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Evolution of the Human species
This is an area of ongoing and interesting debate. Of course we all feel involved in this debate as we are all humans.
This is a summary of some of the most comprehensive evidences which support the evolution of the human species
from a common ancestor from about 6 -7 million years ago. It is not the full story.
Biochemical Evidence
Perhaps the strongest evidence for evolution of the human species is biochemical evidence. DNA is the genetic
instructions for making us. DNA is found in all life. However, as organisms become more closely related their DNA
becomes more and more similar. We share 60% of the same DNA as a banana, 93% of the same DNA as a rhesus
monkey, 97% of the same DNA as a gorilla and 98.8% of the same DNA with a chimpanzee. So in fact chimpanzees
are more related to humans than they are to gorillas. Scientists have calculated the average rate of genetic
mutations as around 70 mutations in each generation. Because we know we are 1.2% different to chimpanzees,
scientists can estimate how long ago we last shared a common ancestor. Current estimates say this was 6-7 million
years ago.
Fossil and anatomical evidence
So the biochemical evidence suggests ancestors of our species separated from ancestors of chimpanzees around 6-7
million years ago. What would we expect to see in the fossil record then? I would suggest fossils which show a
change from ape characteristics to human characteristics over 6 million years. Luckily archaeologists have discovered
and accurately dated many fossils of early hominids. These fossils do show several gradual adaptations from ape like
anatomy to human like anatomy which occur in periods throughout the last 6 million years.
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Anatomical changes for bipedalism
Anatomical changes to increased intelligence
Adaptations to a modern human omnivorous diet.
Bipedalism
Bipedalism is walking on two legs and the fossil record shows that it evolved around 6 million years ago. In the fossil
record we see adaptations of the pelvis which became shorter, wider and bowl shaped to hold the internal organs.
We also see gradual changes to the angle of the femur so that the early hominid legs are under the centre of mass.
Lastly we see adaptations to the foot which became more robust and less generalised. When we look at the fossil
record we see these gradual changes in the structure of the pelvis, the feet and the legs.
Increased intelligence
The relative size of the brain is the primary way we can estimate an organism’s intelligence from fossil records. The
brain is a soft tissue which rarely fossilises but the size of the brain can be seen from the size of the skull and brain
case. The fossils in the image below are organised by date with the oldest on the left and modern human on the
right. We can see a gradual increase in the size of the brain case with time which is consistent with natural selection
for the adaptation of greater intelligence. It is important to understand that the variation in the skulls represents
phenotypes outside of the human range which means that these can’t be just atypical humans. They are distinct
species.
Embryology
Embryology is sometimes cited as an evidence for evolution. However in recent times this has come under scrutiny
as some text book diagrams showing similarities between embryos are very inaccurate. None the less embryology
does evidence evolution as embryos will often contain developmental features that don’t turn into functional organs
in a fully developed baby. These developmental features would have become working organs in our ancient
ancestors. For example Human embryos contain primitive gill slits which in a fish embryo become working gills. In a
human they become other structures including sections of the lower jaw and face. Another interesting human
embryological structure is a small tail. This is largest at day 31-35 and becomes smaller until it is just the tiny bones
of the coccyx.
Geographical distribution
The first modern humans (Homo sapien sapien) are believed to have evolved in Africa around 200,000 years ago.
Modern human fossils are found only in Africa until around 100,000 years ago. After this time there was a wave of
migration out of Africa. One of the first populations of modern human out of Africa was the ancestors of the
Australian aboriginals. These people travelled through and populated Asia before arriving in Australia around 70,000
years ago. Shortly after this, climate changes isolated Australia from Asia making the Aboriginal population isolated
for some 70,000 years. This makes the Aboriginal population of Australia the oldest continuous population in human
history. During this time the Aboriginal people have undergone natural selection and adaptations to the unique, dry
and harsh Australian environment. One of these is an adaptation towards an ectomorphic body types which
dissipates heat easily. This adaptation is seen in many other desert populations around the world. Some traditional
Australian aboriginal populations also have an adaptation to extreme cold desert nights which allows their surface
temperature to drop towards 12oC while maintaining a stable internal temperature of 37oC. This adaptation is
nothing like shivering, which is found in all populations and instead is a change in the blood circulation at the skin
surface.
Many other examples of adaptations to environments can be seen in human populations. One example includes the
epicanthic fold found in many Asian and some African and European populations.