The Sediments of Time

by Meave Leakey

  Birthing human babies is a challenging compromise between the size and shape of the brain and the pelvis, and our babies are much less developed at birth than those of other primates.

  If we model the length of gestation for a human foetus to be developmentally equivalent to that of a chimpanzee at birth, the gestation would be twenty-one months. Nature’s solution is for the human foetus to gestate for as long as possible in the womb and then continue to develop on the other side of the birth canal in a “secondary altricial” state. This is all about our big brains: at birth, a human baby has a brain that is 25 percent of the size of an adult’s, but this doubles a year later to 50 percent. But in all other primates, the growth of the brain slows at birth. In other words, the brain of the human baby continues to grow at the fast foetal rate for twelve months after birth.

  In general, mammals follow one of two alternative reproductive strategies. Altricial birthing allows the birth of more than one infant at a time: usually a whole litter can be accommodated in the womb to maximise the number of offspring born so enough of them survive to propagate the species. Altricial babies are born blind, deaf, and unable to fend for themselves, like newborn puppies. An extreme example of an altricial baby is a joey—the infant kangaroo that, still in foetal form, struggles its way up into its mother’s pouch at birth, where it latches on to a teat and keeps on growing until it is much more fully developed. Precocial babies, in contrast, are generally born one at a time. This enables the mother to keep them in the womb for longer. Because they are born fully developed and independent, they are more capable of escaping predators and surviving right after birth, much like a newborn gazelle is able to get up and run after its mother in a matter of minutes.

  Human babies are unique in that they fit neither pattern completely. They are secondarily altricial because we have reverted from the common primate pattern of precocial births to a more altricial state in order to compensate for the constraints imposed by the combination of a bipedal pelvis and a big brain. In some respects, human babies are still precocial—they soon open their eyes and they can hear and respond to stimuli—but they are altricial in their complete dependence on others and their undeveloped motor coordination.

  Another consequence of the unique shape of our pelvis is that, unlike all other primates, humans find it next to impossible to have unassisted births. While a baby’s head is narrowest between the ears and widest from the forehead to the back of the head, the human birth canal begins at the inlet with its widest point from side to side but shifts midway by 90 degrees so the long axis is front to back at the outlet rather than side to side. Consequently, the baby must contort and twist around during its passage through the birth canal so its widest dimensions (the shoulders and head) are always aligned with those of the changing shape of the birth canal. The baby, which enters the birth canal facing sideways, exits facing backwards. A chimpanzee or monkey mother can easily reach down to guide her forward-facing baby out of her own birth canal, clear her infant’s breathing passage, and begin to nurse. Because of the precocial development of the foetus, it can grab hold of its mother and assist in its own delivery once its hands are free. For a human baby, this much exertion so early on is impossible. Moreover, because the baby is born facing backwards, the mother risks injury to its spinal cord, nerves, and muscles by pulling against the natural curvature of its spine. Complications to delivery, such as the umbilical cord being wound around the baby’s neck, can likewise be solved only by a helping third party, so humans in all but exceptional circumstances rely on assistance at birthing. For millennia, experienced grandmothers or elder women have acted as midwives to assist with the risky business of birthing human babies.

  The abbreviated gestation period for a human baby can be explained by problems arising from the combination of two key human traits—bipedalism and a large brain. Extended childhood, as well as an extended life history long after menopause, are likewise unique to our species, and both are related to our unusually large brain. The extended childhood serves several functions. Delaying the growth to a full-sized adult body permits more energy to be spent on the calorically costly growth of the brain before puberty. The body then catches up at the end of adolescence with a final growth spurt. During childhood and adolescence, the smaller body size ensures that the young are nonthreatening to adults and that they are the recipients of social goodwill and teaching. Our large brain allows us to be an unusually innovative species, and our ability to speak enables us to form a collective body of ever-increasing knowledge that far exceeds that which could be achieved in the span of a lifetime. This body of knowledge is passed from generation to generation, and speech is a pivotal force in making us who we are today. Needless to say, modern humans therefore need an extended childhood for all this learning, and grandparents, with the accumulated wisdom of age and a postmenopausal period free from the constraints of childbearing, are traditionally the best-positioned members of a social group to make this contribution.

  Having adult females around for such a long time who are not reproductively active may seem at first glance to be a poor strategy for a species’ survival. But if we look more closely, we find that this is a very successful strategy as it reduces the interbirth interval for reproductive females. On average, chimp mothers are able to bear young only every five and a half years (although this can vary between four and six and a half years) because each infant must be fully weaned and independent before the mother can turn her attention to a newborn. Human mothers are usually able to bear young at twice this pace. The two-year-old is nowhere near as developmentally advanced when the new sibling arrives, but the grandmother can help. With their long postmenopausal period that allows for collective nurturing, human grandmothers significantly increase the fertility of the group.

