Reconstructing Ancestral Lifeways Using Humans and Non-Human Primates.
- THE LIVES OF EARLY HOMINIDS
- Finding out what early hominids actually did and how they acted is a tricky task, particularly since it is difficult to imagine what the actual landscape might have been like hundreds of thousands - or even millions - of years ago. While the fragmentary remains of the past are often ingeniously interpreted by researchers, additional information is often required to corroborate some of the conjectures which arise. Two additional ways in which ancestral pathways are examined rely upon ethographic and ethnographic comparison.
- Ethographic comparison relies upon inferences drawn from animal studies, especially those of the African Great Apes (i.e., gorillas, chimpanzees and bonobos). While none of the ape models are expected to provide exact replicas of hominid behavior (since apes themselves are very derived forms), the likelihood of some models can be tested through field observation. Consistent approaches by a majority of primate groups to particular ecological dilemmas strongly suggests that response to be a likely scenario in hominid evolution as well.
Ethnographic comparison draws upon human behavioral variation through the study of different cultures. Again, while no existing societies are expected to conform to an exact hominid lifestyle (no modern humans living today are evolutionary throwbacks), many insights have been made through the study of hunter-gatherer social groups. The fact that there appear to be many commonalities between culturally different hunter-gatherer societies suggests that ecological constraints often provoked similar responses from different people in the past.
- Behavioral Patterns
- One of the most important and intriguing questions in human evolution is about the diet of our earliest ancestors.
The presence of primitive stone tools in the fossil record tells us that 2.5 million years ago, early hominids (A. garhi) were using stone implements to cut the flesh off the bones of large animals that they had either hunted or whose carcasses they had scavenged.
Earlier than 2.5 million years ago, however, we know very little about the foods that the early hominids ate, and the role that meat played in their diet. Current situation due to lack of direct evidence.
Nevertheless, paleoanthropologists and archaeologists have tried to answer these questions indirectly using a number of techniques.
- 1. Primatology (Studies on chimpanzee behavior)
2. Anatomical Features (Tooth morphology and wear-patterns)
3. Isotopic Studies
- Early Hominid Environment and Ecology
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Paleoecological reconstruction is possible through the study of correlates to environment and ecology. Plants and animals which existed in particular types of environments are carefully extracted and catalogued as fluctuations inthe biosphere over a period of time. Added to this is the use of oxygen isotopes, which indicate worldwide temperature fluctuations. More recently, analysis of aeolian (wind) dust deposition has provided a more detailed record of climate change and seasonality.
All of these forms of evidence point towards an increasingly cold and dry environment with greater seasonality during the late Miocene and Pliocene eras. Reduction in forested areas most likely spelled to end for many Miocene hominoid species.
The hominids successfully adapted to open savanna and woodland environments, developing a series of different strategies for predator defense, foraging, and social behavior. One of these behavioral adaptations was possibly a shift to accomodate quantities of meat in the diet, to augment plant resources.
- Meat Eating, Cooperation, and Seasonality
- The inclusion of meat in the hominid diet has long been cited as a prominent factor in the evolution of human-like social organization. Many of the earlier theories took this concept and ran with it, from Robert Ardrey's portrayal of the Killer Ape, to the "sanguinary habits" which convinced Raymond Dart of our evolutionary history of bloodlust. From a more contemporary functional ecological perspective however, meat is a precious commodity for modern hunter-gatherers, providing a bonanza of calories and proteins which are otherwise difficult to acquire. The desirability of meat, however, obscures the reality of day-to-day existence: hunting parties, more often than not, return emptyhanded. Bearing that in mind, there are two factors which make a system of foodsharing desirable - if not absolutely necessary - for a reliance on meat in the diet to be feasible:
high payoff for success: success often brings enough food for the entire social group
low individual success rate: successful hunts are likely unpredictable and irregular, making success for any individual hunter relatively low at a particular point in time
These conditions suggest that a network of foodsharing would greatly benefit all participating parties by providing a buffer against starvation (or malnutrition) due to individual irregularities in success. Such a system would be an obvious example of reciprocal altruism, where individuals who did not participate in foodsharing were rapidly excluded from subsequent events.
An additional factor influencing the increasing amounts of meat in the hominid diet may have been accentuated seasonality in the environment. The dry season decreased resource variety and abundance, causing many animals to divert their foraging strategies to exploit more of a single food item, or a greater variety of foods they may not have sought out before. These might include underground storage organs in plants, nuts, or other specialty food items to compensate for an overall decrease in resource abundance. Robert Foley (Cambridge University) proposed that during human evolution, two modes of adaptation were exercised by the hominids.
The robust Australopithecines developed masticatory specializations which allowed them to exploit tougher plant materials, while the more gracile species relied upon a diversified diet - which included more seasonal meat consumption.
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When Did We First Use Tools?
