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Evolution Index

The origins of life
Evolution and Fixity
Natural selection
Lamarkian Evolution
Antibiotic Resistance
Industrial melanism
Palaeontology : The study of fossils
The C-14 Decay Curve
In Search of Deep Time
Evolution of the Horse
Punctuated Equilibrium
The classification of living organisms : Taxonomy
Kingdoms
The Hominids
The Changing Trees of Human Evolution
Genetic verses Cultural Evolution
Phenylketonuria (PKU) Fact Sheet
Cystic Fibrocis (CF) Fact Sheet

Topic Chapters Index

 

The long childhood: Neotony

The retaining of immature characteristics into adulthood. Often with delayed sexual maturity.

Genes not only control the final aspects of the phenotype (eg hair colour, blood type) but they also control the rate and extent of development. A small mutation in a developmental gene could lead to a dramatic change in appearance (morphology).

There is strong evidence to suggest that this is what happened in human evolution.

  • Flat face

  • Central position of foramen magnum

  • High relative brain weight

  • Persistence of cranial sutures

  • Thin skull bones

  • Orbits (eye sockets) under cranial cavity

  • Lack of body hair

  • Tooth row structure, small canines and no gaps between the teeth

  • Late eruption of teeth

  • Prolonged infantile dependency

  • Prolonged growth period

 

EVOLUTION

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Humans: Neotonous, bipedal African apes

Trends in the evolution of the hominids

  • Upright walking (bipedality).
  • Reduction of front teeth.
  • Enlargement of cheek teeth.
  • Development of technology.
  • Increase in brain size.

An ape?

For taxonomists to place us in the super family Hominoidea (apes and men) we must have a number of shared features.

  • Large primate.
  • Tailess primate.
  • Y-5 cusps of the molar teeth.
  • DNA more similar to the apes than any other type of animal.

Bipedality

Putting your best foot forward! Why bipedality?

Several theories have been proposed:

  • Freeing the arms to carry things: Provisioning, carrying offspring.

  • Increased visibility in a habitat increasingly covered by grassland.

  • Reaching for food.

  • Regulating body temperature, the upright posture exposed less surface to the tropical sun.

  • It is more economical than knuckle walking used by the other great apes. In a more fragmented woodland the hominids would have to travel further per day for resources.

Characteristics

  • Head held vertically with the foramen magnum (lit. "big hole" hole for the spinal cord) entering underneath the cranium.

  • Longer legs, shorter arms.

  • Reduced lower back with pronounced lumbar curve (lumbar vertebrae are the ones below the ribs and above the pelvis). This holds the torso vertically.

  • Short wide pelvis, particularly broad in females (an adaptation for child bearing). The opening for the birth canal is wider than it is long, the opposite of quadrupeds).

  • Stronger head to the femur (thigh bone) where the ball and socket joint fits into the pelvis.

  • Longer femur.

  • Increased angle of femur so the knees are closer together (the valgus angle).

  • Arched feet acting as shock absorbers. The weight is taken on the heal, transfers round the outside of the foot, across the ball of the foot and out via the big toe.

  • Large, non-opposable big toe for thrust.

  • Straight fingers and toes. Loss of tree branch grasping adaptation.

When?

The earliest hominids show signs of bipedality. The above characteristics are shown more or less in Australopithecus afarensis (3.5Ma), but there are still some non-bipedal features.

The smoking gun

The Laetoli footprints. In 1976 a bed of volcanic ash was uncovered, it was dated at 3.6Ma. Soon after it had fallen a shower of rain made it wet. Animals walking across it left their footprints, including two Australopithecines walking together. One large, one small perhaps a male and a female.

 

African origins

  • All the early hominid fossils are found in Africa.
  • Hominids did not migrate out of Africa until 1.6Ma (lower jaw of H. erectus found in Georgia is currently the oldest fossil hominid outside Africa).

 

Big brains

Though brain size (cranial capacity) is not an absolute indicator of intelligence (what is intelligence anyway?) it is an indicator which is possible to determine its change from fossils.

Endocasts (imprints from inside the cranium) can also show the changes in the surface structure of the brain adding another dimension to the evidence.

  • The earliest hominids had brains no bigger than the apes (about 380-500 cm3).

  • About 2Ma a rapid brain expansion occurred in H. habilis (app. 700cm3) which continued in H. erectus (850-1100cm3).

  • Big brains need more energy (our brains consume 30% of our food energy)

  • The brain expansion coincides with a change in diet and technology perhaps triggered by a climatic change leading to a change in the vegetation.

  • The first stone tools appear (perhaps earlier hominids used tools of wood or other material which did not preserve well - chimpanzees and bonobos certainly do today)

  • Meat increases in the diet of the Homo line whilst Australopithecines specialised in plant food.

  • The increased energy in the diet permitted brain expansion.

  • Improved technology improved butchering of meat.

  • Increase intelligence (is brain size a sign of this?) improved technology.

  • A positive feedback loop in evolution?

  • In Homo sapiens the brain size is about 1350cm3.

 

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