The fossils of a female adult and juvenile male were discovered in a cave not far from Johannesburg.
Researchers tell the journal Science that the creatures fill an important gap between older hominids and the group of more modern species known as Homo, which includes our own kind.
The team has assigned the name Australopithecus sediba to their finds.
“It’s at the point where we transition from an ape that walks on two legs to, effectively, us,” lead scientist Professor Lee Berger of the University of the Witwatersrand told BBC News.
“I think that probably everyone is aware that this period of time – that period between 1.8 and just over two million years [ago] – is one of the most poorly represented in the entire early hominid fossil record. You’re talking about a very small, very fragmentary record,” he explained.