a three-year-old child (the Taung child) with a mixture of ape and human features. He called this species Australopithecus africanus (Southern ape); on eventual discovery of further specimens, it was hailed as “the missing link.” Subsequent finds showed that a great range of upright-walking, big-brained hominins used to roam this planet, sometimes in overlapping territories. Some were our forebears; others were different branches of an increasingly bushy looking hominin family tree.

  These links are “missing” only in the sense that these hominins are now extinct. This is not to say, however, that the fossil record is complete and no information is missing. In fact, new evidence is unearthed quite frequently, and the record misses all manner of things that do not have sufficient permanence. Whereas teeth and stone artifacts can exist for millions of years, soft tissue and plant material quickly decompose. Whole civilizations built on bamboo would leave little trace in the remote future. A lot of information is undoubtedly missing from the record, much of which will never be established, and scholars debate about what does exist. For example, disputes occur between paleoanthropologists who recognize a distinct set of fossils as a new species and those who argue that it should be considered part of normal variation of an already described species. Some paleoanthropologists are more inclined to “split,” and others are more inclined to “lump.”3 Putting such arguments aside, the current picture of the hominin past is detailed and intricate, featuring many distinct hominin species, and there is no frantic search for a vital piece missing from the puzzle. Figure 11.2 shows the variety of hominins that are currently thought to have roamed the Earth over the last six million years. I will discuss them in turn further below.

  How can we breathe life into fossils and begin to understand their minds and behavior? A typical starting point is to compare them to extant animals living in similar environments. Surviving apes have long served to inspire speculation about the life history, appearance, and behavior of our ancestors. It is tempting to simply assume the last common ancestor with animals, the point of departure on our way to evolve the current gap, was “chimpanzee-like.” However, this assumption is problematic. For instance, should we assume the last common ancestor was more like a common chimpanzee or more like a bonobo? As we have seen these two species of Pan are quite different from each other, yet they are both equally closely related to us. How could we know whether our ancient ancestors were aggressive, male-dominated apes like common chimpanzees or oversexed egalitarian creatures like bonobos? In fact, it is quite possible they were neither. We have changed dramatically over the last six million years, and both chimpanzees and bonobos have had the same span of time to evolve their own idiosyncrasies. Simple analogies, then, do not suffice.

  As we saw in Chapter 3, the distribution of traits among closely related species can be used to make inferences about the homological origin of these traits in shared ancestors, and the last common ancestors of great apes probably had a sophisticated mental repertoire. It is likely that they could think beyond what was immediately available to their senses; visually recognize themselves and reason about hidden displacement; make tools, copy others, and recognize being copied; maintain multiple social traditions; and reason by exclusion. With such mental attributes at its disposal these common ancestors embarked on a journey that eventually led to our human minds.

  FIGURE 11.2.

  Currently widely recognized hominins of the last six million years. “Splitters” would subdivide some species, such as Homo erectus, into different species such as H. erectus, H. ergaster, possibly H. pekinensis, and H. georgicus, whereas “lumpers” merge them. Some “lumpers” consider Neanderthals, for instance, to be a subspecies of Homo sapiens (i.e., H. sapiens neanderthalensis). Bars represent estimated age of fossils. Elongated bars indicate that multiple fossils of different ages have been found. Bar shades represent common assignment of distinct genera.

  The study of what happened on that subsequent journey, of how we evolved the capacities that today set us apart from even our closest living relatives, has to rely on other kinds of clues, such as fossils and genetics. I am neither a paleoanthropologist nor a geneticist, so I can only sketch this path from my cautious reading of the currently available literature.

  There are numerous theories about the subsequent anthropogenesis—about how, when, and why our ancestors became human. Some scholars highlight the role hominin bodies had, such as the capacity for aimed throwing, better cooling of the brain, and the ability to run long distances. Others have argued that changes in our sociality—collective punishment to enforce rules, the consequences of pair bonding and cooperative breeding—were the p
rime movers of our success. Yet others see specific innovations as the key, such as the control of fire and the development of cooking, or the invention of the baby sling or of symbols. Chances are, most of these factors played some part, but it is not clear how prominent a role each played in determining our ancestors’ path. Below I sketch the major facts currently known about our journey4 and evaluate their implications for the emergence of nested scenario building and the urge to connect our minds. Let’s have a brief family reunion.

