"A good analogy is the chimps and gorillas," he said. "There are a bunch of places in West Africa where both of them live in the same area. But they don't interbreed, they keep to themselves."
The common ancestor of H. habilis and H. erectus remains a mystery, Spoor added.
However, he and other scientists still believe that modern humans evolved from H. erectus, perhaps via an intermediate form. (Related: "'Missing Link' Human Skull Found in Africa, Scientists Say" [March 27, 2006].)
Chris Stringer studies human origins at the Natural History Museum in London. He was not involved in the study.
The new research accurately dates and identifies the two fossils, "throwing light on the early evolution of humans," he said by email.
He also noted that H. erectus may still have evolved from H. habilis, but different adaptations and lifestyles could have allowed some populations to live alongside each other for hundreds of thousands of years.
"One possibility is that the larger and perhaps more mobile erectus species was an active hunter, while habilis scavenged or caught small prey," he said.
The other fossil is an exquisitely preserved skull said to belong to H. erectus. Dated to 1.55 million years ago, it is the smallest H. erectus skull found so far.
The find suggests that H. erectus could vary tremendously in size, said study co-author Susan Antón, an anthropologist at New York University.
"One way of reading that is that there is a lot of size difference between males and females, which would be a lot more sexual dimorphism than we previously thought," she said.
In primates, she noted, large size differences between males and females can be related to sexual selection, mate competition, and reproductive strategy.
For example, a large male silverback gorilla has several smaller female mates, whereas in gibbons males and females are similar in size and shape and mate in pairs.
Previously, scientists believed H. erectus was more like modern humans in terms of size difference between males and females.
"If this is sexual dimorphism and a lot of sexual dimorphism, then it's probably telling us something about behavior that was somewhat less like what we are today," Antón said.
Ian Tattersall is an anthropologist at the American Museum of Natural History in New York who studies human origins. He was not involved in the study.
He questioned whether the fossils are correctly assigned to H. habilis and H. erectus.
Nevertheless, he said, they indicate at least two lineages of the genus Homo overlapped in eastern Africa.
"All of this is really contributing to a picture of diversity in early hominid evolution in this time period," he said. "And it's another problem for the notion of linearity."
Rather, he added, "the history of hominids in this time period was one of experimentation—of different ways to be hominids."
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