Authors: Ian Tattersall
Partially reassembled replica of an “Oldowan” stone tool, consisting of a pebble of fine-grained volcanic rock with several sharp flakes successively chipped from it. Replica by Peter Jones; photo by Willard Whitson.
Whoever it was, exactly, that had made the Gona tools and the Bouri (and Dikika) cut-marks, these extraordinary finds are witness to a revolutionary behavioral innovation among hominids. Extensive coaching of a bonobo called Kanziâa star in ape “language” experiments, and a cognitively admirable representative of his speciesâfailed to teach him to hit one rock with another at exactly the angle and force necessary to detach a sharp flake. He rapidly got the idea of using such flakes to cut a cord that held a piece of food just out of his reach; but he never really picked up the principles of shaping stones. Eventually he developed a preference for throwing a rock on the floor to shatter it, and then picking through the fragments to find a sharp one. This may actually have had as much to do with Kanzi's hands as his brain and learning capacities. Making stone tools is not only hard on the hands, but it also requires a hand that is capable of holding objects precisely.
Our hands, with their broad palms, long thumbs, and ability to oppose the thumb to the tips of all the other fingers, are ideally structured to manipulate objects. This ability demands the rearrangement of a whole host of palm muscles to promote delicate movement rather than strength. The hands of living apes are, in contrast, very differently proportioned. They are much longer and narrower than human hands, and the muscles and tendons are arranged to flex the long fingers with enormous powerâwhich is exactly what you want when you spend most of your life hanging on to tree branches. What's more, because of the apes' knuckle-walking proclivitiesâwhereby they bear the weight of the front of the body on the outside of the flexed fingers when they're ambling around on the groundâthe tendons of the apes' hand flexor (closing) muscles are shorter than those of the straightening extensor muscles, making it impossible to extend both the wrist and fingers at the same time. This kind of strongly flexing hand is far from ideal for making tools, an activity that requires hugely precise movement and placement of the digits.
Exactly why the early hominid toolmakers already possessed hands that were up to this unusual task is not clear. Logically, there must have been some advantage to losing the specialized grasping capacities of the
a development that we already see quite strongly expressed in the hand bones from Hadar despite the still-curved fingers. And chances are there were no knuckle-walkers in their ancestry. But whatever the countervailing benefit was, it wasn't the ability to make stone tools, which as far as we can tell only started to be done well after the australopiths and their various physical characteristics, including the manual ones, were already well ensconced. Still, even though we are in the dark as to the exact circumstances, there is a really important lesson to be gained from this episode in human evolution: you can't use a structure until you have it. As a result, most of our so-called “adaptations” actually start life as “exaptations”: features that are acquired through random changes in our genetic codes, to be co-opted only later for specific uses. Natural selection is, quite simply, in no position to drive new features into existence, no matter how advantageous in theory those features might be. This is yet another reason why the idea that the Dikika hominids butchered animals using naturally fractured stones is attractive, for it is otherwise very odd that there is no evidence of stone tools or their use in the long lapse of geological time between the Dikika and Bouri/Gona finds.
EARLY HOMINID LIFESTYLES AND THE INTERIOR WORLD
Emerging from the deep forests into the forest edges and the adjacent woodlands and bushlands was a major commitment for the early hominids. And it was one with multiple consequences. Not only did this ecological shift involve a radical change in diet and locomotion, but it also brought with it a huge increase in vulnerability to predators. What's more, it laid bare several fundamental differences between our ancestors and their close ape relatives. While apes move around on all fours when they venture into savanna environments, the hominids evidently responded to the new challenges not only by standing upright, but also by adopting a radically new dietary strategy. The consequent changes made enormous new demands on their various bodily systems. Let's see how they adjusted.
STAYING WELL FED
A major imponderable in paleoanthropology is how the early hominids contrived to cope with a new diet that evidently included animal fats and proteins. Even today, we meat-eating humans have digestive tracts
more closely resemble those of our vegetarian ancestors than they do those of carnivores. And our teeth, small as they have become, are essentially those of plant-eaters, emphasizing grinding rather than carnivorous cutting. But at some point those ancient vegetarian bipedal apes became interested in animal carcasses: an interest based on the potential of those cadavers to provide food. Dealing with this unfamiliar dietary supplement posed a host of problems.
Red meat straight from the carcass would have been indigestible for those early hominids, whose stomachs would not have been filled with the highly concentrated acids that allow modern carnivores to break down bone and muscle tissues before delivery to their short intestinal tracts. One possibility is that our forebears used those multiply bashed rock cores to pound red meat in order to break down the muscle tissue and render it somewhat more digestible. Another is that they avoided muscle tissues altogether, and concentrated on eating the internal organs of dead animals. We know this happened at least sometimes in the early days of the hominids: a 1.7-million-year-old skeleton from northern Kenya shows distortions of the bones probably caused by an excessive intake of vitamin A, most plausibly from a carnivore's liver. But a specialized interest in offal seems unlikely, not only because these organs provide the preferred foods of the primary carnivores and scavengers with which the hominids must have competed, but because of those telltale cut-marks. Some of those marks, at least, must have been made in the process of removing red meat from the limbs, for they would not have been made where they were in the process of simply opening the abdominal cavity and removing organs.
