Chapter Summary and Key Concepts
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, such as environment or climate, can impact the evolutionary paths of species
- understand how internal factors, such as social behavior and “learned” culture, can impact the evolutionary paths of species
- know the physical characteristics (skulls, dentition, postcranial features) that place hominins on a distinctively human pathway
- understand how inferences are made from hard tissue to determine 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, adaptation, 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. Some are 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.
The Human Evolutionary Record
Human evolution includes several major splits, producing multiple lineages of contemporary, related species. The last common ancestor of humans and African ape lived probably between 8 million and 6 million years ago, and following this split with the other apes the ancestral human lineage developed bipedal locomotion (walking on two legs). A number of “cousin” species to the line that led to us lived at the same time, yet only one descendant survives today – us.
THE PRIMATE ANCESTORS OF APES AND HUMANS
Prosimians, monkeys, apes, and humans are all classed together as Primates. Currently, there are more than 200 living species, largely tropical and sub-tropical. Humans, the exception, are adapted to almost every habitat on Earth. Primates share distinctive traits, resulting from common ancestry. Most 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 trend 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, which have larger brains and more developed visual senses, more altered, or “derived,” from earlier primate characteristics.
Overview of Primate Evolution
The major primate adaptive dispersal 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.
Early anthropoids have a number of dental and cranial (skull) features that link them to present day monkeys (New and Old World) and apes. These Oligocene anthropoids are ancestral to all monkeys, apes, and humans.
Our Ape Ancestry: The Comparative Anatomical and Genetic Evidence
Because we shared a common ancestry with other apes for many millions of years, humans should properly be described as apes, which diverged from other apes, with the African great apes our closest living relatives. Humans differ from living apes 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 ENVIRONMENTAL BACKGROUND
Changes in climate had profound effects on the environment in Africa: temperature, rainfall and therefore flora and fauna. Some of these relate also 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.
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 the early australopithecines. Between 3 and 1.8 million years ago, “gracile” forms. Additionally, two new hominin lines appear alongside them. First were the “robust” australopithecines, sometimes called Paranthropus, with slightly 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.
Second, the genus Homo emerged between 2 and 1.8–1.7 million years ago: H. rudolfensis, H. habilis, and H. ergaster (considered by some to be an early form of H. erectus); they probably evolved from one of the non-robust australopithecines, 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 first recognizable objects modified by hominins date to approximately 2.5 million years ago – flaked and battered stone artifacts from the Middle Awash valley, Ethiopia. The earliest archaeological materials and sites are referred to as the Oldowan industry. 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 this is debated.
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.
FOOD PROCUREMENT AND DIET
Fossil animal bones at Oldowan sites are often 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 all part of an adaptation for efficient hunting. Conversely, 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.
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.
THE BEHAVIOR OF OLDOWAN HOMININS
Stone tools are the best preserved evidence for hominin behavior, but many behaviors would have left few visible artefacts, though some of these can be deduced from the evidence.
Hominin social organization can be inferred from modern primate behavior and hominin fossils. Sexual dimorphism suggests that males competed for females. It has been suggested 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, such as seeds or roots and tubers. The more gracile australopithecines and early Homo forms may have had a more generalized diet. Bone chemistry may clarify this.
Like chimpanzees, hominins probably had other material culture that would not be 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.
Art, ritual, and language
Currently, there is no direct evidence for symbolic behavior among Oldowan hominins. However, increase in brain size and expansion of the prefrontal cortex in Homo habilis shortly after 2 million years ago might indicate 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.
Key Controversy: Classifying the Primates
Key Controversy: Modern Apes as Oldowan Toolmakers?
Key Controversy: What were Oldowan Tools Used For?
Key Discovery: Ardipithecus ramidus and Other Early Fossils
Key Discovery: Australopithecus garhi: The First Stone Toolmaker?
Key Method: Reconstructing Paleoenvironments
Key Method: Dating Early Hominins and their Archaeology
Key Sites: Hadar and Laetoli: “Lucy,” the “first family,” and fossil footsteps
Key Site: Olduvai Gorge: The Grand Canyon of Prehistory
Key Sites: Regional Overview of Major Oldowan Sites
Evolutionary Concepts & Terms
- descent with modification
- genotype, phenotype
- punctuated equilibrium
- ecological niche
- adaptive radiation
- Charles Darwin
- Alfred Wallace
- Mary Leakey
- Louis Leakey
- Richard Potts
- Glynn Isaac
- Lewis Binford
- Robert Blumenschine
- Kathy Schick
- Henry Bunn
- Leslie Aiello
- Peter Wheeler
- Lawrence Keeley
Primate and hominin evolution
- lower primates, higher primates
- derived characteristics
- opposable thumb
- stereoscopic color vision
- dental formula
- gracile, robust
- bipedal locomotion
- A. anamensis
- A. afarensis
- A. africanus
- A. garhi
- A. ramidus
- A. aethiopicus
- A. boisei
- A. robustus
- H. rudolfensis
- H. habilis
- H. ergaster
- H. erectus
- Oldowan industrial complex
- cobbles, cores, flakes, debitage, hammerstones, anvils
- heavy-duty tools
- light-duty tools.
- reduction sequence
- chaînes opératoires
- sexual dimorphism
- angled knee joint
- arched foot
- antigen-antibody reactions
- molecular clock
- genetic divergence
- oxygen isotope chemistry
- deep-sea cores.
- terrestrial dust in deep-sea cores
- carbon isotopes in soil carbonates
- carbon isotopes in bones and teeth
- paleobotanical evidence
- fossil animal remains
- Swartkrans, Sterkfontein Kromdraai, South Africa
- Laetoli, Tanzania
- Gona, Ethiopia
- Omo, Ethiopia
- Fejej, Ethiopia
- Hadar, Ethiopia
- West Turkana, Kenya
- East Turkana (Koobi Fora), Kenya
- Chesowanja, Kenya
- Kanjera, 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
- home bases
- central place foraging
- carnivore kill sites
- opportunistic scavenging
- stone caches
- processing sites
- cut marks
- favored places
- “Man the Hunter” hypothesis
- Early Access model
- “The Expensive Tissue” Hypothesis
- experimental archaeology
- microwear studies
- site formation processes
- isotopic studies
- landscape archaeology