Mr Easteal, of the Australian National University in Canberra, arrived at the conclusion after recalculating the frequency at which natural genetic mutations occur. If he is correct, the earliest fossils that appear to be human - dated about 4.4 million years ago - cannot be human after all.
Mutations of a given gene in a group of related species are thought to occur at a fairly constant rate as they evolve. Working from fossil evidence indicating when evolutionary changes occurred, scientists have worked out an average genetic mutation rate for human development.
Mr Easteal told New Scientist magazine: "We didn't come down from the trees - they went up into the trees."