humanevolution

Human Evolution

Humans are a type of mammal known as Primates

Primate Classification:

Prosimians

Anthropoids

New World Monkeys (i.e. marmosets)
Old World Monkeys (i.e. baboons)
Gibbons
Orangutans
Gorillas
Chimpanzees
Humans

Primate Characteristics:

Ability to move the four limbs in various directions
Grasping power of the hands and feet
Independent mobility of the digits
Well-developed hand-eye coordination
Nails on the fingers and toes
Flexibility of the spine to allow twisting and turning

The monkeys and apes also have the following characteristics

Highly developed brain
Relatively long period of post-natal growth
Considerable parental care

Fossil evidence of human evolution is rare - why do you think this is?

The Evolution of Humans

In 1924, Brithis anthropologist Raymond Dart announced that a fossilized skull was the remains of an early human.
He named this "ape-man" Australopithecus africanus (meaning "Southern Ape of Africa")
Australopithecus was in fact a hominid and walked fully erect

Australopithecus also had human-like hands and teeth

In 1974, an Australopithecus skeleton was found that was 40% complete with skull fragments

This skeleton was named "Lucy" after the Beatle's song Lucy in the Sky with Diamonds
The skeleton was 3.2 million years old
She was petite - only 1m tall
Her head was about the size of a softball

How is "Lucy" Different from Modern Humans?

Though most anthropologists would agree that Homo sapiens is closely related to living apes and, like them, had an ape ancestor, they disagree on which of the living apes is most like that long extinct ancestor.

Most recently, a three-million year old skeleton known as "Lucy" provides the oldest conclusive evidence of small bipedal hominids. Some of the bones are complete, and together with bones from other sites, they provide a basis for reconstructing the body build of Australopithecus. The living species most like the hypothetical ancestor is the pygmy chimpanzee.

Dentition

Australopithecus' canines are only slightly smaller than the chimpanzee's, but the molars and premolars are much larger. The jaw is still very apelike. These changes may relate to a new savanna diet that includes tough, chewy plants.

Body Form

Lucy's humerus (first arm bone) is much shorter than the average pygmy chimpanzee's. Lucy's shoulder probably hung lower than the chimpanzee's

The chimpanzee has a much longer and narrower pelvic girdle. The shorter, broader pelvis and longer lumbar region (lower back) of Australopithecus are characteristic of bipedal hominids.

Lucy's femur (firs leg bone) has a long neck, indicating that the ball-in-socket joint was oriented for bipedalism.

From the skeletal remains of Lucy, it has been hypothesized that her center of gravity was lower, further providing evidence that Lucy was bipedal

Proposed Model of Human Evolution

Taxon	Approximate Time (years ago)	Adaptations, Behavior, and Habitats	Fossil and Archaeological Evidence
Hominid ancestors	8-5 million years	* Relatively large-bodied apes distributed in Central and Eastern Africa across forest-woodland areas	* No fossil evidence yet, but when found, it's expected to be a group ancestral to humans and chimpanzees
Australopithecines	4-2 million years	* Bipedal on the ground (occasionally arboreal) * Open savanna, and mosaic grassland and woodland habitats * Fibrous plant diete that may have also included meat	* Extensive fossils in Eastern and Southern Africa * Large teeth and jaws
Homo habilis	Pliocene - Pleistocene: 2 - 1.5 million years	* Improved bipedalism * Tools to procure and process food * Habitats in drier areas indicating larger home ranges * Scavenging and active animal hunting	* Skeletal changes and increase in brain size * Early stone tools
Homo erectus	Early-Mid Pleistocene: 1.5 - 0.5 million years	* Entry into new habitats and geograpic zones * Definate preconcepton of tool form (they didn't randomly find sharp rocks - they processed it to make the ideal tool) * Manipulation of fire * Increased level of activity and skeletal stress	* Fossils found in formerly unoccupied areas of Africa and outside of Africa * Development of stone tool industry * Archaeological hearths * Increased cranial and postcranial development
"Arcaic" Homo sapiens	Mid Pleistocene: 500,000 - 150,000 years	* Geographic divergence and ecological adaptations * More complex tools	* Old World distribution with some distinct regional morphologies * Bifacial axes: Acheulean-Mousterian stone tool industries
H. sapiens neanderthalensis	Late Pleistocene: 150,000 - 35,000 years	* Large and robust individuals * More social complexity and development of ritual * Increasingly sophisticated tools	* Massive cranial and postcranial development * Intentional burrial of the dead * Increased number of stone tool types
H. sapiens sapiens	Late Pleistocen to Recent	* Decreased levels of activity and skeletal stress * Expansion of technology * Development of complex cultures * Increase in population size	* Appearance of "anatomically modern" humans * Technology ranging from Paleolithic stone tools to putting a man on the moon * Beginnings and expansion of agriculture

Australopithecus - the First Humans

Fossil evidence, such as Lucy, indicate that Australopithecus was about 1 meter tall and walked erect. However, the cranial capacity is only about one-third that of a modern human. There are many species of Australopithecus: A. ramidus, the oldest Australopithecus which was found only 75 km from Lucy and in association with forest-dwelling mammals, challenging the idea that bipedalism evolved in response to living in a savanna-like environment; A. afarensis, the larger and better-known species (this is Lucy's species). Other, larger-boned species of Australopithecus, such as A. robustus, have been proposed based upon skull and teeth fragments.

