by Meave Leakey
This robustness lends itself to the hominin’s species name, robustus. Originally assigned to the genus Australopithecus, these southerners are now considered to belong to a separate genus, Paranthropus (“next to man”). Through comparisons of the fauna and because the robust creature is never found in the same cave deposits as the more gracile A. africanus, we know that the robust hominin is the younger of the two. What we don’t know is what it evolved from. For years, scientists argued whether robustus evolved in South Africa from A. africanus or from the lineage of anamensis-afarensis in East Africa where P. robustus had a relative that strongly resembled it: an even more impressive megadont.
The East African “robusts” had even bigger cheek teeth, an even bigger crest along the top of the skull, and even more sweeping cheekbones that are adapted to a similar type of diet requiring prolonged heavy chewing. When Mary found the most famous and first of these East African megadonts, she and Louis gave Dear Boy the species name boisei in recognition of Charles Boise, who supported them financially for many years. And my very first encounter with a hominin discovery was the Paranthropus boisei skull KNM-ER 406 that Richard and I came face-to-face with in the riverbed on our first memorable camel safari in 1969. The robust australopithecines were clearly common in the Turkana basin, where P. boisei crops up in most of the sediments younger than two million years until it died out sometime less than one million years ago. Although the hyper-robust males crop up again and again, the slighter females occur rarely. Only one partial skull has been found in East Africa. This female, KNM-ER 732, lacks the sagittal crest and massive muscle attachments of the male counterpart. We found it shortly after Richard and I found the P. boisei male skull in the riverbed.
The most impressive megadont of all is nicknamed the Black Skull for its gorgeous dark colouration. This impressive specimen was discovered in West Turkana in 1985 in 2.5-million-year-old sediments at a site called Lomekwi. This was soon after the Turkana Boy was discovered at Nariokotome and Richard had shifted focus from the east side of the lake. The field crew was exploring sediments about halfway up the lakeshore roughly opposite Koobi Fora. It was Alan Walker who found a pile of scrappy-looking bones that several of the crew had seen and dismissed as an unremarkable bovid, which naturally caused them considerable mortification when they realised what they had missed. Although they assumed at first that they were fragments from an Australopithecus skull, by the time Richard returned to Nairobi with the news the following day (happily for me, on my birthday), he had already realised that he had “some odd hominins” to show me and that he “might have to change his theories a little.”
Pieced back together into an almost complete gorilla-like skull, the individual (probably male) epitomizes many of the robust characters of its line. It had a reduced dish-shaped snout (although it still protruded in contrast to the later Paranthropus), and an enormous sagittal crest that allowed masses of room for muscle attachments and compensated for the lack of space in a very small brain. Although the Black Skull is missing all but one tooth, the broken-off roots show that the cheek teeth were huge. These big molar teeth and the prominent bony ridges for attaching big muscles are the defining features of the Black Skull and, like those of P. boisei and P. robustus, are indicative of a diet of tough fibrous foods that required plenty of chewing. The Black Skull is named Paranthropus aethiopicus not because Richard chose it but because the species had already been found. In 1968, Camille Arambourg and Yves Coppens discovered a large mandible that they named aethiopicus—which remained largely ignored in a museum. This mandible shares the same robust features as the Black Skull and also lacks teeth.
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HOW CAN WE interpret this megadont version of hominin? It is universally accepted that the differences between Australopithecus and Paranthropus are driven by dietary changes—Mary and Louis’s Dear Boy skull was also known as Nutcracker Man. But it is only comparatively recently that anyone took a long and close look at the megadont’s teeth using new technology and refined techniques that allow us to reconstruct in far greater detail the actual composition of its diet. The results from two of those techniques—examining the microscopic scratches and the chemistry of the teeth—have given us much to think about.
Scientists have long scrutinised teeth under a microscope to count the scratches and pits that scar the enamel from chewing particular types of food as a way to identify diet. But nowadays, powerful machines can do this tedious job of counting and measuring with ease, precision, and speed. Sure enough, compared to A. africanus, the greater number of pits and the larger and more numerous scratch marks suggest that Paranthropus must have been chewing on harder foods. But the chemistry of their teeth reveals a more complicated picture.
The isotopic analysis pioneered by Thure Cerling and his colleagues that proved so instructive to us at Lothagam has also been used to help explain why a group of hominins evolved such megadont teeth. These new methods are so precise that by taking a series of microscopic samples from different levels of the surface enamel Thure is now able to distinguish the isotopic composition of the enamel at different points in the individual’s life. These analyses showed that the isotopic values were surprisingly variable, changing markedly both seasonally and in different years. This is consistent with a highly variable climate, but Paranthropus, with its whopping teeth, had such extreme and specialised chewing adaptations that everyone had assumed their energies were focused on one particular type of food. But Thure concluded that Paranthropus was not a dietary specialist and was instead eating a large variety of savannah-based foods, including a variety of grasses and sedges. In fact, Paranthropus had an extremely flexible diet. In Thure’s words, these counterintuitive results “may indicate that its derived masticatory morphology signals an increase, rather than a decrease, in its potential foods.” In plain English, becoming a successful generalist and broadening one’s diet in a highly fluctuating climate is a sure way to maximise the chances of survival!
