It's a pattern that scientists have seen before. On land, for example, there is evidence that ancient humans were responsible for the massacre of mammoths and other megafauna across the globe. "We see this over and over again," Heim said. "Humans enter into a new ecosystem, and the largest animals are killed off first. Marine systems have been spared up to now, because until relatively recently, humans were restricted to coastal areas and didn't have the technology to fish in the deep ocean on an industrial scale.