Homo erectus wasn't the first human species to leave Africa 1.8 million years ago, fossils suggest

Early, ancestral members of the human lineage may have left Africa earlier than widely thought, a new study of fossil teeth suggests.

Modern humans, Homo sapiens, are the only living member of the human lineage, Homo, which is thought to have arisen in Africa about 2 million to 3 million years ago and first left that continent a few hundred thousand years ago. But many other extinct human species previously roamed Earth, such as Homo habilis, suspected to be among the first stone-tool makers, and Homo erectus, the first to regularly keep the tools it made.

Scientists investigated fossils excavated from the medieval hilltop town of Dmanisi in the Republic of Georgia. Archaeological excavations there about 35 years ago unexpectedly revealed that Dmanisi is one of the oldest-known sites for ancient human species outside Africa, with the five skulls recovered from there dating to approximately 1.8 million years ago.

The fossils of Dmanisi have drawn intense debate because of the unusual level of variation they display. Many researchers have suggested these specimens all belong to H. erectus, with the anatomical diversity seen between the specimens resulting from factors such as natural differences between the sexes. Other scientists have argued that the Dmanisi fossils represent two distinct human species. One, dubbed Homo georgicus, seemed more closely related to predecessors of humans known as australopiths, while the other, Homo caucasi, appeared more similar to early human species.

Resolving this controversy might reveal whether H. erectus was the first human species to leave Africa, or if others preceded it, study co-author Victor Nery, a historian and archaeologist at the University of São Paulo in Brazil, told Live Science.

Previous analyses of the Dmanisi fossils mostly focused on the skulls. In the new study, published Dec. 3 in the journal PLOS One, researchers instead concentrated on similarities and differences among the teeth.

The scientists analyzed 24 teeth from three individuals at Dmanisi. They compared them not only with each other, but also 559 teeth from other species, including australopiths, early humans such as H. habilis and H. erectus, and modern humans.

The teeth appeared to split into two groups, one closer to australopiths, and the other more similar to early humans, the researchers found. The differences between these groups was especially evident in teeth from the upper jaw.

These dental discoveries suggest "there were likely more than one species that occurred in the Dmanisi region," study co-author Mark Hubbe, head and professor of anthropology at the University of Tennessee, Knoxville, told Live Science.

The scientists noted the dissimilarities between these two groups of teeth was akin to the levels of differences seen between the sexes in chimps and gorillas. That raises the possibility that these represent the teeth from both sexes in one species. However, the researchers argued the Dmanisi fossils did not come from just one human species, since the more australopith-like group had relatively large third molars, in contrast to the trend in humans of smaller third molars when compared to their relatives.

"I agree with the authors that Dmanisi probably has more than one lineage represented," Chris Stringer, a paleoanthropologist at the Natural History Museum in London who did not take part in this study, told Live Science. The large but small-brained "skull looks much more primitive than the others — at least [H.] habilis-like, if not australopithecine. The others might still represent a very primitive form of [H.] erectus, which has been the mainstream view so far."

If one accepts the new study's conclusions that there were two species present at Dmanisi at the same time, then the biggest implication is that "there was an earlier, and more 'primitive' species that migrated out of Africa than generally thought, which is quite interesting," Karen Baab, a paleoanthropologist at Midwestern University in Glendale, Arizona, who did not participate in this work, told Live Science.

If human species did leave Africa before H. erectus, these early humans "could have given rise to remote descendants like [H.] luzonensis, [H.] floresiensis and Meganthropus," Stringer added. (Fossils of Meganthropus, an extinct primate, were first found in Indonesia in the 1940s, and scientists have long argued whether it was an ape, an australopith, or a member of an early human species.)

Still, Baab cautioned that these new findings do not conclusively prove there was more than one species at Dmanisi. For instance, she noted the new study's analysis of teeth from the lower jaw suggested these fossils might belong just to H. erectus, and not two species.

Although the new study argues that the simplest explanation for its results is that multiple species existed at Dmanisi, the simplest explanation might actually be "to propose a single, albeit highly variable species, where some individuals retain more ancestral features and others are more derived in the direction of later Homo erectus," Baab said.

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