The role of A. afarensis as the stem from which the other hominids arose is in some dispute. About 2 million years ago, after a long million year period of little change, as many as six hominid species evolved in response to climate changes associated with the beginning of the Ice Age. Two groups developed: the australopithecines, generally smaller brained and not users of tools; and the line that led to genus Homo, larger brained and makers and users of tools. The australopithecines died out 1 million years ago; Homo, despite their best efforts (atomic weapons, pollution) are still here!

With an incomplete fossil record, australopithecines, at least the smaller form, A. africanus, was thought ancestral to Homo. Recent discoveries, however have caused a reevaluation of that hypothesis. One pattern is sure, human traits evolved at different rates and at different times, in a mosaic: some features (skeletal, dietary) establishing themselves quickly, others developing later (toolmaking, language, use of fire).

Homo habilis

Enlargement of the human brain is first evident in fossils dating back to the australopithecine times, about 2.5 million years ago. Skulls have been found with brain capacities of about 650 cubic centimeters (cc), compared to about 500 cc for A. africanus. Simple stone tools are sometimes found with the larger-brained fossils. After walking upright for over two million years, hominids were finally beginning to use their brains and hands to fashion tools.

The hominid radiation that included Homo habilis was part of a more general burst of speciation among African animals about 2.5 million years ago. Africa's climate became drier during that period, and the transformation of the fauna was associated with adaptations to savannas. Homo habilis coexisted for million years with the smaller-brained Australopithecus.

According to one hypothesis of human origins, the late australopithecines and Homo habilis were distinct lines of hominids, neither evolving from each other. If this scenario is correct, then the australopithecine line was an evolutionary dead end while Homo habilis was on the on the path to modern humans, leading first to Homo erectus which later gave rise to Homo sapiens.

Homo erectus

The first hominid to migrate out of Africa into Asia and Europe was Homo erectus ("upright man"). The fossils known as Java Man and Peking Man are examples of this species. H. erectus lived from about 1.8 million years ago until abut 300,000 years ago. Fossils covering that entire range are found in Africa, where H. erectus continued to live contemporaneously with H. erectus populations on other continent. The migrations of H. erectus were probably not mad dashes to claim more territory - if H. erectus simply expanded its range from Africa by a mile per year, it would reach Java in about 15,000 years. The gradual spread may have been associated with a change in diet to include a larger proportion of meat. In general, animals that hunt require more geographical territory than animals that are strict vegetarians.

Homo erectus was taller than H. habilis and had a larger brain capacity. During the 1.5 million years that H. erectus existed, the cranial capacity increased to 1200 cc.

The intelligence that evolved during the African origins of H. habilis enabled these humans to survive in the colder climates of the north, once migration began. Homo erectus resided in huts or caves, built fires, clothed themselves in animal skins, and designed stone tools that were much more refined than those of H. habilis. In anatomical and physiological adaptations, H. erectus was poorly equipped for life outside the tropics, but made up for the deficiencies with intelligence and social cooperation.

Homo erectus is also credited with the one of the most important innovations of our evolution - the use of FIRE.

Homo sapiens neanderthalensis

Homo sapiens neanderthalensis is the name given to an enigmatic group of hominids known as Neanderthals. They were once thought to be a different lineage of the genus Homo that lived almost exclusively in Europe - however, there is an abundance of evidence that they belong as a sub-species of the species H. sapiens. H. sapiens neaderthalensis had a much more robust body form with a large brow ridge, giving it an appearance to early anthropologists as being brutish and stupid. However, there is much evidence to indicate that they had complex cultural practices, including the burrial of the dead, and very intricate toolmaking skills. Much of the current research on Neanderthal skulls addresses an intriguing question: Did Neanderthals have the anatomical equipment necessary for speech?

The Emergence of Homo sapiens: Out of Africa, but how?

Two hypotheses differ on how and where Homo sapiens originated.

The Out-of-Africa Hypothesis proposes that some H. erectus remained in Africa and continued to evolve into H. sapiens, and left Africa about 100,000-200,000 years ago. From a single source, H. sapiens replaced all populations of H. erectus. Human populations today are thus all descended from a single speciation event in Africa and should display a high degree of genetic similarity. Support for this hypothesis comes from DNA studies of mitochondria: since African populations display the greatest diversity of mitochondrial DNA, modern humans have been in Africa longer than they have been elsewhere. Calculations suggest all modern humans are descended from a population of African H. sapiens numbering as few as 10,000.
The Regional Continuity Hypothesis suggests that regional populations of H. erectus evolved into H. sapiens through interbreeding between the various populations. Evidence from the fossil record and genetic studies supports this idea.

Which hypothesis is correct? Scientists can often use the same "evidence" to support contrasting hypotheses depending on which evidence (fossils or molecular clock/DNA studies) one gives more weight to. The accuracy of the molecular clock, so key to the out-of-Africa hypothesis, has recently been questioned. Recent studies on the Y-chromosome seem to weaken the regional continuity hypothesis by indicating a single point-of-origin for our species some 270,000 years ago. Continued study will no doubt reveal new evidence and undoubtedly new hypotheses will arise. It is a task for all of us to weigh the evidence critically and reach a supportable conclusion, whether we are scientists or not.