Evolving Around the World

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Evolving Around the World
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Camels On the Move Camels are exotic
animals that we typically associate with the
deserts of Asia or Africa. Camels have long been
domesticated, and nomadic peoples still use them
as beasts of burden and as sources of meat, milk,
wool, and fuel. Camels are well adapted to hot,
dry habitats. They can travel up to 100 miles per
day in the desert, and they can survive after
losing over 25 percent of their body weight in
water. When water is available, they may drink
more than 50 liters at a time! These animals are
notorious for their habit of spitting
foul-smelling stomach contents when annoyed.
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One Hump or Two? There are two
species of camelsthe one-humped Dromedary or
Arabian camel (Camelus dromedarius) and the
two-humped Bactrian camel (Camelus bactrianus).
An easy way to remember which is a Dromedary and
which a Bactrian is to rotate the first letter of
the name so it lies flat to show the profile of
humps. Contrary to popular belief, the hump is
not used to store water it is actually a
reservoir of body fat used for energy.
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The two camel species live in different parts of
the world. The Dromedary is found the Middle East
and North Africa in dry, flat desert regions. The
Bactrian is found in Central Asia in mountainous
regions and arid high plains, such as the Gobi
desert. Both species exist today primarily as
domesticated herds, although a few wild Bactrian
populations still roam in Central Asia.
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Tell-Tale Features of Camels Camels have
several characteristic features. These include a
long neck with a relatively small head, a cleft
or divided upper lip, a three-chambered (rather
than four-chambered) stomach for digesting their
vegetarian diet, and oval-shaped red blood cells.
Camels feet rest on tough, fleshy pads, rather
than on hooves. They have only two toes, which
are splayed apart to provide sure footing on
sandy or rocky terrain. Camels also
have a characteristic stride. They dont trot, as
horses do instead, they pace, moving both legs
on each side in unison. This allows them to take
very long strides and is efficient for traveling
long distances. It also causes their body to sway
from side to side, so that camels are difficult
to ride and sometimes referred to as ships of
the desert.
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Other Camel-like Critters Four
other species also have these distinctive camelid
or camel-like featuresthe alpaca, the guanaco,
the llama, and the vicuna. These animals, too,
are grazers living in arid high plains and
deserts. However, they occur naturally only in
the Andes Mountain region of South America, an
ocean away from the African and Asian homes of
the camels.
guanaco (Lama guanicoe)
alpaca (Lama pacos)
vicuna (Vicugna vicugna)
llama (Lama glama)
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Explaining Similarity How can we
account for the shared features of camels and the
four South American species? The theory of
evolution predicts that multiple shared features
among species reveal relatedness of the organisms
and their descent from a common ancestor that
likewise had these features.
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The idea that camels and the four South American
species descend from a common ancestor raises
some puzzling questions, however. If there was a
common ancestor of these six species, where did
it originate? How could its descendants have come
to occupy such widely separated geographic
distributions, with camels in Asia and the four
other species located an ocean away in South
America? A key question is whether the time and
place of origin of the proposed camel-like
ancestor is compatible with the geographic
distribution of its presumed descendants. If so,
this will provide support for the hypothesis that
all camelidsthat is, camels plus the four South
American speciesdescended with modification from
a common ancestor. In the panels that follow,
you will explore fossil data and geographic data
in search of evidence to support, or refute, this
hypothesis. Use your Data Sheet to record
information about the fossils.
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Ancient Fossil Camelids
Poebrotherium (37-24 MYA) was a goat-sized animal
common in open woodlands of what is now South
Dakota. It had the characteristic camelid
two-toed foot, having lost the two lateral
toes. It did not yet have pads on its feet.
Protylopus (about 45 MYA) was a rabbit-sized
animal that originated in North America. It is
the earliest animal known to have had some
camelid features. However, Protylopus still had
four-toes, hooves, and teeth better suited for
browsing than for grazing.
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Procamelus (about 10 MYA) had similar camelid
features.
Protolabis (about 20 MYA) was the size of a
llama. It too had camelid features, including
padded feet with two toes, and high-crowned teeth
suited for grazing on grasses.
Protolabis and Procamelus were also North
American groups with clear camel-like features.
The earliest camel-like fossils found in Asia
date to around 6 MYA and resemble Procamelus.
Might Procamelus or one of its descendants have
migrated from North America to Asia six million
years ago? What route could such a migration have
followed?
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Pliauchenia (about 5-2 MYA)
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Hemiauchenia (about 9 MYA)
Hemiauchenia macrocephalus (large-headed llama)
was another North American species with typical
camelid features. It is the earliest fossil known
to resemble todays South American camelid
species. The earliest camelid fossils in South
America date to around 2.5 million years ago.
Might Hemiauchenia or one of its descendants,
such as Pliauchenia, have migrated from North
America to South America at this time? What route
might such a migration have followed? .
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Aepycamylus (about 10 MYA)
Oxydactylus (about 20 MYA)
Oxydactyus and Aepycamelus were North American
camelids that became extinct from 10-20 million
years ago. They had long necks and legs and
probably browsed on tree foliage like giraffes.
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Camelops (about 2 MYA)
Camelops hesternus was another North American
camelid. Its fossils have been found as far north
as the Yukon-Alaska border. It became extinct
during the last ice age12 thousand years ago,
along with all the other remaining camel-like and
llama-like species in North America. Exactly why
all the camelids became extinct in North America
at this time is unknown.
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Ancient Geography Today, 53 miles of the
Bering Sea separate Siberia in eastern Asia from
Alaska in North America. During an ice age six
million years ago, however, water levels in the
Bering Sea dropped about 300 feet. This gradually
exposed a 1,000 mile-wide grassland region known
as the Bering Land Bridge. This land bridge
connected North America to Asia for several
thousand years.
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Does the timing of the Bering Land Bridge
correspond to the time when camel-like fossils
first appear in Asia?
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North America and South America also were
separated by ocean at one time. Around 2.5
million years ago, however, the Isthmus of Panama
connecting these two continents was raised above
sea level by colliding continental plates. This
land bridge still connects the North and South
American continents today.
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Is the timing of the Isthmus of Panama
consistent with the time of appearance of the
first camel-like fossils in South America?
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• Questions
• Review your Data Sheet and the slides as
  necessary to answer the following questions.
  Write your answers on a separate sheet of paper.
• Are the fossil data compatible with the idea that
  the various camelid species descended with
  modification from a common ancestor? Explain.
  Which fossil species is the most likely candidate
  for such an ancestor, and where did it originate?
• 2. Do the fossils show changes across time in
  their features toward those typical of camelids
  today? Describe how the features changed over
  time.
• 3. Would it have been physically possible for
  descendants of a camelid ancestor to spread to
  the parts of the world where camelids are found
  today? If so, when would these migrations have
  occurred, and what paths could they have taken?
• 4. Where did the camelids spend most of their
  evolutionary history?
• 5. In addition to fossil distributions and
  geographic data, what other kinds of evidence
  could be used to test the hypothesis that
  camelids are related and descended from a common
  ancestor?