Assessment Statements
Outline the method for dating rocks and fossils using radioisotopes, with references to 14 C and 40K.
Define half-life
Deduce the approximate age of materials based on a simple decay curve for a radioisotope.
Describe the major anatomical features that define humans as primates.
Outline the trends illustrated by the fossils of Ardepithecus ramidus, Australopithecus including
A.afarensis and A. africanus, and Homo including H. habilis, H. erectus, H. neanderthalensis and
H. sapiens.
State that, at various stages in hominid evolution, several species may have coexisted.
Discuss the incompleteness of the fossil record and the resulting uncertainties about human
evolution.
Discuss the correlation between the change in diet and increase in brain size during hominid
evolution.
Distinguish between ‘genetic’ and ‘cultural’ evolution.
Discuss the relative importance of genetic and cultural evolution in the recent evolution of humans.
Dating Fossils with Radioisotopes
Most of our understanding of our human origins has come from looking at
the remains of our fossil ancestors.
Fossils record the appearance of our long-dead ancestors and their
locations tell us where they may have lived, but if we can also estimate the
age of the fossils, we can begin to construct our human history.
Elements exist in different forms called isotopes, which have different
number of neutrons in their atoms.
Carbon exists as different isotopes carbon -12 (12C) accounts for 98% of carbon
atoms, while carbon 14 (14C) is found in tiny quantities.
Carbon 14 is known as a radioisotope because it is unstable and slowly decays to
stable carbon 12.
The rate of decay of any isotope is constant.
For carbon 14 it is known that after 5,700 years, half the unstable atoms in a
sample of carbon will have decayed, so this time period in known as its half-life.
Graph showing the decay of radioisotope carbon 14.
If the percentage of the remaining carbon 14 in a specimen is 50%, then one
half-life has passed.
If the percentage is 25% then two half-lives have passed.
Carbon dating of fossil specimens with these percentages would us that the
first fossil is around 5,700 years old and the second is 11,400.
Carbon and potassium are used for dating fossils.
Carbon 14 has a half life of 5,700 years, and is useful from dating fossil
material 1,000-10,000 years old.
Potassium 40 has a half life of 1.3 billion years and is used to date very old
fossil material or rock.
All naturally occurring carbon and potassium are mixtures of isotopes of known
proportions, so during its lifetime an organism will accumulate both forms in its tissues
as it feeds or photosynthesizes.
When organisms die no more atoms are accumulated so the naturally occurring ratio
will begin to change.
As radioisotopes decay, their proportion in the organism will decrease and the
proportion of the stable form will increase.
To date fossil material, the proportion of different isotopes is measured to assess the
number of half-lives that has passed, this is radiocarbon dating.
Humans are Primates
Taxonomy is based on shared characteristics.
Humans are classified as primates, a group within mammals, which includes lemurs,
gibbons, monkeys and great apes.
Humans share many physical and genetic characteristics with all these animals.
Humans are most closely related to the great apes (orangutan, gorillas and chimps.
We share the majority of genes with chimps but the base sequences still differ in many
positions. Chimps are the closets related to humans based on sequence comparison.
Shared Characteristics
Long limbs and grasping hands with opposable thumbs allowing for object manipulation.
Extremely mobile joints at shoulder and hip, the ball and socket joints giving great flexibility
of movement.
Rotating forelimb, which increases the use of the hand.
Retention of collar bone, which stabilizes the shoulder allowing body weight to be
supported by the arms when moving through trees.
Trends in the Human Fossil Record
Hominids are a group of primates that walk
on two legs.
Modern humans are members of this group
alive today but many other species of
hominids have existed in the pat, and some
of them were alive at the same time.
Our knowledge of these species has been
gained entirely from the fossils that we’ve
found.
Ardipithecus ramidus
4.4-4.3 mya in Ethiopia, East Africa
Foramen mangnum is more ventral, indicating a trend towards bipedalism.
Brain size unknown
Teeth seems to be intermediate between apes and Australopithecines
canines are smaller and more blunt than apes
but molars are large and ape-like
The point at which the spinal column meets the skull is the foramen
magnum.
A ventral foramen magnum allows for a more upright body position and
bipedal walking.