My favourite fossil monkey shows an interesting parallel evolution—the ancestors of the Ethiopian gelada baboons that, using their remarkably dexterous fingers, dine exclusively on all that grass has to offer in the Ethiopian Highlands. The geladas’ ancestors are called Theropithecus, a ground-dwelling baboon that found itself in much the same predicament as the australopithecines as the climate dried and habitats opened between three and two million years ago. Theropithecus brumpti lived between 3.5 and 2.5 million years ago and preferred closed woodland. The males sported fearsome daggerlike honing canines more than an inch long in a long snout—the typical sexually dimorphic primate pattern. But its later relative, Theropithecus oswaldi, had to make some serious compromises to survive. And survive it did: T. oswaldi is the most common and most widespread fossil monkey known in East Africa. Its range extended from South Africa to Europe and across to India, and it survived for more than 1.5 million years until extreme climate fluctuations and competition from diverse grazing bovids eventually drove it to extinction.
The evolutionary secret to this phenomenal success shows many parallels with Paranthropus. Through this 1.5-million-year period, the snout is reduced, the cheek teeth are enlarged, and the canines become quite small for a monkey. The lower incisors of a late T. oswaldi are so reduced that they are usually missing altogether in older individuals. Since this is a baboon, not a hominin, the canines still hone and are not reduced to the same degree—but considering the continuing need for canines in sexual display, the changes to the snout and canines are remarkable. The molars adapt to the selective pressure to withstand more abrasive material and more chewing in a different way from Paranthropus because the starting shape of the tooth is very different. So instead of developing thicker enamel on a flat grinding surface, T. oswaldi’s molars resemble those of grazers such as hippos: with an increasing number of convolutions and indentations and a higher crown to resist wear through the animal’s lifetime. To power the grinding in this evolving jaw, T. oswaldi has a sagittal cres
t running along the top of the skull that is bigger and better than that of P. robustus, and it accommodates the need for muscles to attach to a skull housing a smaller baboon-sized brain. The youngest T. oswaldi, less than a million years old, are also very big animals—comparable to female gorillas. This increase in body size is probably a response to predatory pressure in open country, and there is much evidence that T. oswaldi was indeed heavily predated upon, including by hominins.
We also find evidence that Paranthropus society was male dominated, which suggests that alternative methods of sexual display and defence must have evolved to compensate for the puny canines brought about by this dietary adaptation. Males in highly sexually dimorphic primate societies (gorillas and baboons, for example) have a problem: only one alpha male enjoys the undivided sexual attentions of a group of females. For the rest of the males, life is about flying under the radar, sneaking sexual favours when the dominant male is distracted, and constantly jockeying with other potential rivals to become the next kingpin. Upon reaching their prime, male gorillas become significantly larger and more powerful, weighing up to a staggering 275 kilos (more than 600 pounds). Their huge broad shoulders and their beautiful coats crowned by the distinctive silver hair on their powerful backs make these imposing ubermale individuals impossible to miss in the crowd. Although an average adult man is bigger than many female gorillas, the silverback is far heavier and ten times stronger than the biggest football players and wrestlers. These great beasts tower over females and nonalpha male gorillas, who view them with a mixture of respect and terror, deferring to them in everything and proffering preferential access to every single resource. A world with too many silverbacks would be brutish indeed. Nature’s solution to this highly stressful situation is ingenious: males have their development arrested. While female gorillas reach sexual maturity by about age seven, males remain sterile until they mature into the silverback state between the ages of eleven and thirteen. But in the interim, blackbacks are highly vulnerable, living either on the periphery of the group or in smaller groups without the protection of a silverback, so predation is much higher.
Among the australopithecines, illustrated here by KNM-ER 406 as robust and Sts5 as gracile, the robust forms show extreme megadont adaptations involving the size of the teeth and the flaring of the cheek bones and crests for the attachment of extra large muscles. Not drawn to scale.
The late Charlie Lockwood, who had an abiding and intense curiosity about the South African hominins, published a compelling study that shows that P. robustus probably also followed a pattern of extended male development like gorillas do. Lockwood went through the South African collection to pick out adult specimens with pieces of face and jaw. From the degree of wear on the cheek teeth, he estimated the age at death of each individual and found that old adult males were significantly larger and more robust than younger males and adult females. His conclusion was that while females attained their full size upon sexual maturity, males followed a pattern of extended development, growing larger after attaining dental maturity like modern male gorillas do. Lockwood also found many more males in his sample, which suggests more males fell victim to predation. When remains are found in caves, as they are in South Africa, the individuals were usually dragged there by a carnivore, as evidenced by the damage found on many of the bones.
One big and unsettled question about the robust hominins is how the South African megadonts tie into the well-dated australopithecine story in East Africa. Two different scenarios are perfectly plausible from an evolutionary standpoint. The first posits that the robust form in the east and south of the continent could have evolved in parallel from different stem species, each taking advantage of similar feeding niches of tough, fibrous vegetation. Other examples of parallel evolution abound. The venomous sting has evolved separately at least ten times: in jellyfish, spiders, scorpions, centipedes, insects, cone shell molluscs, snakes, cartilaginous fish (stingrays), bony fish (stone fish), mammals (male platypuses), and plants (stinging nettles). On an even grander scale, the radiation of pouched mammals (marsupials) in Australia parallels the evolution of placental mammals elsewhere in the world to an extraordinary degree and fills many of the same niches. Although many of the placental mammal and marsupial pairings look quite different, their adaptations are often very similar.