It is hard to imagine life without tools—finding food with our bare hands, eating it raw with our teeth, seeking a cave or a tree for shelter. In fact, our reliance on tools is reflected in our brains and bodies. The areas of our brains responsible for things like controlling our hands are enlarged compared with other primates.
Our hands themselves are different, with proportionately longer thumbs and other anatomic changes that allow us to touch our fingertips and hold tools with more skill.
The dawn of tool use was a crucial turning point in human history: It let our ancestors take control of their lives by finding food in places that were off-limits to their ancestors. But scientists still have hardly any clues to how that evolutionary transition took place.
The most reliable record of our technological history comes from the tools themselves. The oldest known hominid tools date back 2.3 million years, to a collection of chipped rocks in Ethiopia. They don't look like much, but with them hominids could butcher an elephant or crack open a wildebeest's bones and suck out the marrow.
Mentally, they're also a big accomplishment: They require a brain capable of looking at an untouched rock and seeing a tool hiding within it.
In recent years, however, some hints have emerged that human technology may have roots reaching back millions of years further into the past. For one thing, chimpanzees and other apes have proved surprisingly gifted at making tools. In order to walk across thorn-covered ground, chimpanzees can fashion sandals out of leaves. In order to eat termites, they can strip sticks to create fishing tools. Unfortunately, a leaf-sandal doesn't leave a fossil. But some researchers believe that the hands of hominids may shed some light on the mystery of tools.
For example, Lucy and her A. afarensis fellows lived a million years before the oldest tools. Despite having curved, chimplike fingers, this hominid also had an elongated thumb that could make contact with its fingertips.
"There's nothing to say that these creatures couldn't make crude stone tools," says Bernard Wood of George Washington University. It's possible that hominids had already become skilled with wood and other materials 3.5 million years ago, paving the way to mental breakthroughs for making stone tools.
As intriguing as this hypothesis may be, however, many researchers think there's not enough evidence to say anything definitive about the evolution of tool use. Tim White, a paleoanthropologist at the University of California at Berkeley, says any speculations "would be strictly X-Files."
- Dexterity and Early Tools
- Early stone toolmaking marks an important juncture in evolution. The Oldowan stone tools from Hadar, Ethiopia, are among the oldest known, dating back 2.3 million years.
In the thumb diagram, the shaded areas show three muscles in the human thumb that chimpanzees normally lack.
A few anthropologists have gotten quite good at what seems a rather odd preoccupation: making stone tools the way our hominid ancestors did. They can find a good rounded stone to be the "core," pick up another, called a "hammerstone," and proceed to knock fine-edged "flakes" that are very sharp and could be used to cut meat and scrape animal bones. These hardy individualists have immersed themselves in stone tool manufacture because the point at which our ancestors began to do this marks a critical juncture in evolution. Animals like the chimp -- and even Darwin's finches -- can use sticks they've found as tools. But even when chimps are given stones and shown how to chip tools from them, what they produce doesn't compare to even the most primitive implements that show up in the hominid record about 2.5 million years ago.
Part of the problem is that chimpanzee hands are not well adapted to such delicate work. Chimp thumbs are much shorter in relation to the rest of their hand than are ours, making a "precision grip" between thumb and forefinger difficult. And recent research using 3-D imaging techniques has shown that a humanlike "power grip" is beyond their capabilities as well. The relative importance of these two grips in early stone tool manufacture and use is an area of active research.
But those tools, known as Oldowan because they were first found in Africa's Olduvai Gorge, also reflect some subtle but important advances in thinking that are beyond the abilities of even the smartest chimps. For example, only human ancestors, not chimps, showed the ability to choose the best fine-grained rocks to use, and to understand where and at what angle to strike the core stone to produce a sharp flake.
The discovery of the tools has raised the still-unanswered question of who these tool-makers were. It would be tempting to say they must be early members of the Homo lineage -- that is, on the road to modern humans -- but there aren't any Homo fossils older than 2.3 million years ago, which is too late to be the fabricators of such tools. Recently, a new species of hominid, named Australopithecus garhi, was discovered in association with fossil animal bones that bear the distinctive marks of stone tools, suggesting another candidate for an early toolmaker. A. garhi dates from 2.5 million years ago, and was a small-skulled creature, indicating that tool use came before any significant expansion in the brain.
Indeed, one hypothesis proposes that tool use, and the higher-quality diet that it made possible -- whether marrow from scavenged bones, termites extracted with bone tools, as recent research from South Africa suggests, or tubers obtained with digging sticks -- may have been a necessary condition to allow the evolution of large brains, which require a huge amount of nutrition.
Whenever and among whomever toolmaking arose, it came long after the transition from a tree-based locomotion style to the upright-walking stance that freed the hands, which took place at least 4 million years ago and possibly as far back as 6 million years ago. It took considerable change in the hands and fingers to create the deft manipulative abilities of the Homo species. Major changes show up in the fossil record of the species known as Homo habilis, or "handy man." These individuals had a mobile thumb joint, powerful muscles to bend the fingers, and large fingertips -- all adaptations that may have made possible the making and use of stone tools, and came just as the human brain was expanding and undergoing reorganization.
