Our early ancestor Homo erectus may have been smart and social enough to build seafaring rafts. This flattering portrait of these early humans is reinforced by new dates for stone tools, reported in tomorrow's issue of Nature, that confirm H. erectus's presence on the Indonesian island of Flores some 800,000 years ago.
Although H. erectus was known from just 600 kilometers away on Java, Flores lies across several deep-water straits, and most researchers were convinced that H. erectus lacked the social and linguistic skills needed to pilot a raft over deep, fast-moving waters. But in 1994, a Dutch and Indonesian group dated stone tools found on Flores at about 750,000 years old--a time when H. erectus was the only type of human in Southeast Asia. The dates were considered suspect due to the lack of human bones at the site and uncertainties of the type of dating used, called paleomagnetic dating.
Now fission-track dating of ash layers confirms these findings. The method uses tiny tracks left in volcanic crystals such as zircon by the spontaneous fission of uranium-238 atoms. Using this method on 50 zircon grains from ash layers just above and below the tool-bearing sandstone layer, Paul O'Sullivan and Asaf Raza at La Trobe University in Victoria, Australia, where the technique was pioneered, got ages of 800,000 to 880,000 years for almost all of the grains.
Researchers who have worked at the site, called Mata Menge, are convinced that H. erectus arrived by raft or other watercraft. Even when the sea level was at its lowest, these humans would have had to cross 19 kilometers of water to get to Flores from the closest island of Sumbawa. That has broad implications for H. erectus in Asia and beyond: "They were intelligent, thinking animals," says lead author Mike Morwood, an archaeologist at the University of New England in Armidale, Australia. "They must have had language for the collective effort needed to achieve this sea travel."
But not all experts are ready to up the early human's intelligence quotient. "Let's be cautious about what conclusions we draw about the navigational skills of H. erectus," says Colin Groves of Australian National University in Canberra. He points out that the tectonics of these volcanic islands is so unstable that there may even have been a land bridge briefly connecting them. Other researchers add that H. erectus might have accidentally drifted over to Flores on a primitive raft of tree limbs and vegetation.