  ACCORDING TO MOLECULAR EVIDENCE, THE last common ancestor of humans and any other living animal lived between 6 and 7 million years ago. Recent analyses suggest that the story may be more complicated than was long assumed. Hominin and chimpanzee lineages appear to have initially split 7 million years ago but then hybridized, finally splitting for good after 6.3 million years. Some such hybridization may also have occurred with other species, such as the ancestors of gorillas. Chimpanzees and humans are more closely related to each other than to gorillas, but some sections of human DNA are more similar to that of the gorilla than to that of the chimpanzee. It is possible that some interbreeding occurred after the initial split from gorillas some 10 million years ago. The subsequent journey to humankind (just like these initial steps) was neither simple nor direct.

  There are several relevant fossils from this time period. The oldest specimen is a skull from the Chad estimated to be over six million years old. This so-called Sahelanthropus tchadensis has a large brow ridge, small canines, and a cranial capacity of 365 cubic centimeters, equivalent to that of a modern chimpanzee. In 2001 researchers reported six-million-year-old fossils of a different probably upright-walking hominin, Orronin tugenensis, dubbed “millennium man.” It is unclear whether these fossils are of an ancestor of humans, of chimpanzees, or of both. A fossil of a potential early hominin from a period following the split from the last common ancestor was described in 2004: Ardipithecus kadabba lived over five and a half million years ago. It appears to be the precursor of a species of which we now know a lot more.

  In 2009 researchers described in detail a near complete fossil skeleton of an over 4.4 million year old hominin species called Ardipithecus ramidus. Its brain capacity was still equivalent to that of a modern chimpanzee (300–350 cubic centimeters), but it already had reduced canines and walked on two feet when on the ground. Yet Ardi still had a grasping toe for climbing trees and had flexible wrists unlike those of chimpanzees. It had been widely assumed that when our common ancestor climbed down from the trees, it walked on its knuckles just as modern chimpanzees and gorillas do. The flexible wrist of Ardi, however, suggests that this was not a shared ancestral trait. Furthermore, there are signs chimpanzee and gorilla knuckle walking differ from each other and perhaps evolved independently. It may be that bipedalism did not evolve from quadruped ground locomotion but rather derived from bipedal clambering in trees, as is frequently observed in orangutans.

  The most common account for the rise of bipedalism and the split between the lines that led to modern apes and us is called East Side Story. Coined by French anthropologist Yves Coppens, the theory points to a massive geological event and its climatic consequences. Some eight million years ago tectonic plate movements resulted in the Great Rift Valley and its peaks on the western rim that now separate East Africa from the rest of Africa. Whereas the west of the continent continued to enjoy the precipitation required for rainforests, the climate and vegetation of the east changed dramatically. As a result, while apes in the west continued to live in the same habitat, their cousins in the east had to adapt to an increasingly different environment, as forests turned into open savannah. Our ancestors had to find ways to make a living away from the trees. In line with this scenario, virtually all early hominin fossils come from this part of Africa.5 It is likely that climate change and the rise and spread of grasslands have repeatedly played major roles in our evolutionary history.

  Bipedalism is not necessarily the obvious solution for an ape adapting to the savannah. For one, it comes with some serious side effects, including back problems and hemorrhoids. Bipedalism also required a realignment of the spine and a narrowing of the pelvis, restricting the size of the birth canal and resulting in painful and dangerous birthing. This, in turn, appears to have forced a gradual shifting of infant skull and brain growth into the postnatal period. As previously discussed, the upside of this development is that social and cultural input could more effectively shape the infant’s maturing brain. Thus this apparent design flaw may have been a crucial step in the evolution of our minds.

