Lisa Ryther
Anthropology 1020
Research Paper:
Mutated Characteristics That Led to
The Evolution of Homo sapiens
Due December 3rd 2012
What Made Us Human?
Several genetic mutations may be to blame. There are an estimated 15 million differences between
the human and chimpanzee genomes. Out of these 15 million changes, only about 10,000 were acted
upon by natural selection. This paper will focus on six specific changes: weakened jaw muscles,
increased energy supplied to the brain, brain expansion, hand development, dietary changes, and
enhanced ability to communicate. All of these changes played a role in the evolution of modern
humans.
Jaw muscles
Jaw muscles may play a bigger role in the evolution of modern humans than one might think. A
chimpanzee can bite off a human finger in one go. Compared to chimps, human jaw muscles are
quite weak. This may have been caused by the mutation of a single gene. The gene in question
encodes a muscle protein. The mutation inactivates the gene, which forces jaw muscles to be
produced by a different version of the protein. This alternate protein causes jaw muscles to be much
smaller.
“This finding, which came in 2004, caused a stir when the researchers argued that
smaller jaw muscles could have allowed the growth of a bigger skull. Primates
with big jaw muscles have thickened supporting bone at the back of their skull,
which arguably constrains skull expansion, and therefore that of the brain too.
‘We are suggesting this mutation is the cause of the decrease in muscle mass and
hence the decrease in bone,’ says Hansell Stedman, a muscle researcher at the
University of Pennsylvania in Philadelphia, who led the work. ‘Only then do you
lift the evolutionary constraint that precludes other mutations that allow your
brain to continue growing.’” (Wilson)
One research team dated this mutation to 2.4 million years ago, while another estimated that the
change occurred closer to 5.3 million years ago. Either way, the mutation occurred after humans
split from their last common ancestor with the chimps. The weakened jaw muscles were obviously
not impairing our ancestors to the point that natural selection was acting against them.
“Why would our ancestors switch to a weaker bite? Steadman speculates that
rather than changes in diet being the catalyst, it could be that our ancestors no
longer used biting as a form of attack. ‘At some point, perhaps through social
organization, this form of weaponry became more optional for our ancestors,’ he
says.” (Wilson)
Energy supply
The brains of most primates require about 8% of their total energy at rest, whereas human brains
require 20% of their total energy at rest. The ability to support that level of energy consumption was
an important mutation. When researchers attempt to isolate a beneficial mutation, they often start
out by looking at mutations that have a negative impact on the same function. The blood supply to
our brain is organized differently than most other primates, which led researchers to study diseases
related to blood flow. One disease, called moyamoya, involves the narrowing of arteries in the brain.
The disease is caused by a mutation to the gene called RNF213. Scientists believe that other changes
to that gene may positively influence blood flow.
“There are more ways to boost the brain’s energy supply than just re-plumbing its
blood vessels, though. The organ’s main food source is glucose and this is drawn
into the brain by a glucose-transporter-molecule in the blood vessel walls.
Compared with chimpanzees, orangutans and macaques, humans have slightly
different ‘on switches’ for two genes that encode the glucose transporters for
brain and muscle, respectively. The mutations mean more glucose transporters in
our brain capillaries and less in our muscle capillaries.” (Wilson)
That means that more of the available glucose can be sent to power the brain, which means that the
human body is wired to promote intelligence over athleticism.
Brain size
Brain size is thought to have increased through a ‘snowball effect’.
“Initial mutations caused changes that were not only beneficial in themselves but
also allowed subsequent mutations that enhanced the brain still further.” (Wilson)
The brain increased in complexity as well as size. Increasing the size and not the complexity of the
brain of any creature would not match the results of human evolution.
“It has become increasingly clear that the human brain is not simply a large ape
brain: Important qualitative and quantitative changes occurred as well. Some of these
changes are a result of broad patterns of adaptation that are inherent in the nature of
life. Some are presumably the result of direct selection for specific behavioral abilities
of various kinds.” (Schoenemann)
Hand development
It is unclear how exactly humans managed to evolve the fully opposable thumb, and the shortened,
straight fingers, but there is a general consensus among scientists that tool use played a large role.
“Although no comprehensive account has been offered, there is general agreement
that the anatomical reconstruction of the hand during human evolution was
somehow linked with tool behaviour. This approach is consistent with evidence that an
elderly hominid (hominin) behaviour was bipedal gait, which would have ‘freed the
hands’ for greater use of tools.” (Young)
Some of the specific changes to the hand include the lengthening of the thumb, and the
shortening of the palm and fingers.
