Named "Little Foot," the skeletal remains are those of a small ape-like creature who fell into a pit in South Africa's Sterkfontein cave complex millions of years ago.
How many years, though, is the question, and teams have put forward an extraordinary range of estimates, from 1.5 to four million.
The date is the key issue as to whether humans rose in East Africa, as a mainstream theory suggests, or in southern Africa -- or possibly in both or other places simultaneously.
That makes it a rough contemporary of "Lucy," the Ethiopian hominid that has the most prominent claim on being our earliest-known ancestor.
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"There is nothing to rule out the idea that (Little Foot) was the forerunner of humanity. Everything is possible," said Laurent Bruxelles from France's National Institute for Archaeological Research (Inrap), who took part in the study.
In the claim for being the cradle of humanity, "southern Africa is back in the race," Bruxelles said.
The technique, called cosmogenic nuclide dating, looks at levels of rare isotopes that are created when soil or rocks are hit by high-speed particles that arrive from outer space.
A first attempt using this method, in 2003, suggested an age of four million years, although it had an enormous margin of error.