The Gap

by Thomas Suddendorf

  In some localities, new evidence suggests, hominins other than Neanderthals and anatomically modern humans persisted successfully until very recently. As noted in Chapter 1 recent genetic analysis of thirty-thousand-year-old fossils found in the Denisova cave in southern Siberia revealed that the DNA belonged neither to a modern human nor to a Neanderthal. Instead, the data suggest that these Denisovans are the Asian descendants of an earlier migration out of Africa some five hundred thousand years ago that split into Neanderthals in Europe and Denisovans in Asia. Moreover, like the Neanderthals, Denisovans appear to have interbred with modern humans. They contributed some genetic material to present-day Melanesians. The picture of hominin ancestry is increasingly becoming more colorful and diverse.23

  Another major find was Homo floresiensis, tiny hominins from the Indonesian island of Flores. They stood one meter tall and were discovered in 2004, at a time when the world was watching Peter Jackson’s film adaptation of Tolkien’s The Lord of the Rings. The label “hobbit” immediately stuck. Indeed, people began to wake up to the notion that Middle Earth is not that outlandish an idea. Until recently in prehistory, a range of distinct hominins have been roaming this Earth, not entirely unlike hobbits, orcs, elves, and dwarfs did in Tolkien’s novel. The hobbits on Flores had small brains (around four hundred cubic centimeters) and yet appeared to have made sophisticated stone tools. The discrepancy led to some skeptical debates about whether these specimens are deformed or diseased modern humans, a debate that also occurred when the first Neanderthals were unearthed. Current evidence suggests these were hominins with attributes akin to Homo habilis or Homo erectus (who, as you may recall, survived on neighboring Java until perhaps as recently as twenty-seven thousand years ago). The isolation on the small island of Flores may have resulted in so-called island dwarfism, a phenomenon observed in many animals. Elephants on Flores were tiny too (yet the local rats and lizards are unusually large). It’s a mysterious isle of flowers, indeed.

  Even if a group of fossils is eventually identified as a human pathology rather than the remains of a distinct species,24 new finds appear with such regularity that we most likely underestimate the great diversity of hominins that used to populate this planet. Throughout most of our prehistory several species of upright-walking, intelligent hominins lived side-by-side with our own ancestors. We are merely the last one standing.

  DARWIN APPEALED TO GROUP SELECTION to explain the rise of human faculties such as morality25 and noted that “at all times throughout the world tribes have supplanted other tribes.” Competition between and within groups might have increasingly selected for enhanced mental scenario building and better linking between minds, including advances in the domains of language, foresight, theory of mind, reason, culture, and morality. When a group cooperates to attack you, the most effective response usually is to cooperate in defense. The burgs and fortresses of old Europe and the Pa sites of Maori illustrate how frequently humans had to do this in recent times. Battle plans and strategies, weapons technology, organization and enforcement, bluffs and deceits, valor and heroism are only some of the qualities for which such persistent threat of conflict might select. It pains me as a pacifist to give so much credit to war and conflict, but it appears quite plausible (given our history of conflict and our current obsessions with less violent intergroup competitions, such as soccer26). However, as so often is the case, plausibility should not be mistaken for proof.

  Such intergroup competition may have had a fundamental role in the creation of the current gap between us and our closest relatives. Conflict and competition may have led not only to the supplanting of tribes but to the extinction of closely related hominin species and subspecies (just as human arrival may have led to the extinction of other megafauna). Indeed, such competition with other hominins may have selected for better scenario building and communication between group members.