Today, the genus name Paranthropus is frequently applied to all the robust australopithecines, which implies that the two robust forms share a single ancestor—most probably the East African anamensis-afarensis lineage. This is the nomenclature we have followed in this book, but it is important to point out that the true evolutionary path from the early gracile australopithecines to the robust forms remains yet another conundrum in need of more fossil data.
If we look at all the robust australopithecines from East Africa in chronological sequence, we see that they exhibit increasingly megadont cheek teeth with corresponding adaptations of the skull. The later specimens have a slightly larger brain, and thus the sagittal crest is never as extreme as in the Black Skull since there is a larger surface area for attaching the large chewing muscles.
The reason that the dietary changes of the robust australopithecines is so significant is because you simply can’t be gutsy and brainy at the same time as brains and guts are both metabolically expensive to maintain. Ruminants have incredibly specialised digestive tracts to break down the tough fibrous walls of plant cells. A cow has four chambers to its stomach—a veritable factory for fermentation—where billions of bacteria, protozoa, moulds, and yeasts do most of the heavy lifting. In the first and biggest compartment, the rumen, every millilitre of rumen fluid contains an extraordinary number of microbes: some 25 to 50 billion bacteria and 200,000 to 500,000 protozoa. These organisms digest the plant fibre and produce volatile fatty acids that are absorbed directly through the rumen wall into the bloodstream and converted into glucose in the liver. They supply 60 to 80 percent of the energy needed by the cow. The reticulum, with its honeycomb-like lining, is the chamber involved with rumination. When cattle ruminate, or chew their cuds, they are regurgitating a bolus of incompletely chewed feed that they then rechew and process a second time in the rumen. The fourth chamber, the abomasum, is the “true stomach,” which functions much like the human stomach, producing acid and some enzymes to start protein digestion. The cow’s small intestines function much like ours, absorbing the digested nutrients (except for the volatile fatty acids already absorbed upstream).
When I first started working for Louis at the Tigoni Primate Research Centre, I couldn’t help noticing the colobines version of this process. Compared to the other monkeys, the Colobus were my hands-down favourites. I liked them the most because they were more relaxed and laid-back, placidly chewing leaves and sitting around with their large bellies protruding as they belched contentedly. Their behaviour was in sharp contrast to the lively, more aggressive Cercopithecus monkeys. Colobines don’t have four stomachs—but they do have two. The first compartment, or fore stomach, functions the same way as the cow’s rumen, where cellulolytic bacteria degrade the fibre. For both ruminants and leaf-eating monkeys, breaking tough fibres into digestible components takes time and requires the food to pass slowly down a long digestive tract. The Cercopithecus monkeys, in contrast, eat a lot of fruit and don’t have the fermentation fore stomach or a long digestive tract because fruits are made of simple carbohydrates that are readily digestible with enzymes available in the acid stomach.
But switching from a diet comprised of mostly plant fibres to one with higher energy yielding fruit can only reduce the gut so far. The australopithecine strategy of becoming bipedal in order to move between the trees and the opening grasslands while foraging on fruits, plants, tubers, eggs, and insects worked well when it first evolved four million years ago. But obtaining sufficient energy from these foodstuffs would have been getting more and more difficult two million years later. Not only were a great diversity of competing species moving into the same feeding niche, bu
t as we shall see, this kind of lifestyle was becoming a risky business as greater seasonal variability and longer-term extreme fluctuations in climate came into play.
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THE AUSTRALOPITHECINES responded to the increased competition and changing climate by specializing in at least two different ways—one was to consume tough fibrous vegetation, for which they had to grow robust features (big chewing cheek teeth, large muscles for mastication, and probably a long gut). As we would expect, however, the brain size of these megadonts didn’t get any bigger than that of afarensis. We already had a glimpse of the second alternative trajectory among late australopithecines in the nonrobust adaptations of Australopithecus sediba in South Africa, whose hands had evolved a precision grip. In East Africa, something similar may have been happening.
In 1997, Yohannes Haile-Selassie found yet another important fossil in Ethiopia’s Afar region—a 2.5-million-year-old skull. This fossil is puzzling. It is conceivably a possible candidate for the female counterpart to the Black Skull, but it is more generally believed to be evidence of further diversity among the late australopithecines. Because the skull lacks the pronounced sagittal crest of the Black Skull, Haile Selassie’s colleagues, including Tim White, attributed this specimen to a new species of Australopithecus, A. ghari (ghari means “surprise” in the Afar language). The combination of large cheek teeth with moderately large canines and the small sagittal crest characterizing A. ghari might arguably be expected at 2.5 million years. Although it is not yet at all clear where this hominin fits, it does emphasise the general trend of increasing megadontia seen in the australopithecines.