American Scientist

In December 1991, at a conference at the Senckenberg Museum in Frankfurt-am-Main in Germany, three then little-known scholars quietly upended the conventional thinking about how humans migrated out of Africa. Leo Gabunia, a soft-spoken, silver-haired man from the Republic of Georgia’s National Academy of Sciences, David Lordkipanidze, the dig director from the Georgian National Museum, and Antje Justus, then a German graduate student, had spent the summer excavating a site in Dmanisi, Georgia. On the last day of the dig, Justus extracted a Homo erectus lower jaw from underneath a skeleton of Smilodon, a large saber-toothed cat. With a preliminary date of 1.77 million years, the Dmanisi jaw seemed to be the oldest clear evidence of H. erectus outside of Africa. Fortuitously, my late husband, anthropologist Alan Walker, was at the conference and had brought a cast of the jaw of the H. erectus skeleton that he had been studying over the summer. When he set the cast down on the table next to the jaw from Dmanisi, the two looked like twins: The Dmanisi fossil truly did appear to be a H. erectus. And later analysis showed that H. erectus was far more predatory than expected and revealed far more about our ancestors’ cooperation and sociality than most had suspected.

Many paleontologists were skeptical of the finds, perhaps assuming that these three scholars they had never heard of, from a country nobody then saw as a gateway from Africa to Eurasia, had gotten something wrong. (I wrote about this debate in the November–December 2000 issue.) The age of the Dmanisi fossils seemed too early for H. erectus to have migrated into Eurasia, because at the time that they were found, the prevailing knowledge was that the species had only just appeared in Africa. Later finds showed H. erectus had occurred even earlier in Africa. These fossils also seemed too early to have moved with their distinctive stone tools into Europe, which was full of ferocious predators.

Eventually, five largely complete skulls of H. erectus were found at Dmanisi. The fourth skull was spectacular: an aged individual who had lost all but one tooth in life, causing a resorption of bone on the lower jaw and palate. The degree of resorption of the bone that once surrounded and anchored the teeth is striking and took a long time to occur. Aside from the sheer riches of the site, this skull implied that H. erectus had had such strong social ties that others had cared for this disabled individual for years, obtaining soft foods or processing foods into mush so that he could eat.

Subsequent work has vindicated the scholars from Georgia. Dmanisi is an incredibly important site for understanding the migration of these hominins out of Africa and their adaptations to a new region and lifestyle. The age of the fossils confirmed that the hominins at Dmanisi were only slightly younger than the oldest H. erectus anywhere. They had left an extensive accumulation of stone tools at the site, and they had lived in an environment with predominantly Ice Age fauna.

New Evidence on Diet and Sociality

Evidence from this Dmanisi fossil reinforced conclusions based on the study of the skeletal remains of another H. erectus specimen found in Kenya known as KNM-ER 1808, which is dated to 1.65 million years. The KNM-ER 1808 fossils were marked by an extraordinary overgrowth of pathological bone tissue around the limb bones, diagnosed as a probable case of hypervitaminosis A. This disease is sometimes caused by overzealous parents who feed their children excessive amounts of vitamins. It also occurs in extraordinary circumstances when an individual consumes an extreme amount of raw carnivore liver at one sitting, or smaller amounts over a period of time. Carnivores concentrate the vitamin A from their prey in the stellate cells of their livers, making carnivore liver poisonous to species not adapted to this diet. The measured content of vitamin A in a single gram of adult polar bear liver can be 20 to 100 times the recommended daily allowance for humans. Before dying, sufferers of hypervitaminosis A experience weakness, headache, sloughing off of skin, joint pain, and loss of hair and teeth.

Dmanisi is an incredibly important site for understanding the migration of hominins out of Africa.

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We know of this disease because of stories from polar explorers such as the Australian geologist Sir Douglas Mawson. When the sledge carrying his supplies fell into a crevasse during the Australasian Antarctic Expedition (1911–1913), Mawson and his companion, Swiss ski champion Xavier Mertz—both already in poor physical shape from the extreme demands of the expedition—had to survive on the soft livers of sled dogs. Both Mawson and Mertz suffered vitamin A poisoning. Mertz died in delirium, leaving Mawson to travel 160 kilometers back to base camp alone and on foot, dragging a sledge he had cut in half with a penknife, that was loaded with the remaining equipment and supplies. Mawson’s book The Home of the Blizzard is a stunning account of endurance and survival.

We have no accounts or stunning dramatizations of the life of the fossil specimen KNM-ER 1808, as we do of Mawson’s ordeal. But the pathological condition of the fossil bones showed that H. erectus had shifted to a predominantly meat-eating diet and that this particular individual probably died of hypervitaminosis A. He would have needed extensive care while disabled for months or years as he grew up to a quarter inch of pathological bone in many places on his limbs. Although we don’t know why the nearly edentulous individual from Dmanisi lost so many teeth, we do know he required help from others to survive.

A Contemporary Canid

It is not only human ancestors who suffered issues while moving into new territories; other mammals faced competition from other species. Dmanisi preserves many carnivores: giant cheetahs, two kinds of saber-toothed cats, a small wolf, a lynx, several kinds of bear, giant hyenas, and a jaguar as big as a lion. There was also an extinct canid, Canis (Xenocyon) lycaonoides, a probable ancestor of the living African hunting dog or painted dog, Lycaon pictus. The modern African hunting dog hunts in packs and has a very high success rate in capturing prey. Both ancient and modern canids are part of a distinctive lineage, with special adaptations for speed (up to 70 kilometers per hour), endurance, a close-knit pack structure, cooperative hunting, and dental adaptations that increase the length of the shearing blades on the teeth for cutting up meat. A fossil cranium of a closely related canid species from the roughly contemporaneous site of Venta Micena, Spain, is strongly asymmetrical because of congenitally missing teeth, leading paleontologists to deduce that this individual must have been assisted by others in obtaining food as it grew to adulthood, much like the H. erectus individuals in the fossil record who must have required care.

