"The number is mounting, and it'll vary depending on whom you talk to," said John Stewart, an evolutionary paleoecologist at Bournemouth University in the United Kingdom. Some researchers argue that the species known as Homo erectus is in fact made up of several different species, including Homo georgicus and Homo ergaster.

"It's all about the definition of a species and the degree to which you accept variation within a species," Stewart told Live Science. "It can become a slightly irritating and pedantic discussion, because everyone wants an answer. But the truth is that it really does depend."