Once tool manufacture and use became an essential component of the human lifestyle, affecting the amount and quality of food an individual could obtain, tool use may have driven the further evolution of the human hand. Individuals whose hands happened to be better adapted for tool use would tend to be better nourished and to successfully raise more children, who in turn would be likely to inherit their parents' hand morphology.
Thus our ancestors' manipulative abilities increased, until the modern human capacities of grasping and manipulating, which allow us to throw a curve ball or play a symphony, were much as they are today.
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- Chimps Make Weapons to Hunt
- Chimpanzees have been seen using spears to hunt bush babies, U.S. researchers said on Thursday (02/22/2007) in a study that demonstrates a whole new level of tool use and planning by our closest living relatives.
Perhaps even more intriguing, it was only the females who fashioned and used the wooden spears, Jill Pruetz and Paco Bertolani of Iowa State University reported.
Bertolani saw an adolescent female chimp use a spear to stab a bush baby as it slept in a tree hollow, pull it out and eat it.
Pruetz and Bertolani, now at Cambridge University in Britain, had been watching the Fongoli community of savanna-dwelling chimpanzees in southeastern Senegal.
The chimps apparently had to invent new ways to gather food because they live in an unusual area for their species, the researchers report in the journal Current Biology.
"This is just an innovative way of having to make up for a pretty harsh environment," Pruetz said in a telephone interview. The chimps must come down from trees to gather food and rest in dry caves during the hot season.
"It is similar to what we say about early hominids that lived maybe 6 million years ago and were basically the precursors to humans."
Chimpanzees are genetically the closest living relatives to human beings, sharing more than 98 percent of our DNA. Scientists believe the precursors to chimps and humans split off from a common ancestor about 7 million years ago.
Chimps are known to use tools to crack open nuts and fish for termites. Some birds use tools, as do other animals such as gorillas, orangutans and even naked mole rats.
But the sophisticated use of a tool to hunt with had never been seen.
Pruetz thought it was a fluke when Bertolani saw the adolescent female hunt and kill the bush baby, a tiny nocturnal primate.
But then she saw almost the same thing. "I saw the behavior over the course of 19 days almost daily," she said.
Planning and Foresight
The chimps choose a branch, strip it of leaves and twigs, trim it down to a stable size and then chew the ends to a point. Then they use it to stab into holes where bush babies might be sleeping.
It is not a highly successful method of hunting. They only ever saw one chimpanzee succeed in getting a bush baby once. The apes mostly eat fruit, bark and legumes.
Part of the problem is this group of chimps is shy of humans, and the females, who seem to do most of this type of hunting, are especially wary. "I am willing to bet the females do it even more than we have seen," she said.
Pruetz noted that male chimps never used the spears. She believes the males use their greater strength and size to grab food and kill prey more easily, so the females must come up with other methods.
"That to me was just as intriguing if not even more so," Pruetz said.
The spear-hunting occurred when the group was foraging together, again unchimplike behavior that might produce more competition between males and females, she said.
Maybe females invented weapons for hunting, Pruetz said.
"The observation that individuals hunting with tools include females and immature chimpanzees suggests that we should rethink traditional explanations for the evolution of such behavior in our own lineage," she concluded in her paper.
"The multiple steps taken by Fongoli chimpanzees in making tools to dispatch mammalian prey involve the kind of foresight and intellectual complexity that most likely typified early human relatives."
- Summary
- Much of the evidence for the earliest hominids (Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus ramidus) is not yet available.
Australopithecus anamensis shows the first indications of thicker molar enamel in a hominid. This suggests that A. anamensis might have been the first hominid to be able to effectively withstand the functional demands of hard and perhaps abrasive objects in its diet, whether or not such items were frequently eaten or were only an important occasional food source.
Australopithecus afarensis was similar to A. anamensis in relative tooth sizes and probable enamel thickness, yet it did show a large increase in mandibular robusticity. Hard and perhaps abrasive foods may have become then even more important components of the diet of A. afarensis.
Australopithecus africanus shows yet another increase in postcanine tooth size, which in itself would suggest an increase in the sizes and abrasiveness of foods. However, its molar microwear does not show the degree of pitting one might expect from a classic hard-object feeder. Thus, even A. africanus has evidently not begun to specialize in hard objects, but rather has emphasized dietary breadth (omnivore), as evidenced by isotopic studies.
Subsequent "robust" australopithecines do show hard-object microwear and craniodental specializations, suggesting a substantial departude in feeding adaptive strategies early in the Pleistocene. Yet, recent chemical and anatomical studies on A. robustus suggest that this species may have consumed some animal protein. In fact, they might have specialized on tough plant material during the dry season but had a more diverse diet during the rest of the year.
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