  Since there is currently no direct evidence for genocides of hominin species, the effect of such conflict is speculation. The oldest indication of violence between people currently comes from archaic sapiens in Spain some 250,000 ago. Several skulls demonstrate healed impact fractures, and several individuals show signs of having been butchered (and presumably consumed). Healed skull fractures have been interpreted as signs of attacks with weapons and have also been reported from Neanderthal and Paleolithic human sites. A Neanderthal in northern Iraq (Shanidar 3), dated at over 50,000 years old, was found with a penetrating lesion to the rib cage. Experiments suggest that the injury was likely caused by a long-range projectile, and the researchers have raised the possibility that modern humans might have been responsible. The jaw of a 30,000-year-old Neanderthal child was found among modern human remains at a cave in southwestern France. It bears cut marks that are distinctive indicators of slaughter. In spite of these few suggestive finds, it is not clear how common and significant violent encounters have been.

  Compelling evidence of violence between humans comes from a twelve-thousand-year-old cemetery in the Sudan that contains a couple dozen individuals with chert projectile points embedded either in or next to their bones. From the subsequent millennia, there are various mass graves and numerous examples of wounds caused by weapons. Recall, for instance, the violent death of Ötzi the Iceman. Despite such finds, it is not possible to properly determine the scale of prehistoric conflicts.

  Over the last decade the evidence had strongly favored an out-of-Africa account, in which modern humans displaced archaic forms wherever they met, over a multiregional account, in which ancient hominins such as Homo erectus evolved into humans in different regions of the world. But there has been a paradigm shift in recent years. As the new Neanderthal and Denisovan evidence shows, some gene flow with existing populations is evident, and there is a good chance more will be unearthed. African data also suggest interbreeding with archaic people. The competing multiregional account is unlikely, since the genetic data point to common descent from East Africa. Compared to chimpanzees, human DNA is homogeneous and not as diverse as a simultaneous evolution in different parts of the planet would suggest. Given that some interbreeding clearly took place, for now, a combination of out-of-Africa and multi regional accounts probably best approximates the complex path to modern humanity.

  Whether through competition, absorption, or other factors, our hominin cousins are no longer discernible, even if many of us carry some of their genes. Our closest remaining relatives now are apes in the equatorial jungles of Africa and Asia. We have come out of the last ice age as clear winners, spreading across the globe. We have subdued much of the natural world and domesticated plants and animals to serve our needs. We wield unheralded powers to create new worlds and have often been unimaginably ruthless in our conquests.

  In this chapter I have tried to outline our current knowledge about our ancestors’ extraordinary journey to becoming human as well as offer plausible scenarios about the forces that may have created the gap, without obscuring the difference between established facts and informed conjecture. In so doing I have exercised the two legs—the ability to generate and reflect on scenarios and the drive to communicate them—that, whenever they first came into being, amplified our skills above those of other animals.

  Our capacities to generate accurate mental scenarios and exchange them widely and efficiently have improved dramatically in recent generations. My mother’s mother came of age in a world without electricity, computers, or cars and with her education limited to Catholic doctrine conveyed by parents, nuns who taught her at school, and priests. She had little opportunity to make herself heard beyond her hometown. By contrast, my children are growing up in a world in which they can access ideas from virtually anyone anywhere, and they can in turn broadcast their own musings and discoveries across the globe. We have come a long way—and there are more changes ahead.

  1A recent study challenges this, putting this date as far back as 142,000 years ago. Incidentally, it has been suggested that Genghis Khan is responsible for one
particular Y chromosome lineage that is present in about 8 percent of men in a large region of central Asia, but in only 0.5 percent of men elsewhere.

  2You can now get your own DNA tested to reveal your deep family tree (e.g., https://genographic.nationalgeographic.com).

  3Even in biology there is debate about what qualifies as a species in living organisms. One simple rule of thumb is that members of the same species should be able to produce viable offspring. Members of different species do not interbreed. Naturally, such a test cannot directly be conducted with fossils unless DNA can be extracted. Instead, arguments typically center on the comparison of small fragments of bone and teeth. Not surprisingly, there are disputes. The discoverers of a new set of fossils have an incentive to “split” rather than to “lump,” because if their find is a new species they are its famous discoverers, and they can even give the species a name; whereas, if their find is yet another specimen of an already described species, there will be less excitement, and they may struggle to find funding for more digging. Even the most skeptical “lumpers,” however, agree that there were a number of distinct hominin species.

