After reading this chapter, students should be able to:
list the names and sequence of hominins, or species that were ancestral to humans, who lived in Africa between 6 million and 2 million years ago.
relate hominin characteristics to both ancient and modern primates.
appreciate how external factors, like environment or climate, can impact the evolutionary paths of species.
know how internal factors, such as social behavior and "learned" culture, can impact the evolutionary paths of species.
understand the physical characteristics - skulls, dentition, postcranial features - that place hominins on a distinctively human pathway.
make inferences from hard tissue to brain development, posture, and diet.
describe how social behavior can be determined from the fossil record, and what behaviors are associated with each species.
explain what paleoanthropologists and archaeologists currently know about hominin tool use, and the types of tools associated with each species.
understand how biological traits interface with and are related to tool-making, social behavior, adaption, and reproductive success.
discuss archaeological sites associated with various hominin species, and how the evidence at these sites has been used to trace or infer physical and behavioral attributes and change through time.
understand how small fragments of evidence can be assembled into cogent arguments for the evolution and phylogeny of hominins.
EVOLUTION AND HUMAN ORIGINS
Charles Darwin used the term descent with modification
to describe changes in the form and behavior of species through time. This process, also called evolution by natural selection
, was the first theory that offered a reasonable explanation of how changes occur over time in species and how new species emerge.
Natural selection occurs when fitness within a particular environment leads to differential reproductive success: within a species, in each population (a group among which mates are found), individuals are born with a range of inherited physical and behavioral traits. Unlike Darwin, we now know that this is due to their chromosomal makeup. These variations are expressed as traits, some more favorable for survival in certain environments. Those with favorable traits that lead to more success in feeding, evading predators, and attracting mates, survive and reproduce at a greater rate, passing these traits on to more offspring. Over time, these traits become more common in a population. If the environment changes, new traits may be selected for and further evolution - or extinction - may occur. The fossil evidence that scientists have now been able to accumulate provides confirmation for the evolution, by natural selection, of life on earth.
Models of Evolutionary Change
In Darwin's time, gradualism
was the favored evolutionary concept: a gradual, steady accumulation of small changes over long periods produces major changes in a species. Punctuated equilibrium
is a more recent model: short periods of more rapid evolution are separated by longer periods of little change (or stasis). This might occur during dramatic environmental changes, when the selection of newly "fit" traits over those no longer advantageous would be strong.
Natural selection is complex, creating branches of closely related species, often through adaptive radiation
, whereby a species "radiates" out, adapting into and occupying different niches in the environment. A niche is not only a physical place, but a way of life: food source, habitat, diurnal versus nocturnal activity. One species can rapidly evolve into many, each occupying a slightly different niche. This happened first among reptiles, then mammals, and eventually primates.
Human evolution includes several radiations, producing multiple contemporary, related species. The human and African ape lineages diverged first, and following this the ancestral human lineage underwent adaptive radiation, at the same time that bipedalism developed. Several "cousin" species lived at the same time, yet only one descendant survives today.
The Human Evolutionary Record
Among human ancestors, tool use and language, supported by the genetically-driven traits of large brains, dexterous hands, and reshaped vocal tracts, were highly adaptive and selected for, as they enhanced survival and reproductive success. In the archaeological record, we can see an increasing reliance on material culture such as stone tools, and on the shared, learned behavior of their manufacture. The species associated with these first "Oldowan" stone tools is subject to debate. The earliest tool tradition initially shows very slow change over time, but later hominins showed more rapid innovation.
THE PRIMATE ANCESTORS OF APES AND HUMANS
What is a Primate?
Prosimians, monkeys, apes, and humans are all primates. Currently, there are 200 living species, largely tropical and sub-tropical. Humans, the exception, are adapted to all habitats. Primates share distinctive traits, resulting from common ancestry. They are arboreal (tree-living), or show arboreal ancestry. All have dexterous, grasping hands and feet, opposable thumbs and great toes, nails instead of claws, share reduced emphasis on olfaction and increased reliance on eyesight, with stereoscopic and color vision. Primate locomotion emphasizes hind limbs and an erect trunk, though front limbs are important in moving through trees and underbrush. Primates also share an evolutionary trends toward larger brains and greater intelligence, particularly among apes and humans. The two major primate groups are the lower primates, the prosimii
, and the higher primates, the anthropoidea
, who have larger brains and more developed visual senses, more altered, or "derived," from earlier primate characteristics.
Overview of Primate Evolution
The primate adaptive radiation occurred after the mass extinction event 65 million years ago which ended the Age of Reptiles. During the ensuing Cenozoic, mammals radiated into environments vacated by dinosaurs, and evolved into multiple forms and lineages. Early primates became arboreal.
