Complex sentence: Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 million years ago.
Split into simpler sentences: Today, the closest living relatives of cetaceans are the hippopotamuses. Whales and hippos share a semi-aquatic ancestor that branched off from other artiodactyls some 60 million years ago.