  4It may be reassuring that several more or less plausible accounts of anthropogenesis are available. Unfortunately, plausible ideas are easier to come by than decisive evidence. For instance, the aquatic ape theory proposes our ancestors returned to the sea (similar to dolphins or seals), or at least the shore, for a significant period of time. Adaptations to a watery environment might explain a range of peculiarities of the human condition, but alas there appears to be no direct evidence hominins spent any periods adapting to aquatic lives.

  5Some recent evidence challenges this widely assumed scenario. Sahelanthropus tchadensis was found to the west of the Great Rift Valley, and Orronin tugenensis appeared to have lived in forested environments.

  6A bit of the brain that folds outwards is called a gyrus, whereas the crease in between two gyri is called a sulcus. The primary visual cortex is divided from the parietal lobes further forward by a deep crease called the lunate sulcus.

  7The Taung child fossil includes a unique natural endocast, but nonetheless there remained debate about the location of the lunate sulcus. Dart’s conclusion was challenged and defended by two of the most influential scientists in this field, Dean Falk and Ralph Holloway. In a personal history Holloway recounts this and many other disagreements with Falk, including a confrontation at a conference in which both proponents were armed with conflicting endocasts of the same Australopithecine skull (one showing an occipital-marginal sinus, the other not)—leading him to speculate about the damage a thrown plaster endocast might do.

  8Curiously, there is also evidence that while Australopithecus feet were functionally like those of modern humans, some contemporary hominin species continued to be able to comfortably climb trees. A recent find of a 3.4-million-year-old foot still had an opposable big toe like that characteristic of Ardipithecus.

  9The claim of hairlessness is somewhat misleading, as it is not the loss of hair follicles but the replacement of thicker terminal fibers by thinner, shorter, and more transparent vellus fibers that creates our lack of apparent fur cover.

  10Given the potential interbreeding between hominins and ancestral gorillas noted earlier, Paranthropus may be the offshoot of such hybridization. This in turn could also mean that our ancestors may have gotten crabs from mating with Paranthropus. But I am merely speculating.

  11There is debate about whether erectine fossils should be split into several species. The most common split is made, based in part on somewhat thinner cranial bones and less pronounced brow ridges, between African and Asian populations, with the former labeled Homo ergaster and the latter Homo erectus. Some splitters even reserve the name Homo erectus only for Indonesian specimens and refer to the Chinese fossils as Homo pekinensis and Georgian fossils as Homo georgicus. In spite of the anatomical and geographical differences, these fossils have much in common, and I adopt a lumper position here and refer to them collectively as Homo erectus.

  12The numbers of fossil specimen on which these estimates are based are often small, leaving statistical comparisons with limited power. For example, in a review by Robson and Wood, estimates are based on one Sahelanthropus tschadensis, eight Australopithecus africanus, ten Paranthropus boisei, six Homo habilis, thirty-six Homo erectus, seventeen Homo heidelbergensis, and twenty-three Neanderthals.

  13Overheating has also been argued to be a key problem for large brains. It has been proposed that elaborate cranial venous circulation evolved to act as a radiator for the increasingly large brain of hominins.

  14The importance of fire for Paleolithic hominins, especially the discovery of how to make fire, is powerfully dramatized in Jean-Jacques Annaud’s wonderful 1981 movie, Quest for Fire.

  15From 164,000 years ago there is evidence of heat treatment of stone to improve flaking properties. Wood tips could be hardened, and adhesives softened; eventually new powerful materials could be formed, such as ceramics and metals. But these events only occurred thousands of years ago, not many hundreds of thousands of years ago.