During the Eocene, primates similar to modern prosimians - lemurs and tarsiers - radiated into the tropical environments of still-connected North America and Eurasia, but were absent in not-yet-connected South America.
During the drier, cooler Oligocene, 35 to 22 million years ago, their habitats shrank and populations dropped, while the "higher primates," or anthropoids, flourished, overtaking diurnal niches and forcing prosimians into a nocturnal life, except on the island of Madagascar.
Oligocene anthropoids are ancestral to all monkeys, apes, and humans. Some, like Aegyptopithecus
, have the dental formula of 126.96.36.199, relating them to later Old World monkeys, apes, and humans. New World monkeys, who later appear in South America, arrived from Africa or North America.
During the Miocene, 20 million years ago, Old World monkeys and apes emerged. Apes, or hominoids, were at first dominant in Africa's forests, then Europe and Asia. Proconsul
is an example of an early hominoid. Later, more evolved apes, such as Kenyapithecus
, may be ancestral to great apes (including humans). Sivapithecus
is a relative or ancestor of the orangutan. Ape species dwindled during the Miocene as the climate cooled and forests declined, while previously less-prevalent monkeys spread and diversified. From this diminished ape stock, early human ancestors emerged 8 to 6 million years ago.
Our Ape Ancestry: The Comparative Anatomical and Genetic Evidence
Because of a shared ancestor 8 to 6 million years ago, humans should properly be described as apes, similar to the African great apes, our closest living relatives. Apes differ from Old World monkeys: they have broadened noses, widened palates, lack a tail, have enlarged brains, and a mobile shoulder joint for arboreal locomotion. Humans differ in their bipedal locomotion, smaller teeth, increased body size, longer legs relative to arms, significantly larger brains, and finely manipulative hand with a long and opposable thumb.
Similarities between humans and apes are now corroborated by DNA studies: we are closest to African apes, less similar to Asian apes, and successively less to Old World monkeys, New World monkeys, and prosimians. The "molecular clock," corroborated by fossils, shows that humans and chimpanzees diverged only 5 to 8 million years ago. Other molecular/biological studies indicate that human and chimp DNA differs by only 1.5 percent and chromosomal similarity is also high.
THE ENVIRONMENTAL BACKGROUND
Changes in climate profoundly affected conditions in Africa: temperature, rainfall, flora and fauna. These in turn relate to major changes in human evolution. While such climate-driven change is not the only factor behind the emergence of the human line, it warrants further investigation.
Climate Change and Early Hominin Evolution
At least three major climatic changes are correlated with hominin evolutionary change:
10-5 million years ago
: major cooling and drying and spread of grasslands, during which the first bipedal hominins emerged from a common ancestor of African apes and humans. By 6 million years ago small-brained, bipedal hominins were present.
3-2 million years ago
: another cooling and drying trend, increased Arctic and Antarctic ice, increased aridity in Africa, coinciding with the emergence of the early "robust" australopithecines and subsequent later robust species, plus the continuation of less robust australopithecines and early members of the genus Homo
. The archaeological record begins, with the appearance of the earliest recognizable stone tools.
Around 1.7 million years ago
: major shift to drier, more open habitats in East Africa, the approximate time of the appearance of later Homo species and the first Acheulean tool industries. Hominins first leave Africa.
THE RISE OF THE EARLIEST HOMININS
Fossil hominins were initially found in South Africa, but later discoveries were made in eastern and central Africa and the Sahara, pushing our origins back to 6 million years ago.
Evidence prior to 4 million years ago consists of pre-Australopithecus
fossils from east Africa and the Sahara. Around 4 million years ago, evidence increases with the appearance of several early australopithecine species, such as A. anamensis
and A. afarensis
, with long arms and curved phalanges, suggesting significant arboreal ties, yet they were clearly bipedal, with a broad, short pelvis, long femur, angled knee joint, and arched feet. They had ape-sized brains, prognathic faces and large dentition. Sexual dimorphism was pronounced. They are not associated with any stone artifacts.
Between 3 and 1.8 million years ago, "gracile" forms continue, with A. africanus
and A. garhi
. Additionally, two new hominin lines appear alongside them. First were the "robust" australopithecines, sometimes called Paranthropus
-- A. aethiopicus
and A. boisei
in East Africa, and A. robustus
in South Africa - with larger brains and massive jaws and teeth. By at least 2.5 million years ago, the first stone tools appear. Robust australopithecines were extinct by 1 million years ago.
