Download Overview There has been a trend towards larger brains in hominins

Chapter 7
The Human Species
Characteristics of Living Humans 1
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Overview
Humans share many features with the other hominoids, but also exhibit a number of differences.
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In addition, humans have a growth pattern that differs from other primates in its extended childhood and
adolescent growth spurt.
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Behaviorally, humans are quite variable.
Distribution and environment
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Humans are the most widely distributed living primate species.
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Humans originally evolved in a tropical environment, but we have expanded into many different environments.
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Biological adaptations have aided humans in new environments, such as cold weather and high altitude.
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Our cultural adaptations have enabled us to range far beyond our biological limitations.
Brain size and structure
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Roughly 20% of a modern human’s metabolic energy is used by the brain.
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Absolute brain size is not as useful a measure of intelligence because larger animals tend to have larger brains.
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An alternative method exists for analyzing brain size: allometry.
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This method looks at the change in proportion of various body parts as a consequence of different rates of
growth
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Brain size increases in a non-linear rate with body size
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The allometric relationship between brain size and body size is quite regular in primates.
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Humans are the exception. Humans have three times the brain size we would expect for a primate of our
body size.
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Are People with Bigger Brains Smarter?
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There has been a trend towards larger brains in hominins for the last 2 million years.
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While we assume that this larger brain size is associated with an increased ability for planning, problem solving,
and language
There is a term for comparing the expected brain to body size given a specific species, the encephalization ratio.
There remains a second question: What about brain size variation within our species?
We need to redefine the value: Because brain size is larger in larger persons we should use the ratio, not the
absolute value
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This is called the brain-to-body ratio.
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There has been a history of racist and sexist interpretations of these differences.
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These include attempts to link IQ with brain size.
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One such set of attempts was by using MRIs
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While 16% of the variation was predictable, once measurements within family groups (stand-in for
environmental factors) was added this value went nearly to 0%
New finding on the link of a protein to large human brain.
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Characteristics of Living Humans 2
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Bipedalism
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The physical structure of human beings shows adaptations for upright walking as the normal mode of movement.
We are bipedal.
Human bipedalism was made possible by anatomical changes involving the toes, legs, spine, pelvis, and muscles
The skeletal structure of the feet of a chimpanzee and a modern human differ.
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In humans the foramen magnum is repositioned farther underneath the skull, so the head is more or less
balanced on the spine (and requires less robust neck muscles to hold the head upright).
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The big toe of the human lies parallel to the other toes, facilitating upright walking:
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The big toe is enlarged and brought in line with the other toes
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In addition, a distinctive longitudinal arch forms, helping absorb shock and adding propulsive spring.
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The pelvis is shaped in the form of a basin to support internal organs; the ossa coxae are shorter and broader,
thus stabilizing weight transmission, reflecting adaptations to upright walking.
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The femur is angled inward, keeping the legs more directly under the body; modified knee anatomy also
permits full extension of this joint.
The spine has two distinctive curves (called lordosis) —a backward (thoracic) one and a forward (lumbar)
one—that keep the trunk (and weight) centered above the pelvis.
Characteristics of Living Humans 3
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Canine teeth
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Human canine teeth are small and do not project beyond the level of the other teeth, serving much the same
function as incisor teeth.
Speculation: No longer need as weapons because we use tools?
Genetic differences between humans and apes
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We are close relatives, genetically and evolutionarily
We share 98% of our DNA, but this means we are 60 million of 3 billion base pairs different (2%)
There are similarities/differences
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FOXP2
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FOXP2 gene in humans is on chromosome 7 in humans and is associated with normal language
acquisition
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This gene is the same in gorillas, chimpanzees, rhesus monkeys, but all have a 2 amino acid difference
(means the mutations in humans occurred after the age-human split)
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For humans the gene seems to have been selected for during the last 200,000 years
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Regulatory genes
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A comparison of the genome of chimps, macaques and humans determined some of the regulatory genes
are missing in humans, 510 sequences.
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One deletion allows for the expansion of the cerebral cortex, another deletion an androgen receptor
resulting in the loss of sensory whiskers and penile spines.
Characteristics of Living Humans 4
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Genetic differences between humans and apes (continued)
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Jaws
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Apes have powerful jaws, humans not so much
Human jaws have been getting smaller as brains got bigger
Genetically, this is associated with a muscle protein called myosin
The MYH16 gene is mutated in humans
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This made it inactive and resulted in smaller jaws in humans
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A gene that codes for the protein prodynorphin (PDYN), a precursor of endorphin, had 5 mutations
since ape-human split
Prodynorphin is involved in behavior, learning, social bonds, memory, and pain experiences
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Five mutations in a 68-base section is a lot!
Sex and reproduction
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A unique aspect of human sexuality often cited is the fact that humans do not have the estrus cycle.
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Most temperate zone animals only mate during certain seasons and only around the time of
ovulation.
In our pets we call this ‘going into heat’.
This means receptivity also aligns with ovulation
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Orangutans also do not have an estrus cycle.
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Bonobos do, but practice sex outside ovulation
Human females cycle throughout the year and often mate at any time during the cycle.