Another possibility that has received a lot of recent publicity is that hominids made flesh more digestible by cooking it over fires. Cooking certainly makes nutrients in both plant and animal foods much more readily available to the enzymes of the stomach; and there is some evidence that people today find it hard to maintain their weight on diets composed entirely of raw food. The big difficulty here, though, is that there is no good evidence for controlled fire before about 800 thousand years agoâand the regular use of fires for cooking seems to have begun only well after that. Still, some authorities think that the increase in average hominid brain size that began some two million years ago could
have been made possible by higher-quality (i.e., higher-fat, higher-protein) diets than plant materials alone could have supplied. The brain is an extremely energy-hungry organ, whose consumption of calories increases with its size. You simply can't maintain a brain that is larger than absolutely necessary without some caloric compensation; so it is argued that some animal protein must have been available in the otherwise pretty non-nutritious hominid diet to fuel the brain's enlargement.
There are various independent indicators that hominids have been eating meat for a long time. Bizarrely enough, one of these comes from the study of tapeworms. Different forms of these ubiquitous intestinal parasites are specific to particular hosts, and it was long thought that humans were first afflicted by them when they domesticated cattle and started living in close proximity to herds. But according to molecular studies, tapeworms got into hominid populations very early on, presumptively as a result of sharing antelope carcasses, and thus remnant saliva, with carnivoresâprobably lions, wild dogs, or hyenas.
This finding fits with studies of stable isotopes (alternative forms) of carbon preserved in the teeth and bones of australopiths. These studies are based on the principle that “you are what you eat.” Most plants fix atmospheric carbon dioxide along what is known as the C
pathway. This results in a low abundance of the carbon isotope
C in the bones and teeth of animals that eat them. Some kinds of vegetation, however, including tropical savanna grasses, use the alternative C
pathway. Eating such resources results in a greater quantity of
C in the tissues of the animals consuming them. The resulting chemical signals, measurable in the teeth, get transferred from animal prey to the predators that eat them. The relative abundance of these isotopes in its tissues thus provides a clue to an animal's diet, irrespective of whether it is or was a primary herbivore or lay higher up the food chain.
Isotope studies have confirmed what was already known from behavioral observations, namely that today all chimpanzees, even ones living in open country, stick to the kind of C
diet furnished by the forests. Numerous australopiths, on the other hand, show a strong C
signal; and since it is improbable that they were all grazing on grass, this signal must have come from grazers they were eating. Plausible candidate victims include such creatures as hyraxes, or young grazing antelopes.
does not mean that early hominids were mostly eating meat; but the isotopic signal indicates that they had departed from their ancestral diet of forest plants, and had become significantly more omnivorous. So unless they were raising herdsâa practice that began only well after
had become its modern selfâthey had to hunt for their meat or scavenge for it. Among living hominoids chimpanzees occasionally hunt, but they don't significantly scavenge. What's more, while chimpanzees mostly hunt cooperatively, the sharing of their kills seems to be much more significant in reinforcing social bonds within the group than it does in contributing to their diet. And when chimpanzees do hunt, they go after animalsâcolobus monkeys, blue duiker, bushbabiesâthat have a forest diet, and thus a C
signal in their tissues.
Something different was clearly happening in the case of the australopiths who, wherever they may have spent most of their time, were almost certainly getting most of their C
component from the cadavers of grazing animals obtained away from the deep forest. Since they were small bodied and not particularly fleet of foot, the most obvious C
source would have been scavenged carcasses; but out there in the open, competition for this relatively rare source of sustenance must have been rough. Even more importantly, dead cadavers quickly turn toxic, and living primatesâ including humansâlack any of the specializations for dealing with this huge problem that full-time scavenging animals such as vultures have. Once decomposition sets in (which in the tropics will not be very long after the initial hunters have departed), the virus, microbe, and other parasite populations inhabiting the carcass skyrocket, and the flesh rapidly becomes not only indigestible but potentially lethal. Little wonder that scavenging is so rare among living primates: one study of chimpanzees in Uganda found that they spontaneously encountered opportunities to scavenge fresh carcasses about four times a year, but only even tasted the meat about one time in ten, or once every two and a half years. All in all, scavenging of old carcasses does not appear to be a very attractive proposition for primates in general; and why early hominids should have taken to it in any major wayâif that's what they didâis not at all apparent.
that nagging C
signal. And one suggestion is that early hominids became meat-
when they began spending significant amounts of time away from the closed forests. Hominids and leopards,
, seem to have had a particularly intimate relationship since the ancestral hominid environment was broadened to include woodland and bushland (one australopith skull fragment from South Africa even bears holes made by leopard teeth). Leopards, fearful that larger carnivores will take over their kills, are frequently seen stashing the cadavers of their prey high in trees for safekeeping while they are off patrolling their territories. Australopiths might well have capitalized on their considerable climbing skills to scoot up trees and steal bits of carcass in the leopards' absence, a risky activity that would certainly have been facilitated by the ability to quickly cut off chunks before making a hasty getaway. In that event, perhaps we shouldn't be surprised by discovering that stone tools were first invented not by members of our proud genus
but by bipedal apes. This perspective does make it possible to see that the use of stone tools could have made achievable the major dietary shift that underwrote the remarkable developments to come. And at the very least, while this story of stealing fresh meat doesn't come close to tying up all the loose ends, it opens up new possibilities.