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Humans occupy a particular place within the animal world.
Humans are mammals since we have warm blood and breast-feed our offspring.
Humans are classified as primates due to a variety of features, such as grasping hands, a highly-developed sense of sight, and other shared features.
We are members of the Hominoidea family, a group that includes apes and humans.
Primate evolution occurred over a long period of time.
The first primates emerged about 65 m.y.a.
The apes, or Hominoidea, first appeared about 25 m.y.a.
Around 17-12 m.y.a, a new group of apes, known as dryopithecines, emerged and ranged in Africa, Asia, and Europe.
At some time after 10 m.y.a., one of these African primates became bipedal.
Fossil and genetic evidence suggest that humans shared an ancestor with chimpanzees about 6 m.y.a.
In addition to upright walking, humans have several other characteristics that are different from other apes.
Most of the characteristics that distinguish humans from other apes cannot be found because they are not preserved in the fossil record.
The ones that are found most readily include upright posture, larger brains, and tools.
Paleoanthropology is the study of human evolution.
Fossils and artifacts are the evidence used.
The oldest human ancestors come from Africa.
The Family Tree
Obtaining information from the fossil record is difficult for several reasons.
Fossil remains of our earliest ancestors are very fragmentary and poorly preserved.
All of the early fossil finds represent only a few parts of a few hundred individuals.
Determining the age of fossils is difficult.
New fossils that modify current ideas are found almost every year.
The terms used to describe early human ancestors are controversial.
Major changes in paleoanthropology have occurred in recent years, including the discovery of several new species.
Our oldest known bipedal ancestor, Sahelanthropus tchandensis, was found in Chad and dates to between 6 and 7 m.y.a.
In Ethiopia, Ardipithecus ramidus dates to 4.4 m.y.a. and is also a biped.
Finds in Kenya also reveal bipedal locomotion and include Kenyathropus platyops and Orrorin tugenensis.
New information on dating and habitat has altered the view of when and where human ancestors evolved.
In South Africa, dates of australopithecines have been pushed back close to 4 m.y.a.
Habitats in the past were more forest-like compared to open plains.
Information about the diets and behavior of human ancestors has been obtained.
Analysis of tooth size indicates that the diets of early human ancestors were mostly plant foods.
Studies indicate that our early ancestors spent some time in trees.
Reduced sexual dimorphism in Homo erectus suggests a more monogamous lifestyle compared to apes.
Early human ancestors diversified, with some becoming more human-like.
Australopithecus afarensis evolved from early human ancestors.
From Australopithecus afarensis evolved more robust australopithecines with large teeth and jaws for processing plant foods.
Homo habilis, the first member of our own genus, emerged about 2.5 m.y.a.
Homo habilis eventually led to the emergence of modern humans.
Homo erectus evolved from Homo habilis about 1.9 m.y.a.
Homo erectus was the first human form to leave Africa.
Homo heidelbergensis evolved from Homo erectus around 600,000 years ago.
At some point after 200,000 years ago, Homo sapiens began to appear.
Until 1970, there was relatively little evidence for the earliest human ancestors.
The human characteristics of upright posture, large brain size, and tool use were thought to have evolved simultaneously.
Hadar is one of the most productive areas of research for human origins.
Located in Ethiopia, this geologically active zone has exposed numerous layers from the Pliocenethat contain some of the earliest human fossils yet discovered.
Donald Johanson and Marice Taieb began a search for early hominin fossils in 1972.
One of the most complete early human skeletons ever discovered was found at Hadar in 1974.
Johanson initially spotted a small arm bone while on a survey walk.
After 2 weeks of searching, his team found almost 100 pieces, representing about 40% of a complete skeleton.
"Lucy" was just under 4 feet tall and had a small brain.
She lived more than 2.9 m.y.a.
Remains of "the first family" were also found at Hadar.
Over 200 bones from at least five adults and two children were found together.
The individuals apparently died as a group, possibly in a flash flood.
The remains indicate that our ancestor lived in groups.
Evidence indicates that they walked bipedally.
Later fieldwork at Hadar revealed information about australopithecine behavior.
An unearthed skull confirmed the upright posture of Lucy’s kind.
Significant sexual dimorphism suggested that australopithecines were polygynous.
In the first half of the 20th century, there was almost no way to determine the age of the remains of early humans and their artifacts.
A few relative dating technique s were available, but establishing an absolute age was generally impossible.
Relative methods relied upon stratigraphic relationships to sequence older and younger materials or relied upon association.
There are many methods for determining absolute dates available today.
The most common techniques rely upon the properties of radioactive decay in certain elements.
Radiocarbon dating is perhaps the best known technique and can date archaeological materials that are up to 40,000 years old.
Radiopotassium dating, or potassium-argon dating, is very important for determining the age of the earliest human remains.
Dating from Olduvai Gorge, which used this technique, revealed dates that were a million years older than previously believed.
Newly formed volcanic rock or ash deposits can be dated.
Potassium-argon dating has a half-life of 1.3 billion years.
In 1976, Mary Leakey discovered fossilized footprints at Laetoli in Tanzania.
The footprints were formed sometime around 3.6 m.y.a when an active volcano covered the area with a layer of ash.
After a light rainfall, animals walked across the layer of wet ash.
A chemical reaction led the tracks to harden.
About 70 human footprints over a distance of 20 feet were made by three individuals.
The footprints were made by individuals who walked on two legs.
