The Sediments of Time

by Meave Leakey

  Application of this technique allows us to go back to the Turkana Boy and compare the new methodology with Holly’s results and see how his true age matches the estimated skeletal age that Alan originally based on a human growth trajectory. Since Alan’s seminal work on the Turkana Boy, another colleague who specialises in the microstructure of teeth, Chris Dean, has also studied the Turkana Boy’s teeth. Chris found that the rate of enamel accumulation in H. erectus was faster than in modern humans. He estimated that the true age of death of the Turkana Boy was approximately nine years—reassuringly close to the dental age that Holly estimated using a different methodology. By comparing the rate of enamel formation of tooth samples of fossil hominins and apes and modern humans and apes, Chris found that although the H. erectus enamel is thick like that of modern humans, it grows fast like modern and fossil apes. Plotting all of his samples on a scatter plot, Chris found that the trajectory of enamel growth of H. erectus is intermediate between ape and human but more similar to that of apes. So it seems that H. erectus grew neither like a modern human nor a modern African ape but like something in between, and this is what we would expect for an animal with a brain size that was also halfway in between.

  Dental records can offer insight into the final part of the life history cycle as well—where humans again depart from other apes. We find a surprising number of old individuals with very worn teeth throughout the hominin record. Wambua’s maxilla of Australopithecus anamensis found in our very first week at Kanapoi is one example. In 1970, I also found an H. erectus mandible with very worn teeth at Koobi Fora. This mandible is what first set me to thinking about how old this individual was when it died and what the life expectancy of H. erectus would have been. In 2004, two researchers from the University of Michigan decided to look at this in detail. Rachel Caspari and Sang-Hee Lee looked at the fossil record to see if there was any evidence of increased adult survivorship over time. By looking at the degree of wear on the teeth of 768 hominin fossils from the past three million years, they were able to obtain a rough measure of the ratio of old to young individuals in the sample. Their results show that this ratio has indeed changed from australopithecines to H. sapiens. The data support the conclusions from our study of the gestation and early childhood patterns: while H. erectus had begun to enjoy greater adult survivorship, this is not very pronounced compared to earlier hominins. The marked increase in longevity that characterizes modern humans is unique to our species and has come into play only relatively recently.

  Nevertheless, the life history pattern of H. erectus does seem to have begun to depart from the norm for primates. In human terms, the Turkana Boy’s development corresponded to eleven years of growth instead of a chimpanzee’s seven—but the Turkana Boy was actually nine. This slightly extended childhood would have been matched by the expectation of slightly increased longevity, which in females probably extended beyond menopause. The most fascinating implication of this is that such a departure from the normal primate mode must have gone hand in hand with heightened social cooperation. Higher primates are intensely social creatures, but the bonds in primate societies are in general based on mutual self-advancement. The exception is the mother-child bond, which is so strong that the mother will put herself in extreme danger to save her child, and she may even carry a dead baby for several days before finally abandoning it: behaviour that can surely be described as grieving. But the success of the human strategy of having postreproductive females to allow increased fertility in the actively reproductive group only works if the females can spend less time mothering each child before the next one comes along. This in turn depends on having social bonds that extend beyond the mother-child relationship. Somebody has to teach the youngsters during their extended childhood when Mum is busy.

  It is very difficult to infer social behaviour from ancient bones, of course. Yet there is compelling fossil evidence that social bonding had already begun to strengthen in H. erectus. The first time Richard and I were alerted to this intriguing possibility was back in 1970. One of the most talented members of our field crew earned his merit badge by sheer audacity propelled by his conviction that fossil hunting was his calling. Bernard Ngeneo turned up one day and began washing dishes in our Nairobi home. Richard and I were completely bewildered when we discovered after a few days that neither one of us had employed him! However, his affable manner, helpfulness, and initiative soon had him in our formal employ. Not very long after he turned up in our kitchen, we found Ngeneo at the end of our driveway, bag in hand, as we were leaving for the field. “May we give you a lift somewhere,” Richard enquired curiously, to which Ngeneo replied, “Yes, please—to Koobi Fora!” Ngeneo manoeuvred himself out of our Nairobi kitchen into the Koobi Fora kitchen and soon afterwards into the camp kitchen of an archaeological dig being conducted by one of our dearest friends and closest colleagues, Glynn Isaac. From there, it was but a small step to find himself on the path between the camp and the dig that was walked along countless times each day by every trained member of Glynn’s archaeology field crew. It is testimony to Ngeneo’s innate talent that it was he who discovered beside this well-trodden path a hominin femur with a startling story.

