Pinpointing when our ancient ancestors evolved humanlike brains is a frustratingly difficult puzzle. Brains almost never fossilize, so researchers must scrutinize impressions in the skull left behind by the brain’s grooves, folds, and bulges. A new analysis of such imprints from five skulls suggests our genus, Homo, developed complex language and advanced toolmaking hundreds of thousands of years later than previously thought. Other researchers disagree with that interpretation, but say the study still sheds much-needed light on brain structures in our genus’ earliest days.
The fossil skulls, discovered in the 1990s in Dmanisi, Georgia, are tentatively identified as the early human ancestral species, Homo erectus. They represent some of the earliest members of our genus, as well as the earliest people to trek out of Africa. The four males and one female, who lived between 1.85 million and 1.77 million years ago, show a wide range of primitive, intermediate, and humanlike brain features, says Shawn Hurst, an anthropologist at the University of Indianapolis who studies primate brain evolution and was not involved in the study. That sheer variability within a single population is surprising, he says. “That’s amazing. … It’s actually a more interesting story than the one they suggest.”
Many anthropologists have assumed that humanlike brain features—especially those having to do with speech and language—developed at about the same time the genus Homo emerged out the genus Australopithecus. Yet no convincing data supported that idea, says the new study’s first author, Marcia Ponce de León, an anthropologist at the University of Zurich. Many scientists have followed in the tradition, thinking “if it’s Homo, there must be something special about its brain,” she says.
In the new study, Ponce de León combined a variety of high-resolution imaging techniques to scan replicas of the underside of the braincases, known as endocasts, and create computerized models of their interiors. Next, the researchers charted the slight grooves and furrows left behind by ancient brain surfaces, giving them rough insight into where certain regions of the mammalian brain would have been located. By mapping out the relative positions of those landmarks and comparing them with the endocasts of living humans and great apes, they reasoned, they could infer whether certain regions were more—or less—developed compared with modern humans.
Two landmarks in particular—the coronal suture, a horizontal groove along the top of the cranium, and the precentral sulcus, a groove that runs along the top of the brain and down the side—caught the team’s eye. In great apes, the precentral sulcus crosses paths with the coronal suture toward the front of the skull. But in most modern humans, this groove is shifted toward the back. That gives more “real estate” to the brain’s frontal regions, including one heavily involved in speech known as Broca’s area.
When the researchers looked at these landmarks in the Dmanisi endocasts, they concluded that the ancient individuals’ frontal lobes were “primitive”—that is, smaller than those of modern humans and shaped more like those of living great apes and earlier Australopithecus species. Because this region is critical to complex speech, long thought necessary for advanced toolmaking, the findings suggest these early members of our genus didn’t possess particularly modern humanlike brains, the researchers report today in Science.
Next, Ponce de León and her team applied the same methods to endocasts of 34 other Homo fossils from various species dating to between 2 million and 70,000 years ago. They found that fossils younger than 1.5 million years often had more modernlike frontal lobes, perhaps making that time period the critical juncture when our lineage developed the capacity for complex language. “We do not know whether these early Homo populations had a modern humanlike language, but they probably [had] some kind of proto-language that … favored the evolution of these brain structures,” Ponce de León says.
Elliot Murphy, a neurobiologist who studies language processing at the University of Texas Health Science Center, Houston, says the new study is “careful and illuminating” and represents a leap forward in our understanding of brain evolution. “If frontal lobe reorganization occurred comparatively late, after Homo initially left Africa, then this provides even less time than typically thought … for language to emerge,” he says.
Others profess skepticism about the team’s interpretations. One sticking point for several was whether it was possible to make any accurate judgement about brain structure based on the landmarks of the coronal suture and precentral sulcus. “When they report the coronal suture is here and precentral sulcus is there, that’s correct, but you can’t predict anything from that,” Hurst says.
Emiliano Bruner, a paleoneurobiologist at the National Research Center for Human Evolution in Burgos, Spain, agrees, adding that spatial constraints of the Dmanisi skulls could account for the spatial relationships between those landmarks without having any bearing on the underlying brain structure. “The authors apparently interpret the spatial relationships between brain and skull as direct evidence of brain evolution,” without considering those cranial constraints or other developmental factors, he says. Ponce de León counters that her team did, in fact, account for such constraints, and still drew the same conclusions.
Hurst points to several other brain landmarks identified but not discussed in detail in the study, such as the relative position of another groove called the inferior frontal sulcus, as evidence of a mix of modern and primitive features among the Dmanisi individuals. “There’s a lot new to explore here which would tell us about the tempo of human brain evolution,” he says.
John Hawks, an anthropologist at the University of Wisconsin, Madison, agrees this variation in brain features is intriguing. “Everybody until now has sort of looked at early Homo brain evolution as if there’s going to be a magic feature that switches [on] in Homo and after that, you’ve got the basic architecture of the Homo brain,” he says. “They are making a claim that that’s not the way it works at all: You’ve got diversity. … I like the idea that diversity is an important aspect of the initial origin of our genus, and we don’t know yet which of these samples are going to be closely connected to our evolution.”