Ancient hominins had small brains like apes, but longer childhoods like humans

April 01, 2020

Human ancestors that lived more than 3 million years ago had brains that were organized like chimpanzee brains, but had prolonged brain growth like humans, new research from the University of Chicago and other leading institutions shows.

That means these hominins -- the species Australopithecus afarensis, made famous by the Lucy and Dikika child fossils found in Ethiopia -- had a mosaic of ape and human features, a hallmark of evolution.

By using precise technology to scan eight fossil skulls from this region, the researchers also resolved a longstanding question of whether this species had prolonged childhood, a period of time unique to humans that allows us to learn and grow.

"As early as 3 million years ago, children had a long dependence on caregivers," said Zeresenay (Zeray) Alemseged, PhD, Donald N. Pritzker Professor of Organismal Biology and Anatomy and senior author of the research, published April 1 in the journal Science Advances. "That gave children more time to acquire cognitive and social skills. By understanding that childhood emerged 3.5 million years ago, we are establishing the timing for the advent of this milestone event in human evolution."

Alemseged, who discovered the Dikika child fossil in 2000 and leads the Dikika field project in Ethiopia, has studied its species for decades and helped design the new research. Widely accepted to be ancestral to all later hominins, including humans, Australopithecus afarensis lived in East Africa more than 3 million years ago and had many human-like features: They walked upright, had brains that were 20% larger than chimpanzees and may have used sharp stone tools.

But many questions about the species remain unresolved, including whether its brain was organized like humans -- which could indicate more complex behaviors, like communication -- and whether it also had protracted brain growth.

When Alemseged discovered the Dikika child, he used a CT scan to examine its skull, and by studying its teeth determined that its age at time of death was around 3 years. To understand how the child's brain was organized, however, he needed more precise imaging technology, so his team used synchrotron-computed tomography -- which uses extremely powerful X-rays to reveal detailed information about a material's structure -- to scan the child's skull and seven other skulls from the same region.

While brains do not fossilize, they do leave imprints on the inside of the skull. With the scans, the researchers could measure endocranial volume, and see the placement of the lunate sulcus -- a fissure that separates the anterior and posterior parts of the brain. This placement differs in humans and chimpanzees -- in humans, who have a large prefrontal cortex, the fissure is pushed further down in the brain. In chimpanzees, the fissure is closer to the front. The scans revealed that Australopithecus afarensis had a lunate sulcus in a similar position to the fissure found in chimpanzee brains.

"This resolves a contentious argument that has polarized paleontologists for years," Alemseged said. "We can now say the organization of the brain was more ape-like."

Did that mean that the species acted more like chimpanzees? Not necessarily. The group of researchers also used synchrotron-computed tomographic scans to count the Dikika child's dental growth lines. Similar to growth rings in trees, these growth lines can show the exact birth and death date of the child. The team's dental experts then calculated the child's age as 2.4 years.

"That allows you to ask how much of the brain was formed at that given age," Alemseged said.

When researchers compared the child's endocranial volume to that of a chimpanzee and humans, they found that brain development in Australopithecus afarensis was protracted, like in humans today. That meant the species had a long childhood, which laid the foundation for subsequent evolution of the brain and social behavior that differentiates humans today.

Alemseged's collaborators on this research were from the Max Planck Institute for Evolutionary Anthropology, Florida State University, the School for Advanced Research, the European Synchrotron Radiation Facility, Griffith University, Arizona State University, the Natural History Museum of London, and University College London.

The Dikka child, known as Selam (which means "peace" in the Ethiopian Amharic language), has been the source of several important papers on human evolution. Alemseged's graduate students are still studying the fossil, looking at the development of its face and shoulder growth. Ultimately, Alemseged hopes to compile everything that he and his lab have discovered about the fossil into one compendium.

"This fossil has played a pivotal role in allowing paleoanthropologists to ask and answer several major questions about how we became human," he said.
-end-
"Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth" was published April 1 in Science Advances.

University of Chicago Medical Center

Related Chimpanzees Articles from Brightsurf:

Like humans, aging wild chimpanzees value their more "positive" friendships most
Like humans, wild chimpanzees focus on fewer yet more meaningful friendships as they grow older, say researchers who studied male chimps over two decades.

Like humans, chimpanzees can suffer for life if orphaned before adulthood
A new study from the Tai Chimpanzee Project in Ivory Coast and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, shows that orphaned male chimpanzees are less competitive and have fewer offspring of their own than those who continue to live with their mothers.

For chimpanzees, salt and pepper hair not a marker of old age
Silver strands and graying hair is a sign of aging in humans, but things aren't so simple for our closest ape relatives--the chimpanzee.

In the wild, chimpanzees are more motivated to cooperate than bonobos
Scientists investigated cooperation dynamics in wild chimpanzees (Tai, Ivory Coast) and bonobos (LuiKotale, DCR) using a snake model.

A rare heart bone is discovered in chimpanzees
Experts from the University of Nottingham have discovered that some chimpanzees have a bone in their heart, which could be vital in managing their health and conservation.

In chimpanzees, females contribute to the protection of the territory
Researchers of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, extensively studied several neighboring groups of western chimpanzees and their findings reveal that females and even the entire group may play a more important role in between-group competition than previously thought.

Cultural diversity in chimpanzees
Termite fishing by chimpanzees was thought to occur in only two forms with one or multiple tools, from either above-ground or underground termite nests.

Similar to humans, chimpanzees develop slowly
Researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have systematically investigated developmental milestones in wild chimpanzees of the Taï National Park (Ivory Coast) and found that they develop slowly, requiring more than five years to reach key motor, communication and social milestones.

The genome of chimpanzees and gorillas could help to better understand human tumors
A new study by researchers from the Institute of Evolutionary Biology (IBE), a joint center of UPF and the Spanish National Research Council (CSIC), shows that, surprisingly, the distribution of mutations in human tumors is more similar to that of chimpanzees and gorillas than that of humans.

Crops provide chimpanzees with more energy than wild foods
A University of Kent study has found that cultivated foods offer chimpanzees in West Africa more energetic benefits than wild foods available in the region.

Read More: Chimpanzees News and Chimpanzees Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.