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Like other primates, single birth is the most common but have additional offspring before previous is fully
matured physically or socially
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Childbirth is difficult in humans and more complex than for apes
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Pelvic anatomy is one reason, but not itself sufficient to expand the need for birthing assistance.
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The enlarged brain is another
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Characteristics of Living Humans 5
Relethford Chapter 7 Page 2
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Social structure
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Humans show variation in family structure: monogamy, polygyny, and polyandry.
Monogamy is the most common marriage (although not necessarily the most preferred)
Humans show variation in institutionalized social structures (economic systems, political systems, and
legal systems).
Biological needs (sex, food, need for family) place limits to human behavior.
Humans show great diversity in other aspects of their cultures (therefore the reason for cultural
anthropology of course!)
Don’t forget the biocultural approach used in anthropology.
Also the concept of holism (that the sum is more than the parts).
Especially during the last 12,000 years, humanity has greatly expanded the range of cultural expressions.
Characteristics of Living Humans 1
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Prenatal growth
This is the period from fertilization to childbirth.
• The zygote multiplies the number of cells to 150 during the first week as it travels to the uterus.
• Between weeks 2-8 the embryonic stage is achieved. The embryo is about 1 inch during this time
• By the second trimester, the fetus has developed and is expanded by length.
• The third trimester is when body weight, head size and brain size increase the most.
The pattern of human postnatal growth
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There are 5 basic stages from birth to adulthood
• Infancy refers to the time from birth to weaning (typically up to 3 years)
• Childhood refers to the time from weaning until the end of the growth of the brain weight (about age 7 years)
• Juvenile stage lays between childhood and adolescence
• Adolescence is the time of sexual maturation and a spurt in body growth (about 10 years in girls, 12 in boys)
• Adulthood is the last stage and its assignment is highly variable across cultures.
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Human growth is usually studied using growth curves.
• One type is called the distance curve
• It is a measurement over time such as height and age
• A unique characteristic of human growth is an extended childhood.
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Characteristics of Living Humans 2
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The evolution of human growth
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It makes sense for humans to grow a large brain early, given the dependency on learning
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Given the limitations of the womb, the best period for rapid brain growth is the first few years after birth
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Remember, large brains are one reason for maternal mortality
The advantage of an extended childhood and delayed maturation is that learning takes time
A shorter infancy means she does not have to nurse and can have a second child earlier
Menopause
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Menopause refers to the permanent cessation of menstrual cycles.
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Other animals decrease reproductive productivity but do not enter true menopause.
Why do humans?
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Hypothesis 1: Mother hypothesis is that, later in life, it is more adaptive to focus on the children
you have, rather that produce more
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Hypothesis 2: Grandmother hypothesis is that women can help their daughters, which is more
adaptive than having their own
One other suggestion is that we simply outlive our egg supply.
Is Human Behavior Unique? 1
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When we talk about humans as unique from other primates, we do not say there is no overlay. We are
talking about the extent of the differences
Tool use and manufacture
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Tool use was once seen as a unique human behavior. The examples to the contrary have been piling up for
decades
Another view is as human as tool-makers taking an object form the natural world and altering it for a new use)
Chimpanzee termite fishing
Jane Goodall’s studies of chimpanzees showed that they would often strip leaves off of a branch in
order to use it for termite “fishing”.
It is not innate, it is a behavior taught to the next generation
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Other examples of tool making
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Chimps will dig underground nests with sticks to hunt for ants
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They have made sponges from leaves to get pools of water from niches of trees
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Leaves have been used as napkins and as toilet paper
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Bones have been used to pick the marrow from another bone
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Excavations in the African tropics have shown that there is a long history of transporting rocks for use as
tools) called manuports
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Latest is the making of spears to hunt bush babies
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Using stones to crack open nuts
Human and chimpanzee tool making
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A difference between human and ape tool use is that humans save and reuse tools while apes do not.
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Several scientists have attempted to teach Kanzi, a bonobo, how to make stone tools. He could make them,
and they were equal in quality to those made by early humans.
Is Human Behavior Unique? 2
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Do apes have culture?
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A growing body of evidence points to cultural behavior in several non-human species (e.g., whales, macaques).
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The comparative analysis of chimpanzee behavior performed by Andrew Whiten and colleagues
demonstrated that, according to the definition used by animal behaviorists, chimpanzees have culture
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Chimpanzee tool making knowledge is passed from one generation to the next through learning.
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Culture is the transmission of behavior one generation to the next culturally
Human culture is characterized by a combination of behavior patterns.
Language capabilities
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What is language?
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Language is symbolic and not just a form of communication
Human languages are characterized by being open systems, being arbitrary and exhibiting
displacement.
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As concerns language, the term displacement refers to discussing events distant in time and/or
space.
Apes and American Sign Language
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Early attempts to teach apes spoken English failed because the apes could not make the same sounds as
humans.
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Washoe was the first ape to learn ASL
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Observations of Washoe's adopted infant, Louis, show that he learned sign language from his mother.
The full extent of ape capacity for learning language remains a strong arena of debate (see p 189).
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