“The human thumb is longer, the palm and fingers are shorter, and the fingers have
lost their curvature. The distal phalanges have gained large apical tufts which support
broad, palmar, fibrofatty pads that distribute pressure during forceful grasping and
whose deformation accommodates the pads to uneven surfaces. Apart from
thickening of the fifth metacarpal and enlargement of its base, the balance of
strength and robusticity has shifted radically, to the thumb, second and third
fingers.” (Young)
These changes enabled them to utilize effectively two unique grips: the precision grip, and
the power grip.
“ ‘In spite of the multiplicity of activities of the hand’, he wrote, ‘…there are only
two prehensile actions: these are called the precision grip and the power grip’. These
two patterns of movement, which are anatomically and physiologically distinct,
provide the basis for all prehensile activities. The precision grip is employed where
precision of movement is required, whereas the dominant characteristic of the power
grip is application of force.” (Young)
The wrist was also adapted to allow freer movement, which improved their ability to throw
and club.
Dietary changes
“Meat eating has always been considered one of the things that made us human, with
the protein contributing to the growth of our brains.” (University of Colorado
Denver)
Rather than continuing as opportunistic meat eaters, humans began to actively hunt and eat meat.
The brain requires a lot of energy, and one way to obtain the necessary energy is through the
consumption of protein. Another dietary change may have contributed toward increasing their
energy supply: the consumption of starch. Most primates subsist on fruit and leaves. These are lower
calorie foods, which require them to spend most of their time foraging for food. Evidence for the
addition of starch in the human diet is found on the genetic level.
“A digestive enzyme called salivary amylase plays a key role in breaking down starch
into simple sugars so it can be absorbed in the gut. Humans have much higher levels
of amylase in their saliva than chimpanzees.” (Wilson)
Chimps only have two copies of the salivary amylase gene, while humans have an average of six
copies. Interestingly, the duplication of the genes appears to have occurred within the last 100,000
years.
“The biggest change in that period was the dawn of agriculture, so Dominy thinks
the duplications happened when we started farming cereals. ‘Agriculture was a signal
event in human evolution,’ he says. ‘We think amylase contributed to it.’ ” (Wilson)
Communication ability
Nonhuman primates maintain order and peace in their social groups through grooming. Grooming
is a one-on-one activity and promotes good relations between participants. Humans typically
maintain their relationships through communication. In a sense, humans have replaced the need for
grooming with communication, which has been beneficial in several ways.
“This increased efficiency arises from at least three key features of language. One is
that several individuals can be ‘groomed’ at once, in contrast to conventional
grooming where only one individual can be groomed at any one time (a problem we
still encounter when we resort to the human equivalent of grooming, namely
cuddling and petting). The second is that it is possible to timeshare with speech in a
way that is not possible with grooming: We can talk and walk or feed, whereas
grooming is an exclusive activity (even in modern humans). Third, language allows us
to exchange information about events within our social network that happened
during our absence: For nonhuman primates, what they do not themselves see, they
never know about.” (Dunbar)
Their enhanced ability to communicate enabled our ancestors to form larger social groups, and
spend less time grooming, which freed up a significant portion of their time. The extra time helped
them to develop ever-improving tool technology, and may have led to the advent of agriculture
itself. Communication is an integral part of what makes us human.
Conclusion
“If we consider all the mutations that led to these pivotal points in our evolution,
human origins begin to look like a trail of unfeasible coincidences. But that is only
because we do not see the harmful mutations that were weeded out, points out John
Hawks at the University of Wisconsin-Madison. ‘What we’re left with is the ones that
were advantageous.’ It is only from today’s viewpoint that the mutations that give us
our current physical form appear to be the ‘right’ ones to have. ‘ It’s hindsight,’ says
Hawks. ‘When we look back at the whole process, it looks like a stunning series of
accidents.’ ” (Wilson)
So many changes were required to make Homo sapiens the species they are today. Some of the most
pivotal changes include small jaw muscles, improved energy supply, increased brain size, hand
development, changes in diet, and communication ability. These changes and many more made us
what we are today. What will we be in another 100,000 years?
Bibliography
Dunbar, R.I.M. "The Social Brain: mind, Language, and Society in Evolutionary Perspective." Annual
Review of Anthropology (2003): 163-181.
Schoenemann, P. Thomas. "Evolution of the Size and Functional Areas of the Human Brain." Annual
Review of Anthropology (2006): 379-406.
University of Colorado Denver. "Anthropologist Finds Evidence of Hominin Meat Eating 1.5 Million Years
Ago: Eating Meat May Have 'Made Us Human'." Science Daily 3 October 2012: Web. 30 October 2012.
Wilson, Clare. "LUCKY YOU!" New Scientist 9 June 2012: 34-39.
Young, Richard W. "Evolution of the human hand: the role of throwing and clubbing." Journal of
Anatomy (2003): 165-174.
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