The Emergence of Homo
Second, the genus Homo
emerged between 2 and 1.8-1.7 million years ago: H. rudolfensis
, H. habilis
, and H. ergaster
, an early form of H. erectus; they probably evolved from one of the non-robust species, perhaps A. garhi
. Larger brains, smaller jaws and teeth and a decrease in sexual dimorphism are among their characteristics.
THE FIRST STONE TOOLS AND THE OLDOWAN
The earliest archaeological materials and sites, dated to 2.5 million years ago, are referred to as the Oldowan industry, after Louis and Mary Leakey's work at Olduvai Gorge in Tanzania. Cores, flakes, hammerstones, and sometimes retouched pieces were divided by Mary Leakey into heavy and light duty tools, and Oldowan and Developed Oldowan industries; she and others argued for complex strategies and preparation, but such arguments are not convincing.
Experimentation has shown that these artifact "types" can be explained by least-effort flaking strategies and variation in raw materials. At Swartkrans and Sterkfontein in South Africa, polished and striated bone and horn fragments associated with Oldowan artifacts have been interpreted as digging implements, reminding us that much of the early hominins' material culture would have been organic and perishable.
Who Made the Oldowan Tools
Perhaps eight hominin species are found in Africa for the period for which the first stone tools are known, both australopithecines and early forms of Homo
. So which made the tools? The fossil record is unfortunately quite sparse. Then in 1997 a new hominin, Australopithecus garhi
, was discovered, which is considered by many to have made Oldowan tools.
The Nature of the Sites
Oldowan sites are usually located in former riverine and lacustrine environments, and in South Africa in limestone caves among wooded grasslands. At some, fossil animal bones are associated with Oldowan artifacts. Debate about site formation has been hot. Mary Leakey and Glynn Isaac suggested that these sites were "home bases," similar to modern hunter-gatherer camps for "central place foraging," where hominins processed and consumed food. Conversely, Lewis Binford suggested they were carnivore leftovers scavenged by early hominins for marrow. Richard Potts stated that hominins created "stone caches," of raw material, while Robert Blumenschine thought that hominins brought scavenged animal parts here for processing. Kathy Schick considers that larger sites might be "favored places," where stones and bones were carried to an area with shade, protection, water, and animal carcasses.
Food Procurement and Diet
Hunters or Scavengers?
Fossil animal bones at Oldowan sites often are broken up or have distinctive cut marks produced by sharp tools. Since the 1980s there has been an ongoing debate about their significance. Before the 1980s, the "Man the Hunter" hypothesis stated that these were hunting sites, and that bipedalism, tool-making, increased intelligence and social cooperation were part of an adaptation for efficient hunting. Conversely, Lewis Binford argued that the sites were carnivore kills scavenged by hominins, who took marrow because little else was left. Blumenschine argued that the body part representation and cut mark location/frequency were most consistent with opportunistic scavenging. Henry Bunn, Manuel Dominguez-Rodrigo, and Travis Pickering disagreed, arguing from the same type of data that hominins hunted or drove predators from nearly complete carcasses. Discovery and evaluation of new sites will lead to a clearer picture, and probably show that behavior was complex, with a range of different strategies.
Food for Thought: Diet and Encephalization
In 1995, Leslie Aiello and Peter Wheeler proposed the "expensive tissue" hypothesis: animals with low-nutrient plant diets and complex digestive tracts devote more metabolic energy to digestion, while carnivores and omnivores with high-protein diets have simpler digestion, allowing energy expenditure on evolving larger brains. Therefore, among hominins, meat/marrow consumption, brain expansion, and tool use may all have evolved together. Homo habilis
sites appear to show increased meat procurement, and the species displays significant brain enlargement, possibly supporting the hypothesis, which could be tested through bone chemistry analysis.
THE BEHAVIOR OF OLDOWAN HOMININS
Stone tools are the best preserved evidence for hominin behavior, but many behaviors would have left few visible artifacts. Some of these, though, can be deduced.
Hominin social organization can be inferred through modern primate behavior and hominin fossils. Sexual dimorphism suggests that males competed for females, and fossil finds suggest they lived in multi-male, multi-female groups of perhaps 30 individuals.
The large, pitted teeth of robust australopithecines suggest a diet of hard, gritty foods. The more gracile australopithecines and early Homo forms may have had a more generalized diet. Bone chemistry may clarify this. Hammerstone blows and cut marks on large mammal long bones show that Oldowan hominins ate at least some marrow and meat. Chimpanzees and baboons prey on much smaller animals, suggesting that tools facilitated new adaptations.
