Over thousands of years, humans have greatly changed animals through selective breeding to make them suit our needs. Early sheep, for instance, had a short, coarse fleece before new woolly breeds were created around 6,000 years ago.
Many modern farm animals can grow faster and have more offspring than their ancestors, and are often far larger. Early wild chickens, for example, weighed about two pounds, but modern farmed chickens can weigh as much as 17 pounds.