  16This is not to say that there was a deliberate exodus. It is likely they simply kept spreading into habitable areas. For instance, climate change turned much of Eurasia periodically into grasslands, and predators may have simply followed their prey species. With subsequent cooling, Homo erectus groups would have had to retreat to accommodating refugia that provided protection in these new areas. Temporary isolations, in turn, lead to local changes. The role of refugia and climate change in human evolution is currently hotly debated, especially in light of the apparent diversity of hominin forms (including diverse Homo erectus) that used to walk this planet.

  17Note that the first successful migrants to Asia did not carry the Acheulean tool technology. Bifacial hand axes appear much later in the East Asian record, and it is unclear whether they represent convergent invention or whether they were carried across by new migrants. Homo erectus’s early migration was done with the more primitive Oldowan tools that had long been used by Homo habilis. This suggests it is not the tools per se that made a difference but the minds that employed them.

  18When models reliably describe the relationship between two factors in the present, one can use one factor to make inferences about the other in the past. For instance, there is a relation between the thickness of primate thigh bones and overall body weight, so the thickness of a fossilized bone can be used to estimate the weight of the individual. However, Dunbar’s estimates are problematic. Neocortex ratio is the volume of neocortex divided by the volume of the whole brain. Although we can measure the cranial capacity of hominin skulls to estimate the volume of the whole brain, how much of this is neocortex remains unclear. Furthermore, group size may be influenced by other variables, such as climate and food availability. Neanderthals had very large brains, but most other evidence suggests they lived in rather small groups in the cold European climate.

  19According to Dunbar, this number is the size we can keep track of, the number of friends we can name, the number of people we invite to funerals and weddings, and the number that, when exceeded, leads to the break-up of a typical hunter-gather group. Whether this is our natural group-size limit is difficult to assess.

  20In some cases heavy objects were apparently moved over kilometers, suggesting cooperation and possibly shared intentionality. However, none of these are conclusive.

  21A mutation of the FOXP2 gene in a British family is associated with profound difficulties in articulation. The gene is implicated in Broca’s area gaining vocal control.

  22There are some problems with such ratios, given that the populations compared are not from the same time period. Consider, for instance, that bones of older individuals are more susceptible to decay because of bone mineral depletion; thus the more ancient the sample, the fewer bones of older individuals may survive.

  23In 2012, strange eleven-thousand-year-old to fourteen-thousand-year-old fossils from a Ch
inese site were described. The skull looked very distinct from that of modern humans, featuring thick brow ridges and lacking a modern chin. They cooked large deer and have hence been dubbed Red Deer Cave People. It is as yet unclear whether they represent any new species or are part of human diversity.

  24To complicate matters, strange fossils may also represent a pathology that is not in existence today.

  25Darwin wrote: “A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection.” However, group selection continues to be a hotly debated topic.

  26We have radically reduced the negative effects of group competition, war, and aggression through sports. They allow us to compete, hone our skills, increase cooperation, and focus our attention; they have taken the place of warfare in many aspects of our lives. Of course, some violence is also associated with sports—think of soccer hooligans—but this is a far cry from actual war.

  TWELVE

  Quo Vadis?

  We are made wise not by the recollection of our past, but by the responsibility for our future.

  —GEORGE BERNARD SHAW

  The greatest adventure is what lies ahead.

  Today and tomorrow are yet to be said.

  The chances, the changes are all yours to make.

  The mold of your life is in your hands to break.

  —J. R. R. TOLKIEN

  HUMANS HAVE LONG THOUGHT THEMSELVES special—distinguished from other creatures on this planet. In some sense we certainly are. We have been extraordinarily successful by sheer numbers: there are more than seven billion of us, and we constitute about eight times the biomass of all other wild terrestrial vertebrates combined. After millions of years of being one of several upright-walking hominins living in small clans armed with stone tools, we now stand alone, wielding unrivalled powers. In a mere five hundred generations we progressed from the Stone Age to smart phones and space exploration. Advances in bio-, nano-, and computer technology are rapidly opening up countless new frontiers.