  This femur, like the Turkana Boy’s, was about 1.6 million years old. On the shaft, thickened bone showed that this individual had suffered, and recovered from, a severe break to its leg. Anyone who has broken a leg bone knows that a biped cannot get along unassisted until it heals. For the bone to have healed as it did, somebody had to have cared for this individual, bringing them food and water, and protecting them from predators. Motives of self-interest cannot explain such behaviour, so the degree of social bonding must have been considerable. It is unfortunate that we were unable to tell from this femur how old the individual was when the break occurred, although we do know that it had reached puberty so it is possible that its mother took care of it.

  Even more compelling evidence of extraordinary caring behaviour was unearthed by Kamoya in 1973. A decade before he would find the Turkana Boy, Kamoya had already discovered a partial 1.7-million-year-old H. erectus skeleton, KNM-ER 1808. We could tell it was H. erectus from its teeth and the partial skull. However, this female skeleton is much less well-known than the Turkana Boy because so little can be said about its morphology. She died from a very severe pathology that is manifested by an extra layer of diseased bone laid down on the surface of her limb bones. Alan Walker was also tasked with studying 1808, and it took him a long time to identify what caused the abnormal calcification of her bones. Eventually, he took thin sections across one of the broken ends of bone and brought the slides to a group of medical professionals at Johns Hopkins University School of Medicine, where he was teaching at the time. Drawing up a list of possible causes, the doctors eliminated those that didn’t fit until a consensus was reached.

  Barring an unknown disease that has since disappeared, they settled on hypervitaminosis A, a massive overdose of vitamin A. Knowledge of just how lethal too much vitamin A can be is common to the Inuit and other polar communities who hunt carnivores. European explorers of the polar regions learnt the hard way when their rations expired and they were forced to eat their huskies for survival. An excess of the vitamin causes the fibrous tissue attachments connecting the muscles to the bone to rip free, and the blood vessels haemorrhage, forming huge clots and further separating the muscle and bone. The intervening space full of blood clots then becomes calcified, leading to an extra layer of bone. It is an excruciatingly painful and debilitating condition accompanied in extreme cases by delirium, disorientation, and dementia as well as severe pain in the joints and stomach cramps. The extent of the blood clotting and the thickness of this calcified layer visible in 1808’s long bones show not only the severity of her case but also indicate that she must have lived for weeks or months after ingesting the toxic dose. The social implications are staggering—there is no way she could have survived that long without round-the-clock nursing, and as an adult female, her bond with her nu
rsemaid(s) cannot have been the exclusive one she had as an infant with her mother, who would have long since passed on. There is only one way to explain how far her disease had progressed: she lived in a social group in which members took compassionate care of the infirm to an extraordinary degree.

  Another interesting aspect of her diagnosis is the question of how she came to overdose on vitamin A. Vitamin A accumulates in the liver and is especially concentrated in carnivore livers where it cumulates through successive consumption of herbivore livers, which is the most likely source for 1808. We don’t know if H. erectus was able to hunt carnivores or if 1808 came across a sickly or dead predator. Either way, the hominin would have had to be able to successfully fend off other scavengers that had a far more deadly arsenal of teeth and fangs in order to obtain this food.

