Download Chapter 27 How Humans Evolved Visual Understanding 1. Figure

Chapter 27
How Humans Evolved
Visual Understanding
1. Figure 27.5
How do the skeletal differences between the chimpanzee and the australopithecine improve the ability to
stand and walk upright for long periods?
Humans have shorter arms, which do not bear any of the body’s weight during locomotion. In the chimp,
the upper limbs are long and heavier, and are designed to bear some of the weight. Chimps are described as
being “knuckle-walkers.”
The bowl-shaped pelvis and inwardly-angled femurs of the human skeleton allow the body weight to be
centered over the lower limbs. In the chimp, the pelvis is much flatter, and the femurs are bowed outward,
adaptations for walking on all four limbs.
Further, in the human, the spinal cord exits from the bottom of the skull, which allows the head to be held
upright. In chimps, the spinal cord exits the skull in a more dorsal, or caudal position as an adaptation to
walking on four limbs.
2. Figure 27.11
From the graph shown, what general conclusions can you draw about brain size in hominids over time?
In general, as hominids evolved over time, their brain size increased up to the maximum exhibited by the
neanderthals. Neanderthals also exhibited the largest body size of any hominid species. In the past 500,000
years, both brain and body size have decreased by approximately 10% with the evolution of H. sapiens.
This correlation suggests that brain size and body size are directly related, and that as body size decreased,
so did the size of the brain.
Challenge Questions
1. What are the major features of primates that allowed them to progress evolutionarily?
Major features of primates that allowed them to progress evolutionarily are binocular vision and grasping
fingers and toes. These characteristics together allowed primates to climb trees and exploit new sources of
food.
2. What are some of the advantages to walking upright?
Walking upright expends less energy and is a faster means of locomotion than walking on all fours. It also
frees the arms for other uses, such as carrying objects.
Walking upright may also have allowed hominids to see over tall grasses and other barriers which may
have allowed them to avoid danger.
Standing upright allowed hominids to be able to reach higher up to pick food from trees.
3. Why don’t we have a clearer picture of how humans (genus Homo) developed?
The earliest hominid fossils extend back 6 to 7 million years, and these exhibit characteristics of both
primitive and modern traits. However, only a few early genera have been discovered and none of these
provides a complete history of the relationship between humans and australopithecines. More extensive
discoveries of ancestral species need to be found and classified to allow a complete mapping of the
evolution of our species.
4. Mitochondrial DNA (which is derived from mitochondria present in the egg cell and so unlike nuclear
DNA, is not subject to recombination at fertilization) is passed, essentially unchanged, from a mother to all
her children through the generations. Likewise, the genes on the Y chromosome (also not subject to
recombination) are passed from father to son. Can you think of other ways that scientists might use this
genetic material to investigate inheritance?
Mitochondrial DNA has been used to identify where the first humans arose. Based on this evidence, humans
likely first lived in Africa and then migrated out to all parts of the world.
Studies of mitochondrial DNA have been used to study these early migrations of humans and to map out
routes. The ancestors of island populations can also be identified using mitochondrial DNA. For example, a
study of the Polynesians has shown that the original founders came from Asia and not from South America
as was previously suggested.