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LARGE size, rather than advanced tools, may have triggered the first human migration from Africa. So say anthropologists who have found the skulls of two early humans alongside primitive stone tools. The pair lived some 1.7 million years ago at Dmanisi in the Republic of Georgia.

The first recognisable 鈥渉umans鈥�, Homo habilis, evolved about 2.4 million years ago, and developed sharp flaked stone tools called the Oldowan tool kit. With bodies about two-thirds the size of modern humans, and brains even smaller, H. habilis never left Africa.

Migration began with the next generation. Fossils from this period found in Africa are called Homo ergaster, while those that made it to Asia are called Homo erectus. The differences between the two are still debated, however. H. ergaster appeared about 1.9 million years ago, and is credited with later developing the advanced Acheulean tools, which are shaped to a preconceived design rather than just being sharp flakes of stone.

The Georgian fossils 鈥渟how clear affinities to the African H. ergaster rather than to the more typically Asian H. erectus鈥�, says excavation leader David Lordkipanidze of the Republic of Georgia State Museum in Tbilisi. 鈥淲e think they are the connecting link between Africa and Asia.鈥� However, the tools suggest the migration from Africa started before H. ergaster developed advanced tools. The primitive Dmanisi stone tools 鈥渟uggest a dispersal driven not by technological innovation but more likely by biological and ecological parameters鈥�, he says.

H. ergaster鈥檚 greater size may have galvanised the migration of humans out of Africa, says anthropologist Susan Anton of the University of Florida in Gainesville. Larger species tend to have larger ranges. The size and anatomy of these first colonisers also allowed them to walk more efficiently than other primates, says Milford Wolpoff of the University of Michigan.