Dr. Rightmire and his colleagues then compared the Dmanisi skulls to many of the early Homo skulls found across the Old World. All of those far-flung fossils fell within the same range of variation that the scientists found at Dmanisi.

Based on this analysis, the scientists declared that all those early Homo fossils belong to a single species — which they suggest should be called Homo erectus. If other researchers find evidence to support this view, said Dr. Rightmire, it would have a big impact on how we understand human evolution. “We’ll have to go back to drawing boards and rethink the origins of Homo right from the start,” he said.

If Homo erectus was indeed a single species, its range would have been tremendous compared with our closest living ape relatives. Chimpanzees, for example, live only across a narrow band of sub-Saharan Africa. What’s more, ape species tend to split apart into new ones.

Two million years ago, for example, a bend of the Congo River cut off the chimpanzees in the southern part of the species’s range. Those southern chimpanzees evolved into smaller, more slender apes that today are more peaceful than their warring cousins north of the river. Their DNA reveals little sign that they have interbred with other chimpanzees over the past two million years — despite living within a few miles of them. As a result, biologists have given them their own species name: Pan paniscus, commonly known as bonobos.

If Homo erectus was like chimpanzees, it would be remarkable for them to hang together across rivers and deserts and mountain ranges as a single species. But Todd R. Disotell, a biological anthropologist at New York University, suggests that early Homo might be more like another primate. “Baboons probably make a good analog,” said Dr. Disotell.