1.Researchers have long suspected that hippos evolved from a family of plant-eating, semi-aquatic mammals called anthracotheres. Researchers could never verify the theory since the only fossils of ancient hippos discovered thus far, dated back just 15 million years. 2. Finding a fossil made them look for more. The teeth serve as a kind of fingerprint for the species, enabling paleontologists to connect ancient creatures to their modern-day relatives. 3. Up to 220 pounds. The hippo.