Domestic   horses now pull ploughs, race in the Kentucky Derby, and carry police. But early horses weren't tame   enough to perform these kinds of tasks. Scientists think the first interactions humans had with horses were far different from those today. Thousands of years ago, people killed the wild horses that lived around them for food. Over time, people began to catch the animals and raise them. This was the first step in domestication. As people began to tame and ride horses, they chose to keep those animals that had more desirable characteristics. For example, people may have chosen to keep horses that had a gentle personality so they could be ridden more easily. People who used horses to pull heavy loads would have chosen to keep stronger animals. Characteristics like strength are partly controlled by the animals' genes. So as the domesticated horses reproduced, they passed the characteristics on to their young. Each new generation of houses would show more of these chosen characteristics. Modern day horse breeds come in a wide variety of shapes and sizes. This variety didn't exist in the horse population before domestication. The Shetland horse is one of the smallest breeds--typically reaching only one meter tall. With short, strong legs, the animals were bred to pull coal out of mine shafts   with low ceilings. Huge horses like the Clydesdale came on the scene around 1700. People bred these heavy, tall horses to pull large vehicles used for carrying heavy loads. The domestication of horses has had great effects on societies. For example, horse were important tools in the advancement of modern agriculture. Using them to pull ploughs and carry heavy loads allowed people to farm more efficiently. Before they were able to ride horses, humans had to cross land on foot. Riding horses allowed people to travel far greater distance in much less time. That encouraged populations living in different areas to interact with one another. The new from of rapid transportation helped cultures spread around the world. The author uses the Shetland horse as an example to show   _  .
it is smaller than the Clydesdale horse horse used to have gentle personalities some horses have better shaped than others horses were of less variety before domestication