What is so curious is that these canids apparently migrated from Eurasia into Africa through Georgia while H. erectus expanded its territory from Africa into Eurasia through Georgia at the same time. And both the canid and the hominin show evidence of cooperative behavior with other conspecifics. These co-occurrences are more than simply an odd coincidence; they may tell us something important about survival and sociality among predatory species.

Big Predators’ Big Adaptation

In 1999, Chris Carbone of the Zoological Society of London and his colleagues looked at how dietary habits were related to body size in a wide range of carnivores. They found that small carnivores ate insects, sometimes plant foods or fruits, and very small vertebrate prey less than half their own size. But animals with weights over about 21–25 kilograms—about as big as or bigger than a Dalmatian—cannot sustain themselves on such a diet. There is a trade-off between the difficulty of obtaining food and the nutritional value derived from that food. Plant foods are easier to obtain but yield much less protein and fat than animals. Those prey species must yield a lot of nutrition to make pursuing them worthwhile. The energetic requirements of larger carnivores can be met only by eating more than 70 percent meat derived from prey much bigger than themselves. This strategy is known as hypercarnivory and requires pack hunting and extreme sociality to succeed. This rule applies to larger felids, canids, and hyaenids; some bears may be an exception.

Then, Blaire Van Valkenburgh, now retired from the University of California, Los Angeles, decided to investigate possible hypercarnivory among the living and extinct large dogs in the family Canidae, working with Xiaoming Wang of the National History Museum of Los Angeles County and John Damuth of the University of California, Santa Barbara. Large canids of North America cluster into three subfamilies: the early, now extinct Hesperocyoninae (40 million to 15 million years ago); the huge, extinct bone-crushing dogs, the Borophaginae (34 million to 2 million years ago); and the current and extinct Caninae, such as foxes, jackals, coyotes, dingoes, and wolves. The team showed that hypercarnivorous species had dental adaptations to increase the length of the shearing blades in their cheek teeth and to decrease the size of the grinding areas in those teeth, used to crush bone or other tough substances. Hypercarnivores also tended to have deeper lower jaws and highly social habits. Growing larger in body size provided an advantage in bringing down prey and defending the carcass from other predators. In arctic habitats, larger body size also improved thermal efficiency. In many ways, bigger size offered benefits.

If our ancestor H. erectus had shifted to a predatory lifestyle, it should have been subjected to similar constraints. An average adult body weight for H. erectus was about 44–65 kilograms, meaning its energetic needs would have forced it into hypercarnivory. When we examine the adaptations of H. erectus, some match the rules of hypercarnivory but others don’t. The species did not have teeth adapted for slicing meat, as hypercarnivorous canids do, but H. erectus did have sharp stone tools that it used on prey, leaving characteristic cutmarks on bones. This stone tool use is perhaps another approach to the problem of getting meat off of a carcass before another predator steals it. Our ancestors also apparently hunted in packs and cared for injured members of the family, at least in some cases. But we don’t see signs that H. erectus was a swift runner like a hunting dog. Nonetheless, these two partial skeletons of H. erectus—one from Kenya and one from Dmanisi—indicate fairly large body size, tool use, strong sociality, and prolonged care for other members of the group. In sum, our ancestors were very probably hypercarnivorous. (See “Meat-Eating Among the Earliest Humans,” March–April 2016.)

Survival of the Cooperative

The co-occurrence of H. erectus and Xenocyon in Georgia does not attest to the domestication of a wolf-like canid into a dog that would live with humans, which did not occur until perhaps 40,000 years ago. The two species do not seem to have lived together or worked cooperatively in hunting or defense of carcasses or territory. The cranial changes that occurred in canids during domestication did not begin at this time, nor is there any indication that hominins managed Xenocyon’s reproductive choices, which would be necessary for domestication. Nonetheless, these two very different carnivorous species may have found cooperation the key to success as each migrated into a new territory full of other carnivores.

Perhaps the most important point is that both H. erectus and C. (Xenocyon) lycaonoides were adapted for enhanced communication and cooperation within a social group, which may have enabled their survival. This kind of behavioral adaptation doesn’t show up plainly in the bones of the animals, but it can be inferred from other qualities, such as care for the injured or disabled and coordinated pack behavior involving other members of the species.

These adaptations to hypercarnivory may have been a behavioral preamble important in later domestication. It is often suggested that the similar social behavior of canids and hominins made the cooperation between them possible and set the stage for their unusual collaborative relationship in the future. I hypothesize that domesticating dogs provided a major advantage for the first modern humans in Eurasia, even though archaic humans such as Neanderthals, Denisovans, and a nonmodern species known as Homo heidelbergensis were there first. Many of the large carnivores of the Ice Age have gone extinct, along with these nonmodern humans, while humans, wolves, and hunting dogs have persisted. To borrow a phrase from the title of a book by my friends Brian Hare and Vanessa Woods, it was the survival of the friendliest. (See the audio slideshow “What Is Intelligence?” for more on Hare’s dog cognition research.)

It is not only legs, claws, and teeth that gave hypercarnivores an advantage. Bonding among individuals, communication skills, and living in close-knit packs may have conferred an edge among hunting species.