New research suggests that climate change helped dogs develop from animals looking like cats to the way they are today. Scientists from Brown University in the USA believe that 40 million years ago, dogs were smaller and they looked like cats. They hid, ran after and killed other animals. As climate changed in the world, the physique 体型）and eating habits of dogs also changed. Thick forests that once covered North America started to become less thick as the earth became cooler.
The appearances of grassland made dogs have fewer places to hide to hunt for food. To get used to the environment, they changed their ways of hunting and slowly grew longer legs. The scientists studied how dogs have changed themselves by looking at the elbows（肘部）and teeth of 32 different species that lived from 2 million to 40 million years ago. The scientists said that the dogs' elbows greatly changed. Dogs from 40 million years ago had elbows that were more similar to those of cats. The function of the elbows was to allow the front paw（爪子）to catch what they could get and hold on to it. The elbows changed to allow dogs to become good runners and then they were able to run after other animals over long distances. Teeth in dogs also changed to be more useful. They became much stronger to be able to hold on to what they get.
①Dogs grew longer legs.
②Climate changed.
③Dogs changed ways of hunting
④Forests became less thick