Like chimpanzees, hominins probably used more tools than the stone forms preserved: animal skins, ostrich eggshells, tortoise carapaces may have been used as containers; bones, horns, tusks, or branches could have been employed as digging tools and clubs, sharpened wood as spears, unmodified stones as projectiles.
Controlled use of fire by Oldowan hominins is controversial. Thermally altered sediments and burned bones are found in layers associated with stone tools and cut-marked bones. Yet in the absence of a discrete hearths or concentrations of burning and artifacts, it is difficult to rule out natural fires.
At present, no Oldowan architectural features are known. Circular stone features once described as hut remains are now thought to be natural. Modern apes build arboreal and terrestrial nests, and early hominins probably had similar behaviors.
Art, ritual, and language
Currently, there is no direct evidence for symbolic behavior among Oldowan hominins. However, prefrontal cortex expansion in Homo habilis
occurred shortly after 2 million years ago, which might mark the beginning of symbolic communication and the introduction of stone tools and cooperative behavior.
RECENT TRENDS IN APPROACHES TO THE OLDOWAN
Recently, Oldowan research has shifted from artifact classification and typologies, toward inferring hominin behavior and adaptive strategies.
Experimental Studies of Stone Tool Use
Experiments using Oldowan-type artifacts for tool-making, animal butchery, marrow processing, woodworking, nut cracking and other activities illuminates the feasibility of certain tools for certain functions, as well as cut mark, fracture pattern, and use wear creation.
Experiments in Site Formation Processes
Geological forces can bury and alter sites. Experiments can help determine if sites are primary (little disturbed) or secondary (drastically changed). Artifact types present and their orientation, among other factors, can reveal the degree of disturbance, for example by water. Other researchers study the affect of site location. Yet others study carnivore kill-sites, determining what would have been available for hominins to scavenge.
Diet and paleoenvironment are reflected in fossil bone/tooth chemistry. Stable carbon isotopes can indicate relative amounts of tropical grass versus shrub and tree foods in the diet. Analysis of A. robustus
teeth shows a diet of trees and shrubs, but also grasses or animals that eat them, different from modern African apes, who almost exclusively eat tropical grass foods. The ratio of strontium to calcium in bones and teeth can indicate the balance of plants and animals in the diet and the extent to which hominins were herbivorous, omnivorous, or carnivorous. At Swartkrans A. robustus
has strontium lower than Homo, suggesting that they were more carnivorous than Homo. Consumption of lily bulbs or hyraxes, with high Sr/Ca ratios, may explain this.
Hominin research now encompasses the study of the whole landscape, as opposed to single sites. A specific stratigraphic horizon along an erosional outcrop is examined over several kilometers for artifacts, bones, and other materials to gain insight on their nature and density, their relationship to nearby waterways, environmental niches, and stone sources. Test excavations can compare surface finds to in situ
materials and help predict patterns of site location.
Key Method: Reconstructing Paleoenvironments
Key Method: Dating Early Hominins and their Archaeology
Key Discovery: Surprise! Australopithecus garhi:
The First Stone Toolmaker?
Key Site: Hadar and Laetoli: "Lucy," the "first family," and fossil footsteps
Key Site: Olduvai Gorge: The Grand Canyon of Prehistory
Key Site: Regional Overview of Major Oldowan Sites
Key Controversy: Classifying the Primates
Key Controversy: Modern Apes as Oldowan Toolmakers?
Key Controversy: What were Oldowan Tools Used For?
Key words and terms Chapter 2
Evolutionary Concepts & Terms
descent with modification
Primate and hominin evolution
lower primates, higher primates
stereoscopic color vision
Hominin tools, cultures
Oldowan industrial complex
cobbles, cores, flakes, debitage, hammerstones, anvils
angled knee joint
oxygen isotope chemistry
terrestrial dust in deep-sea cores
carbon isotopes in soil carbonates
carbon isotopes in bones and teeth.
fossil animal remains.
Swartkrans, Sterkfontein Kromdraai, South Africa
West Turkana, Kenya
East Turkana (Koobi Fora), Kenya
Olduvai Gorge, Tanzania
ST Site Complex, Peninj, Tanzania
Nyabusosi (Site NY 18), Uganda
Senga 5A, Zaire
Mwimbi, Chiwondo Beds, Malawi
Ain Hanech and El-Kherba, Algeria
The Casablanca Sequence, Morocco
central place foraging
carnivore kill sites
"Man the Hunter" hypothesis
Early Access model
"The Expensive Tissue" Hypothesis.
site formation processes