Paleoanthropologists have discovered that while younger fossils of the genus Homo indicate a derived, humanlike brain organization, the earliest Homo genus brains found outside of Africa retained a primitive, great ape-like organization, contradicting the long-standing assumption that fully derived, modern human brains existed at the emergence of the genus.
The study, published April 8 in Science, provides breakthrough insights on one of the classic endeavors in paleoanthropology: how the modern human brain evolved and when these structural innovations occurred.
"We have been thinking about these ideas, I would say, almost 30 years, since the very beginning of our scientific career," Christoph Zollikofer, senior author of the study and a professor at the University of Zurich, told The Academic Times. "But at that time, it was simply not possible to realize our goals. There were no computer tomographs, no MRI machines, no data from great apes. So it was just an idea."
Homo — the Latin word for man — is the genus that encompasses modern humans, Homo sapiens, and all of their extinct, ancestral relatives, including Homo erectus and Homo neanderthalensis.
This study is adding to the ever-growing literature that is seeking to explain human ancestry and evolution; for example, researchers in March found that early humans showed signs of innovation in more locations than previously thought.
One of the greatest challenges in paleoanthropology continues to be the fact that brains do not fossilize, and so in order to learn more about the development of Homo's brain, scientists study endocasts, the natural or virtual fillings of hominin fossil brain cases. "Hominin" refers to modern humans, extinct Homo species and all immediate ancestors.
"The only thing we have are the empty brain cases," Zollikofer said. "And sometimes they are preserved as real sediment fillings, sometimes they're kind of hollow, and then you have to use computer tomography to do a virtual reconstruction of the endocasts.
Up until now, the scientific community had not studied the endocranial morphology of the well-preserved fossils from Dmanisi, a town and archaeological site in the Kvemo Kartli region of the country of Georgia. These fossils are dated to about 1.85 million to 1.77 million years old, making them some of the earliest members of the genus Homo generally, as well as some of the oldest hominin fossils in Eurasia, according to the study.
"These fossils represent a single population, and because they are well preserved, you get a very good idea of the variation that you have within this group, and about what kind of brain organization they represent," Zollikofer said.
Examining the endocasts of these fossils using craniocerebral topography revealed "a primitive frontal lobe organization, featuring an ape-like anterior location of the inferior precentral sulcus relative to the coronal suture," according to the study.
"For the basic machinery of the brain, you need brain areas for primary computational analysis, say for sensory impressions," Zollikofer said. "So vision olfaction, auditory signals, but also from the body itself, so from the body surface. Then you need another kind of neural machinery to produce the motor output movements, everything from hand movements, fingers, and tongues especially."
The neural machinery related to the tongue touches on a neural area, known as Broca's cap, that is closely related to language, according to Zollikofer. Broca's cap, located in the inferior part of the frontal lobe, is a key part of the neural machinery that plays a fundamental role in language production and comprehension.
"These areas are highly specialized in humans," he said. "And because they are specialized, they are great, relatively great, and they occupy more terrain in the brain compared to the basic machinery."
"Then there are also areas that we call association areas in the frontal lobe and in the parietal lobe," he said. These frontal lobe areas are involved in complex cognitive tasks including social cognition and tool use, according to the researchers.
"All these areas are much larger relative to the rest of the brain in humans compared to great apes," Zollikofer said. "So, what we actually see in those fossils is a key shift of the basic machinery, which is shifted backwards relative to these enlarged frontal areas."
"This backwards shift, you'll see a border between two regions that moves backward," he said. "And interestingly enough, people didn't pay attention to that feature earlier on, and this is the key point of our analysis — just look of how things get displaced over time looking at older and more recent fossils."
"So, basically," Zollikofer said, "we can pinpoint when the areas that are responsible in now-modern brains for language or spoken language evolved. That's one of the key points of our analysis."
Finding a primitive brain organization in the Dmanisi fossils is striking because these fossils represent people outside of Africa, Zollikofer said, which means the Homo genus must have left Africa before the modern human brain emerged. This indicates that the derived frontal lobe organization emerged relatively late, "clearly later than the first dispersals of Homo from Africa," according to the study.
In addition to examining the Dmanisi fossils, the researchers comprehensively revisited and revised African and Asian fossil material to produce a new evolutionary timeline for hominin frontal lobes and create a new understanding of the evolutionary timeline of the hominin brain overall.
The materials in this revision included fossils originally found in Java, Indonesia, dated to be younger than 1.5 million years old and representing the youngest Homo erectus specimens from Southeast Asia. Homo erectus, translated from Latin as "upright man," is an extinct species of human from the Pleistocene period that highly resembled modern humans.
Using high-resolution imaging and three-dimensional modeling techniques, the researchers concluded that the first Homo populations to disperse from Africa, potentially as early as 2.1 million years ago, retained a primitive frontal lobe organization, as shown by the Dmanisi fossils.
Then, somewhere between 1.7 million and 1.5 million years ago, modern brain structures evolved in Africa, which led to a second dispersal out of Africa. After this second dispersal, somewhat later than 1.5 million years ago, modern-looking people with modern-looking brains — Homo erectus — were then located in Southeast Asia, according to Zollikofer.
"These findings tell us some surprising insights," Zollikofer said. "Now we know that those hominins with the ancestral type of brains, they were relatively small-structured, like in a great ape. They were not just incapable, they were very capable: they moved out of Africa, and we know that they produce all kinds of foods and that they took care of elderly people."
"So one message from this research," he said, "is to never underestimate the potential of an ape-like brain. This, in a way, closes the gap between us modern humans and more ape-like ancestors."
Additionally, based on this research, paleoanthropologists now have a better understanding of when the frontal lobe brain areas evolved, especially those related to language, according to Zollikofer — though they do not know whether those people that characterize the second dispersal from Africa actually spoke a language.
"But we call that the language-ready the brain," Zollikofer said. "It provides all the structures necessary for modern human spoken language. And we now know that these structures evolved between 1.7 and 1.5 million years ago."
He added that there are many known fossils that have yet to be analyzed, "and now it will be interesting to see how their potential brain structures looked" in the context of these findings.
"And then, of course, the big unresolved question is how and why, exactly, modern humanlike language evolved," Zollikofer said. "This is just the first link to stone in a big mosaic."
The study "The primitive brain of early Homo," published April 8 in Science, was co-authored by Marcia S. Ponce de León, Thibault Bienvenu, Silvano Engel, and Christoph P. E. Zollikofer, University of Zurich; Assaf Marom, Israel Institute of Technology; Paul Tafforeau, European Synchrotron Radiation Facility; José Luis Alatorre Warren, University of Zurich and Harvard Medical School; David Lordkipanidze, Georgian National Museum; Iwan Kurniawan and Toetik Koesbardiati, Museum of Geology, Jalan Diponegoro; Delta Bayu Murti, Airlangga University; and Rusyad Adi Suriyanto, Gadjah Mada University.