Australopithecus afarensis
3.9-2.9 mya in East Africa
Most well known specimen is Lucy
Ape-like face with flat nose and protruding jaw
Large, tall lower jaw
Brain size: 375-550 cm3
Large molar teeth similar to A. ramidus
Australopithecus africanus
3.3-2.5 mya in Southern Africa
Slightly flatter face but still with large, tall lower jaw
Brain size:420-500cm3
Smaller canines but molars still large
Homo habilis
Used simple tools
2.5-1.9 mya in eastern and southern Africa
Face more flattened than Australopithecus, smaller lower jaw bone
Brain size: 500-800cm3
Smaller teeth, hips form distinct pelvic bowl
Homo erectus
1.8-0.3 mya in Africa, Indonesia, Asia, Europe
Found in Europe and Africa so must have migrated.
First hominids to use fire
Face further flattened, skull more rounded with large brow ridges
Smaller lower jaw
Brain size: 850-1100cm3
Homo neanderthalensis
150,000-30,000 years ago in Europe and western Asia
Face furhter flattened but still large brow ridges
Rounded skull but lower forehead
Brain size:1200-1625cm3
Large teeth and jaw muscles, limbs short relative to torso
May have interbred with H. sapiens in Europe
Homo sapiens
130,000 years ago
Flat face with no brow ridges
Reinforced lower jaw producing a chin
Rounded skull with high forehead
Brain size: 1200-1500cm3
Smaller molars, skeleton is less robust than other ancestors
In at least 2 cases, different hominid species lived in the same place at the same
time- A. africanus and H. habilis , and H. neanderthalensis and H. sapiens co-existed
in Europe.
There is a lot of debate about the degree to which the two species intermingled, but
the definitely inhabited the same regions for thousands of years,
At the same time, their populations were small and spread out over 3 continents, so
they may not have ever met.
Australopithecus sediba skull
bears both human and ape
traits.
Found in 2011, 2 million years
old.
Both human and ape
characteristics. Missing link?
Our understanding of human evolution is based on the fossil record, but fossilization is a rare event
and discovering a fossil is also rare.
The fossil record for humans is very patchy
Over the years new fossils have been discovered, dating techniques are more accurate, and genetic
analysis has been applied. The branching of the human evolution tree has been modified many
times.
Some evolutionary paths contain 18 or 19 examples, but some may be variations of the same
species.
Diet and Brain Size in Hominid Evolution
All the hominid fossils found fit into a sequence showing an increasing degree of
adaptation to bipedalism, and increasing brain size relative to body size.
Large brains require a lot of energy to enable them to function and there is
evidence to correlate the increase in brain size with changes in diets to our
ancestors.
H.Sapiens uses 20% of the total energy consumed by the body.
Human ape ancestors lived in northeast
Africa in the Great Rift Valley.
Until about 5 million years ago this was
covered in dense forest, but movements of
the Earth created a wide valley, and volcanic
activity coated the plains with thick ash,
which prevented growth of trees.
Vegetarian tree-dwelling apes, which ate soft leaves and fruit, had to adapt or become
extinct.
Australopithecines adapted by developing stronger jaws and teeth to deal with tougher
vegetation such as stems, roots, but as the savannah expanded, the variety of plants
decreased and grazing animals started to play a big role in the diet.
Early hominids needed new strategies to access this rich source of meat, which
provided an increased supply of protein, fat and energy.
Natural selection favored individuals with large brains who could develop new
strategies for hunting and work in groups to kill large animals.
Meat provided the nutrients to build a larger, thinking brain as well as the energy
to fuel its activity
The increase in meat in the diets shows a positive correlation with
increased brain size and also with the development of more
sophisticated tools for hunting.
Genetic and Cultural Evolution
Genetic evolution of humans is the change, over millions of years, in the
genome population.
Genetic evolution occurs as a result of mutations and these may be
inherited.
Genetic evolution includes the change to bipedalism, changes in teeth
and increase brain size.
All these features are passed on from parents to their offspring.
Cultural evolution, on the other hand, does not involve genetic
changes and can take place over a very short periods of time, even
within a generation.
Cultural evolution involves changes in behavior, social organization,
ideas, communication, teaching and learning.
These are acquired characteristics that do not have a genetic basis,
though the ability to acquire them may be genetically determined.
Cultural evolution does not result in speciation.
The rise of agriculture brought about an enormous alteration in social
organization for humans.
Instead of living in small mobile hunter-gatherer groups, humans began to
settle in large fixed communities, and the rise of different religious faiths
further affected new social organizations.
The invention of the printing press had a huge impact on communication.
TV, the telephone, and internet.