  Looking at the life history of ancient hominins is an illuminating way to explore how human our ancestors were and when they became more like us. But if we are to learn anything about brain size relative to body size and how the mechanics of birthing were accomplished, it is essential to have skeletons associated with skulls. We know from Lucy’s skeleton and other fossils from South Africa that the australopithecines, while bipedal in stature, retained small brains and did not suffer the complications of birthing faced by modern humans. But what about our direct ancestors, Homo erectus?

  The jewel in the crown of the H. erectus sample is the Turkana Boy. This is the remarkably complete skeleton that began as an unpromising matchbox-sized piece of skull found by Kamoya on a short Sunday stroll from his Nariokotome camp in 1984. Because the excavation extended into a high bank, tons of soil that covered the horizon in which the bones were buried had to be removed, but the dividends were unimaginable as bone after bone emerged. Five field seasons later, however, the cost of continuing the enormous excavation simply outweighed the remote chance that we might find the missing hand and foot bones, which are particularly informative and which we desperately hoped for. But even without these missing appendages, we had an amazingly complete skeleton, skull, and mandible of a single individual. This is what makes this find so valuable: having all these elusive bones from a single individual permits many deductions that are impossible to make from single limb bones or unassociated skulls. Frank Brown reconstructed the geology for us, and the samples of tuff that he sent to Ian McDougall came back with a date of between 1.51 and 1.56 million years. This was later narrowed down to a most likely age of 1.53 million years.

  One of our first questions was how old the Turkana Boy was when he died. We could quickly see that he was not yet fully grown because all of his lower teeth had erupted except for the wisdom teeth (third molars) and the milk deciduous canines were still in place in the upper jaw—although we could see the adult canines just peeping out of the sockets. Alan Walker was soon asking my children and the local youths helping out on the site to open wide. But Louise and Samira were both dentally younger than the Turkana Boy—and none of the other children could tell us how old th
ey were in years because they were only able to say that they were “born the year of the bad drought” or some other seminal event that their parents remembered. Alan then turned to the mouths of some of Louise’s friends and provisionally settled on a working age of eleven to twelve based on the sequence and timing of human dental eruption today.

  This date differed somewhat from our initial ballpark estimate based on the long bones. In juvenile mammals, it is the shafts on our bones that grow the most while the articular ends (epiphyses) do not grow at the same rate. The two parts join together only when the growth process is nearly complete. In humans, the lower epiphysis on the upper arm bone (the humerus) is the first of the long bones in our limbs to fuse. In the Turkana Boy, this bone was almost completely fused whereas none of the other epiphyses had begun to fuse. The Turkana Boy was therefore a subadult who had not yet reached puberty and probably still had to undergo the final growth spurt. This timing, based on comparisons with modern humans, suggested a skeletal age for the Turkana Boy of eleven to fifteen years at the time he died.

  The question was: did the Turkana Boy grow along a human growth trajectory or not? This would make a difference in how old he was when he died. Irrespective of his absolute age, his relative age would remain the same whatever growth trajectory he was on—he was about 65 to 75 percent of the way towards being fully grown. But a chimpanzee needs fewer years to arrive at the same point of maturation than a human. Had Alan looked into the mouths of chimpanzees rather than children, he would have decided that the boy was approximately seven and a half.

  Once the Turkana Boy’s bones had been cleaned of matrix and the fragments reassembled, Alan enlisted the expertise of several colleagues to help him get to the bottom of this conundrum. The first order of business was to figure out the stage of foetal development at birth. Would a H. erectus baby be precocial, like a chimp, or secondarily altricial, like us? This would depend on the size and shape of the birth canal, and on the development of the brain from birth through adolescence. Remember that the human brain is 25 percent of the adult size at birth, doubles during the first year, and doubles again to reach its adult size at the end of adolescence. In contrast, a chimpanzee’s brain is already 40 to 50 percent of its adult size at birth.

  The bones of the Boy’s skull were not yet fused together and were so superbly complete that Alan was able to make a remarkably accurate reproduction of the inner surface of the skull and mould an endocast. Because the brain fits snugly into the skull, this endocast gave a very precise size for the Turkana Boy’s brain. When he died, his brain was 880 cc, and regardless of whether he followed an ape or human growth trajectory, his adult brain would have grown to only about 909 cc. This is two-thirds the size of an average modern human brain (1,350 cc) but more than double that of a chimp (400 cc). We might then expect that for H. erectus females, birthing had already begun to present difficulties.

  Alan faced a more formidable challenge next. From the partially complete subadult male pelvis, he needed to estimate the size and shape of an adult female birth canal. Alan reconstructed the pelvic bones as best he could and arrived at a pelvis shape much like ours only narrower. By his calculations, the baby’s head at its widest point could have been no larger than 110 mm, which would accommodate a brain size no larger than 231 cc. To start life at birth with a brain size of 231 cc and arrive at an adult brain size of 909 cc, the brain would have to quadruple in size. In short, it would be impossible for the growth to follow an ape trajectory and end up with a brain this large. Alan concluded that the female birth canal would have been too small to have allowed the passage of a H. erectus baby that was born precocially—with its brain size already nearly half grown as happens in a modern-day African ape. H. erectus would have already had to adopt the secondarily altricial strategy of later humans.