  Bipedalism did not make us fast sprinters—our ancestors’ speed would have been no match for savannah predators such as lions and hyenas. In fact, there is increasing evidence that early hominins were seriously preyed on. How did they manage? One advantage is that bipedalism frees the hands to carry things, potentially opening up new opportunities for defense. A human hand is capable of power and precision grips that allow effective clubbing and throwing. Early signs of these adaptations appear in Ardipithecus (and are well developed in early Homo some two to three million years later). Striking with rocks and sticks may have allowed otherwise defenseless early hominins to fend off carnivores, such as the many species of saber-toothed cats that roamed the savannah. At times, throwing rocks may have been sufficient to repel predators even before they attacked. Rather than the image of “man the hunter,” some researchers have therefore argued that “man the hunted” may have driven initial steps in our evolution.

  It is tempting to use my scenario-building mind to expand on this story and highlight how selection would have led to increased mental scenario building and foresight. It is easy to imagine that natural selection had particularly long teeth back then, giving those who were prepared to defend themselves a distinct advantage over those who were not. Having appropriate defenses ready when it mattered most would have been crucial, possibly leading to the carrying of objects such as stones and clubs for defensive use. Alas, we do not know whether they actually used such defenses, and signs that hominins carried arms only appear millions of years later.

  THE ARDIPITHECUS FOSSILS COME FROM an East African region known as the Afar Triangle in Ethiopia, a source of many extraordinary fossil finds. In 1974 Donald Johanson and colleagues discovered a near-complete skeleton of a hominin now known as Australopithecus afarensis, famously called Lucy. The following year the team unearthed a group of thirteen individuals of the same species. They were a successful genus that survived for over two million years.

  FIGURE 11.3.

  Reconstruction of “Lucy,” Australopihtecus afarensis, 3.2 million years old. (All skull reconstructions by Bone Clones (www.boneclones.com), photographed by Sally Clark.)

  When Raymond Dart discovered the first Australopithecine, he appeared to have found the missing link not only between ape and human bodies but also between their minds. The primary visual cortex of humans is relatively smaller than in other primates, and so the boundary of this area—the lunate sulcus6—is relatively forward in apes and more backward in modern humans. Dart reported that the primary visual cortex of Australopithecus resembled a human’s more than an ape’s; he concluded that these hominins had devoted more brain resources to storage and association of information than to vision. Unfortunately, inferences about ancient brains are based only on endocasts—casts of the internal bone of the cranium—and there have been some persistent disagreements about what might be gleaned from them.7

  The oldest known Australopithecines, A. anamensis, lived some 4.2 million years ago. A. afarensis roamed Africa from 3.9 to 2.9 million years, weighed between thirty and forty kilograms, and stood a little over a meter tall. Their cranial capacity was similar to or slightly larger than that of modern apes, with a mean of 458 cubic centimeters (their South African relatives, A. africanus, had a mean of 464 cubic centimeters). Their pelvis and legs are appropriate for bipedal locomotion, and the question of their bipedalism was apparently settled once and for all by a most extr
aordinary find: Mary Leakey, while playing Frisbee, stumbled across their fossilized footprints. When a volcano erupted in northern Tanzania 3.7 million years ago, covering the ground with ash, subsequent rain turned it into something akin to plaster. Numerous animals walked across this surface and left permanent tracks. Among them were the footprints of an upright-walking hominin family.8

  Australopithecines were not only walking upright but also beginning to lose their fur. Evidence for this derives from a surprising source: lice. These notorious parasites tend to be specialized to particular hosts and cannot survive long without them. Most primates host only one type of louse. Humans, however, host three. One is the head louse. With humans’ loss of fur,9 it seems to have evolved to survive on the head, leaving the crotch available for another species of louse. The pubic louse, DNA comparison suggests, migrated to our ancestors some 3.3 million years ago. The closest relative to the human pubic louse is the louse specialized on gorillas, so we apparently got crabs from the ancestors of gorillas. More importantly, what this finding means is that 3.3 million years ago our head and pubic hair were already sufficiently separated by hairless body regions to enable two types of lice to live in distinct habitats on the same host. In other words, Australopithecines were not covered entirely with fur. (Incidentally, the third human louse is the body louse that lives in clothes. This species diverged from head lice between 170,000 and 83,000 years ago, suggesting that by that time humans had proverbially left the Garden of Eden and were regularly wearing clothes.)