Background Information

The horse has evolved from Hyracotherium, a small creature standing less than 0.4 m tall, to the modern-day horse, a much taller animal standing approximately1.6 m tall. Examine the figure below, which shows the evolution of the horse.

(a) Draw a graph showing changes in the height of the horse over time.

Copyright © 2003 Nelson Chapter 11 Origins of Evolutionary Science 333

Student Worksheet

Additional Case Study: Evolution of the Horse

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Time frame Diagram Description of horse

Equus • This horse lives in the grassy plains area.The modern-day horse • It eats grasses and is classified as a grazer.

• It is approximately 1.6 m tall. • It has a single visible digit.

Pliohippus • This horse lived in areas with shrubs 1.6 million years ago and on the grassy plains.

• It ate grasses and is classified as a grazer.• It was approximately 1.0 m tall.• It had a single visible digit.

Merychippus • This horse lived in areas with shrubs and on the 5 million years ago grassy plains.

• It ate leafy vegetation and grasses and is classified as a grazer.

• It was approximately 1.0 m tall.• It had three visible digits.

Mesohippus • This horse lived in less thickly wooded areas.24 million years ago • It ate leafy vegetation and is classified as a browser.

• It was approximately the size of a small dog, about 0.6 m tall.

• It had four visible digits.

Hyracotherium • This horse lived in thickly wooded areas.37 million years ago • It ate leafy vegetation and is classified as a browser.

• It was approximately the size of a small dog, about0.4 m tall.

• It had four visible digits.

(continued)

(b) Describe changes in leg length and the width of the digits.

(c) Do horses have thumbs and fingers?

Case Study Application Questions

1. Hyracotherium lived in a dense forest area dominated by a rich undergrowthof ferns. What do you think the advantages of small body stature might be inthis type of environment?

2. A change in vegetation to grasslands brought about changes in the shape ofthe horse’s leg. What changes in vegetation could have caused the evolution ofthe horse?

3. The teeth of the browsers, such as Mesohippus, were covered with a thick layerof enamel. The teeth of the modern-day horse, by comparison, have lessenamel and appear to be much wider and flatter. Provide reasons for some ofthe changes in teeth.

4. Why would changes in the environment have caused changes in the horses’predators?

334 Chapter 11 Origins of Evolutionary Science Copyright © 2003 Nelson

LSM 11.6-4

Observation Questions

(a)

(b) Over time, the legs became longer. The main digit became wider and theother digits became narrower until, in Equus, they became vestigial.

(c) Hyracotherium has four digits (fingers) and may have had a small, vestigialthumb. Equus has a single visible finger on its forefeet, plus other vestigialfingers.

Case Study Application Questions

1. A small body would have permitted the Hyracotherium to hide in the under-brush from its predators.

2. As the forests changed to grasslands, the horse had to change from eatingleaves and shrubs to eating grass. This change would cause changes in thetooth structure (grass is coarser than tree leaves and ferns) and changes in thelegs and neck (grass-eating horses have to lower their heads to the ground;leaf-eating horses can eat without stretching).

3. Grazing horses grind their coarser food on their wider and flatter teeth.

4. Forest predators use different methods of catching their prey than grasslandpredators. Consequently, as the environment changed from forest to grass-land, the predators would have to adapt as well or become extinct.

Copyright © 2003 Nelson Chapter 11 Origins of Evolutionary Science 335

Student Worksheet Solutions

Additional Case Study: Evolution of the Horse, Solution

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Hei

ght o

f Hor

se (m

)

40

0.5

1

1.5

2

30Millions of Years Ago

20 10 0

336 Chapter 11 Origins of Evolutionary Science Copyright © 2003 Nelson

Instructional Master

Fossil Evidence of Horse Evolution

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10 000 years ago

14 to 7 million years ago

about 20 million years ago

about 30 millionyears ago

Fossils usually occur inchronological order in rocklayers (called strata). If thelayered sediments have notbeen disturbed, more recentfossils are found near the sur-face while older fossils arefound deeper in the fossilbeds.

about 60millionyears ago