  Alan’s conclusions lean heavily on his pelvic reconstruction, which suggests a slender-hipped physique that could not admit a very large-brained baby. But because of the fragmentary nature of the Turkana Boy’s pelvis, Alan’s reconstruction was part science, part art—a fact he admits to in his gripping account of his study of the skeleton, The Wisdom of the Bones. Since this groundbreaking study by Alan and his colleagues in 1993, additional evidence has come to light from new scientific technology that reopens the question of whether H. erectus was already following a life history pattern like ours or whether it grew up more like an ape.

  It turns out that a critical piece of evidence already existed—found in 1936 at Mojokerto in Java. This specimen is a well-preserved cranium of a young H. erectus that likely lived at approximately 1.5 million years ago (the exact location of this early find is unclear, constraining attempts to date it through modern techniques). Because this specimen has no teeth, it was hard to estimate its age, and for a long time, it was speculated that it died when it was between five and six years old. Only recently, CT scanning techniques have shown this to be far too old an estimate and suggest that the child was probably only about twelve months old when it died. The giveaway was two small bones on the base of the skull right in front of the foramen magnum that fuse together right around puberty. The suture between these bones (the spheno-occipital synchondrosis) gives a good estimate of the child’s developmental age, but it is often difficult to see this clearly in a fossil. Using CT scanning, however, it became clear that this suture in the Mojokerto skull was still unfused. Suddenly, we had a one-year-old baby with a brain size we could reliably estimate—and this allowed us to learn much more about infant brain development.

  The Mojokerto baby had a brain capacity of 663 cc—which, for a one-year-old H. erectus on a human growth trajectory, seems implausibly large. If H. erectus grew on a human trajectory, the brain at age one would still have to double before puberty, bringing it to more than 1,300 cc, which is the size of an average human brain. But if it grew like an ape, it would already be 70 percent of an adult’s size. When we compare the baby’s brain size to an average size for all the adult H. erectus brains from Asia, the Mojokerto baby has a brain that is 72 percent of an adult’s. This strongly implies a trajectory that is as close to that of a chimp’s as to a human’s and that the Mojokerto baby was more likely born as a precocial infant or, at most, partially secondarily altricial. It is probable that the fragmentary nature of the Turkana Boy’s pelvis—combined with the extrapolations that Alan had to make about its maturation into adulthood as well as what the female H. erectus pelvis would have looked like—introduced a considerable degree of error and explains Alan’s contrary result.

  Dental development can also tell us a great deal about the childhood phase of life history. Alan enlisted the expertise of Holly Smith, who specialises in life history patterns and determining the age of a fossil individual from the development of its teeth. She began by mapping out the sequence of tooth development in a monkey, a human, and an ape. Because the Turkana Boy’s pattern of tooth development in no way resembled the macaque monkey, she discarded this from her comparison. Every human’s teeth erupt in roughly the same order at roughly the same age. Instead of only looking at which teeth had already erupted, Holly could get a far more detailed sequencing by using X-rays to look at the development of the roots and crowns of the teeth within the jaw. She was able to plot a chart that showed what stage of development each tooth would be at for every year. At age five, the human adult front teeth and the first molars have fully formed crowns but only partially formed roots. On the other hand, the premolars and second molar have incompletely formed crowns, and the roots have not even started to develop, and the third molar has neither crown nor root formed. In contrast, a chimpanzee grows its teeth more quickly and not in the same order. Because it is much more prominent, the canine starts to grow a little later and takes longer to fully develop.

  Holly then superimposed the sequence and timing of the Turkana Boy’s tooth development over her charts of what age the corresponding state of development would be in a chimp and a human. Neither one was a perfect fit, b
ut the majority of the Turkana Boy’s teeth had developed approximately as much as those of a nine-to-ten-year-old boy or a seven-year-old ape. His canine developed more like ours than a chimp’s, which makes sense because it was relatively much smaller than a chimp’s. Holly concluded two things: first, that the Turkana Boy would have been about nine and a half years old when he died; second, that he grew up on a time scale intermediate between chimps and humans so he would not have been comparable to a nine-year-old human in terms of his social development. In Alan’s words, the Turkana Boy had “the height of a fifteen-year-old, with the brain of a one-year-old . . . a staggeringly different view of our ancestors.”

  New technology using sophisticated microscopes has also yielded important information about age and dental development. If you look at a tooth in cross section under a powerful microscope, you can actually count the daily increments of enamel secretion. Much like rings formed in a tree trunk at regular intervals, these daily growth increments form clear lines known as the striae of Retzius. On the enamel surface, these Retzius lines manifest themselves as perikymata: horizontal lines and grooves running across the circumference of the tooth that are clearly visible under the microscope. They give us a chronometer that is both accurate and perfectly preserved, and allows us to compare the rate of accumulation of enamel on the teeth of humans, apes, and fossil hominins. Because we can count the daily increments, we can count the number of days it takes for each perikymata to develop and estimate the length of time it takes for each tooth to grow. This gives us an accurate means of determining the actual age of an individual until the teeth are fully formed.