Studies suggest that two of the individuals were 4 feet 8 inches tall.
Evidence from the earliest hominins suggests that bipedalism was the first major adaptation.
The brain size of early hominins was small.
No tools from early hominin sites have been found.
Swartkrans is one of several caves in the center of South Africa that contain both human and animal fossils.
The limestone bedrock in the area is easily dissolved by running water, which creates underground caves.
Materials such as bones end up in the caves through surface wash and other processes.
The accumulated materials eventually harden, creating breccia.
Although there are problems studying these materials, remains of more than 150 individual hominins of several species have been found.
There are several problems associated with studying breccia.
Removal of the fossils from the deposits is difficult.
The limestone deposits provide little that can be dated directly.
The fossils from Swartkrans provide insight into human evolution.
Paranthropus robustus and Homo erectus individuals have been found.
Evidence of stone tool and fire usage is present.
The presence of so many individuals may indicate that they were prey to predators.
Hunters or Scavengers?
A major controversy in paleoanthropology is the issue of how early hominins obtained meat.
Determining diets is difficult since remains of meals are not well preserved.
Chimpanzees and baboons occasionally hunt, kill, and consume small animals.
By the end of the Pleistocene, humans were major large-game hunters.
Evidence from numerous sites suggests that humans brought back animals parts to a common location and removed the meat and marrow with stone tools.
In one instance, a leg bone of an animal had been broken into 10 pieces to obtain marrow.
Tiny scratch marks made by stone tools are also visible on fossilized animal bones.
Evidence has been interpreted to suggest both hunting and/or scavenging, with no consensus being reached.
Examining the size and type of animals and their bones at various locations sheds evidence on the issue.
Animals at hominin sites and sites of modern foragers are similar, suggesting that Olduvai hominins were hunting.
At hominin sites, complete skeletons from small animals and leg bones of large animals were found.
This suggests that hominin were hunting small animals and scavenging large ones.
Olduvai Gorge, located in northern Tanzania, is one of the most famous prehistoric sites in the world.
Two million years ago, the area was a large bowl-shaped basin which trapped rainfall, forming lakes and wetlands.
Along the shores of the lakes lived many creatures whose remains fossilized when they died.
Members of the Leakey family have been working at Olduvai Gorge since 1931.
Louis Leakey found stone tools at the site.
Finally, in 1959, Mary Leakey discovered the first hominin skull, Paranthropus boisei, dating to 2 m.y.a.
At the time, this find was much older than specimens found in other parts of the world.
Olduvai Gorge received acclaim and funding, which supported more extensive investigations.
Olduvai Gorge provided the first clear documentation that crude stone tools and the bones of very early hominins occurred at the same point in geological time.
Over 70 layers with stones and/or bones have been located at the site.
The stone artifacts had strong, sharp edges and were made from raw materials from 6 miles away.
The tools are known as Oldowan tools.
Other evidence at Olduvai sheds light on other aspects of hominin behavior.
Two sites at Olduvai were places for butchering animals.
Most of the living floors were occupied during the wet season.
The Leakey Family
Louis Leakey made significant contributions to the field of human evolution.
Leakey was trained in archaeology and began work at Olduvai Gorge in 1931.
He was guided by the belief that Africa was the cradle of humanity, in contrast to the generally accepted view that East Asia held that distinction.
He married Mary Nicol in 1936 and together they began their life together as researchers.
Leakey became the director of the National Museum of Kenya in 1945.
The Leakey family made important contributions to the study of human origins.
Mary Leakey discovered Zinj in 1959.
The Leakeys pioneered the study of early stone tools and animal remains in East Africa.
Mary’s son Richard became a very famous paleoanthropologist in his own right.
The First Tools
Chimpanzee tool use provides a good model for that of early humans.
Chimpanzees in the wild have frequently been observed making and using wood and stone tools.
Unmodified wood, bone, and stone objects may have been used long before the appearance of deliberately modified stone tools, but these either cannot be identified or have not survived.
Intentionally modified stone tools appeared first in Africa between 3 and 2 m.y.a.
Their appearance is probably associated with the increasing importance of meat in the human diet.
Stone tools provide useful cutting edges for a species that lacks both sharp teeth and claws for slicing meat, shredding plants, or digging.
The earliest dated stone tools were found in central Ethiopia and date to 2.6 m.y.a.
Early stone tools were created by striking one stone against another in a process called percussion flaking.
The flakes were used for cutting and for making other tools and the cores were used as well.
The right kinds of raw materials needed to be used to create the best tools.
The pebble tools found are called Oldowan tools.
Studies have shown that between 1.9 and 1.4 m.y.a. the tools were made by right-handed individuals, indicating changes in brain organization.
Detailed studies have revealed how some of the Oldowan tools were used.
Tools were used for cutting meat, slicing soft plant material, and scraping and sawing wood.
Evidence of woodworking suggests that wooden tools were also being made.
Images and Ideas
Some speculation about the evolution of human behavior can occur.
Brains are very costly compared to other body parts.
Humans are born with relatively underdeveloped brains, which requires greater parental care.
A long period of infant dependency fosters a strong bond between mother and child.
These factors mean that humans require food that is denser in calories and nutrients than for most other species.
Differences between humans and other apes are represented in the characteristics that differentiate males and females.
Women have significantly larger breasts, have softer skin, have higher voices, and lack the body hair that often characterizes other primate females.
The vagina points forward to allow face-to-face intercourse.
Women have concealed ovulation.