  The Turkana Boy might also have needed some assistance, and a disability might explain the circumstances of his death. Why a youth entering the prime of his life should come to such a precipitous end facedown in a swamp is hard to fathom. But his vertebrae show a marked scoliosis, an asymmetry resulting from an S-shaped side-to-side curvature to his spine. In 2006, flash flooding in the Omo River produced a twenty-foot-high wall of water that swept away countless livestock and homesteads. Such a circumstance could easily have trapped an individual with scoliosis, who may have been impeded from moving fast enough to get out of the way of the impending torrent. The massive sedimentation accompanying such flash floods would also account for the extraordinary preservation of the Turkana Boy’s bones. Because he was swept into a swamp and submerged in shallow water, scavengers did not find his carcass, and it was buried and fossilised. As the water receded and the swamp dried, hippos walked all over the mud-covered boy. We found imprints of hippo feet in the excavation, and the weight of the hippos snapped some of the boy’s ribs, causing them to lie vertically in the sediment. As we shall see, mobility is one of the key advantages that H. erectus capitalised on as it evolved. His scoliosis was a disability that would have probably impeded his survival to adolescence without the support of a social group.

  We can say with some certainty that H. erectus was on the threshold of becoming human when the Turkana Boy was alive. His tall slender frame already closely resembled that of any modern desert-dwelling human—but his brain had an awfully long way to go. Although the Turkana Boy’s remains answered many questions about his kin, I remained mystified. What were the selective pressures that drove the evolutionary changes for H. erectus to realise this remarkable potential?

  14

  Growing Brains

  Our home on the edge of the Rift Valley is graced with fabulous views and large open spaces. Thin fingerlike ridges dotted with whistling thorn acacias roll south away from our house along the edge of the escarpment, and vegetated valleys run down to the plains on the floor of the rift below. The alkaline Lake Magadi peeps out from behind the great hulk of Mount Olorgesailie, and the opposite flank of the Rift Valley, the Nguruman escarpment, rises majestically up from the plains, occasionally merging with the hazy sky. On clear days, you can see Lake Natron, which marks Kenya’s border with Tanzania, and the distinctive shapes of the ancient volcanoes in Tanzania that stud the skyline—the still active Ol Doinyo Lengai; Mount Kilimanjaro, with its flat cap of snow; the broad-based and towering Gelai; and countless other dormant volcanoes. It is a breathtaking panorama—one that changes endlessly with different light conditions.

  I take our dogs for long walks here as often as I can. Our most recent dog was typical of the dogs from Turkana and a far cry from the pedigree species bred for specific traits. She was lean, independent, and catlike, and had the unenviable name of Fuzzy (bestowed on her by Seiyia despite the dog’s minimalist sleek fur adapted to the dry heat of the desert). Fuzzy, like the dogs who came before her, would invariably catch the scent of a reedbuck, hare, or some other delectable treat. She would take off in high pursuit, leaving me in the dust pondering the improbability that our ancestors could have ever caught anything without the benefits of a bow and arrow, catapult, or a gun. Most mammals compete for survival using their strength, agility, and speed. In contrast, humans are weak, slow, and awkward. There has been a persistent bias in our thinking that humans must have therefore won out by using their superior cognitive abilities rather than any athletic competency (brains over brawn). But none of this has ever explained how the brains grew so big in the first place, and ever since it was established that bipedalism and manual dexterity preceded brain expansion, a convincing theory of what really happened has been elusive.

  Although we don’t know nearly enough about the earliest Homo, including Homo habilis, it seems that by the time H. erectus enters the picture, a strategy for securing meat with relative ease and safety had been adopted. The modern human brain, which is only 2 percent of the body’s weight, consumes a greedy 20 percent of the body’s metabolic resources. However, the total basal metabolic rate of modern humans is no more than the average for a mammal of similar size. Something had to give, and indeed, the human gastrointestinal tract is unusually small for a primate of similar body size. It is our consumption of meat that allows us to obtain enough nutrients to feed our great brains in spite of this drastic reduction in the gut, thus opening a whole new realm of possibility for an enlarged, calorically expensive brain to evolve with all the resulting benefits.

