Scientists have built robots that can walk like a toddler, the first machines of their kind that can move with a realistic human gait.
The researchers say the breakthrough will lead to machines with increasingly human characteristics which could one day become the everyday servants depicted for decades in science fiction.
The robots learnt to walk on different surfaces without programming information from their inventors at the Massachusetts Institute of Technology (MIT) in Cambridge, Cornell University in New York and Delft University of Technology in the Netherlands.
They also walk like humans, by tilting themselves slightly to one side as they pick up one leg to move it in front of the second which is simultaneously used to push the machine forward.
Because walking has to be altered according to the type of surface, the robots have to take the ground on which they stand into account. A robot called Toddler, made by researchers at MIT, can teach itself to walk on a new surface in under 20 minutes, or about 600 steps, said Russ Tedrake, a researcher at the institute's department of brain and cognitive sciences.
"[The robot] is one of the first walking robots to use a learning program, and it is the first to learn to walk without any prior information built into the controller," Dr Tedrake told the American Association for the Advancement of Science in Washington DC.
As well as walking on a variety of smooth surfaces, the robots may soon be taught to stride over more difficult terrain such as uneven sand, pebbles or even snow and ice.
Calculations of the energy the robots use suggest they are energy-efficient, consuming about the same number of calories a human walker would to cover the same distance, the researchers wrote in the journal Science .
All the robots in the study move using a principle called passive-dynamic walking, which has been used for a century or more in walking toys, passive mechanical devices with moveable joints that can walk down a slope under the force of gravity.
The discovery will also aid development of a new generation of robotic artificial legs for amputees.