  Homo erectus is at the centre of this enigma. Despite its living in harsher and more uncertain times than our earlier ancestors, its evolving progressively bigger brains over the course of a million years, and its successfully conquering new territories, the awkward tools erectus uses get no more advanced. However, did H. erectus kill anything with these unwieldy tools? Or, if it was scavenging instead of hunting, how could H. erectus compete successfully with other scavengers and ferocious carnivores?

  It had always been Louis’s opinion that early Homo obtained meat by scavenging, not hunting. To prove this, he tried to reconstruct events “exactly” as they would have been. This is vintage Louis, and imagining the scene never fails to make me giggle. One day at dawn, Louis and Richard removed every single article of clothing from their persons and strode off onto the Serengeti plains in search of some meat. They were armed with the sort of weaponry that Louis imagined H. habilis to have used: easily available natural objects, in this case a long bone from a giraffe to serve as a club, as well as its jaw bone, which would presumably have been a lethal-enough blunt object when applied with force. They soon spotted the telltale circling of a large number of vultures that signified a recent kill. It was a fresh zebra, and they settled down to wait for the pride of lions to finish breakfasting before making their move. Then, as Louis expected they would, the lions went off to drink, and Louis and Richard rushed in ahead of the hyaenas, vultures, and jackals. Louis had used some of the time while they waited for the lions to leave by fashioning a stone tool, which he proudly boasted took him just thirty-five seconds. While the elder Leakey kept the other scavengers at bay by energetically brandishing the long bone club, the younger rushed in and hacked off a leg using the stone tool. At this point, the hyaenas and vultures got rather angry, and father and son had to run for it—but they did so in triumph, with the joint of zebra.

  For a long time, I and many others also assumed that H. erectus was predominantly scavenging. But what aroused my curiosity the most was that the specialist carnivores become extinct in Africa after 1.9 million years: the sabre-toothed cats, the false sabre-tooths, and many of the hyaenas all die out as H. erectus multiplies and spreads across the planet. Something obviously happened to push these successful predators to extinction while the less specialised lion, leopard, and cheetah remained. I didn’t think that it could be coincidence that the spectacular and rapid rise of H. erectus, the superflexible generalist, coincided so precisely with this phenomenon because having erectus scavenge their kills would not have been sufficient to wipe these specialist carnivores out. I continued to puzzle over what adaptation made our ancestor so
astoundingly successful.

  But when I first heard Daniel Lieberman give a lecture, something clicked. Dan is a brilliant scientist with an approachable manner, curly hair, and a perpetually amused look on his face that matches his great sense of humour. He also has an amazing lab full of specialised treadmills and other exercise equipment hooked up to computers that he uses to look at the ways muscles and bones interact. Dan, who is passionate about human evolution, is a committed marathon runner. All those grueling long runs led Dan to think long and hard about the limits of physical endurance, and his obsession is what led to his collaboration with another functional and evolutionary morphologist, Dennis Bramble, who is based at the University of Utah. The pair came up with an inspirational explanation for how the comparatively puny H. erectus may have overcome relatively large and fast herbivores with sharp horns and deadly hooves without being injured.

  What I completely failed to comprehend as my dogs raced happily away after the fitter antelopes who always escaped unharmed is that I actually stood a better chance of catching the beast than they did. The piece of the puzzle that I overlooked is that I was comparing human athletic prowess with that of sprinters. It is true that we are poor competitors in the fields of strength and speed—but in persistence running, we are in fact the stars of the whole animal kingdom. If I (before I became a grandmother anyway!) had kept after the antelopes, they would have tired rapidly. If I chased them at midday, they would have tired even more quickly and collapsed of heat exhaustion. Poaching restrictions aside, I could have bounded up to them and clobbered them over the head with a nearby rock were I so inclined.