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Brain study debunks an assumption about ancient humans

Advanced technology allowed scientists to upend an old idea.

Humans and apes may be closely related, but brains are where we differ. Human brains are larger and structurally different, especially in the areas that allow us to do things apes can’t, namely complex cognitive tasks like talking about the future and past.

But when did this change happen in our evolutionary history? A new study of fossil hominin braincases reveals early species in the genus Homo had a “much more primitive brain” than previously thought, study author and anthropology professor at the University of Zürich Christoph Zollikofer tells Inverse.

This finding was published Thursday in the journal Science.

What’s new — The earliest species in the genus humans belong to, Homo, are thought to be Homo habilis and Homo erectus. Homo habilis lived 2.4 million to 1.4 million years ago, and Homo erectus lived 1.89 million to 110,000 years ago. Previously, scientists thought these early humans had brains much like modern humans.

However, this new comprehensive comparison of over 300 digital brain reconstructions from Homo fossils, modern humans, and apes shows that the evolutionary change from a more primitive brain to a more modern human brain most likely happened in Africa between 1.7 and 1.5 million years ago.

This makes the early evolutionary history of the genus Homo and our brains more complex than scientists realized.

Here’s the background — There are significant differences between the ways brains of great apes, such as gorillas and orangutans, and humans are organized, says Marcia Ponce de León, the lead author of the study and an anthropologist at the University of Zurich.

The study team used computed tomography and virtual reconstruction to examine early Homo skulls. M. Ponce de León and Ch. Zollikofer, University of Zurich

“These differences are manifest on the external surface of the brain,” she tells Inverse. The fold and indentations of the brain leave an imprint on the bone of the braincase, so even though the brain itself isn’t preserved, it can be digitally reconstructed.

This topographic reconstruction of the brain is called an endocast. Understanding when and how these changes happened is vital because the different organization of human and ape brains separates our ability to have language and higher cognitive functions.

The frontal lobe of humans compared to apes is quite different. This reorganization has historically been considered a hallmark of the genus Homo.

“Since the first description of the Australopithecus child fossil from Taung... there has been a tradition of searching for derived (modern human-like) features of the brain of fossil hominins,” Zollikofer says.

In the 1960s, anthropologist Phillip Tobias proposed that one of the earliest members of the genus Homo from Africa, Homo habilis, had a brain that was similar to a modern human brain. This “idea has prevailed” since then, Zollikofer says.

How they did it — Million-year-old brains can be difficult to study because brains don’t fossilize, so researchers need to look at the digital reconstructions instead to trace changes in brain morphology over time. Ponce de León and the other researchers worked for the past 15 years or so to examine brain evolution in early Homo from around the world, but had to wait until the technology became more advanced.

“Only in the late ‘90s with the advent of computed tomography and magnetic resonance imaging, was it possible to quantify brain-endocast relationships in living species,” Ponce de León says.

Now, they’ve been able to compare how brain organization has changed over time in modern and fossil specimens by employing three-dimensional modeling.

What they found — Comparing the arrangements and shifts of grooves in the endocast, which represent changes in brain reorganization particularly in the frontal lobe, suggests the early evolutionary history of the Homo brain was more complicated than previously thought.

A Dmanisi cranium. Paul Tafforeau, ESRF

The team of researchers looked at the endocasts of rare hominin fossils from Africa, Southeast Asia, and a site called Dmanisi in the country of Georgia. The Dmanisi fossils represent the oldest known Homo population outside of Africa at between 1.85 and 1.77 million years old, Ponce de León says. It turns out that the Dmanisi people did not have the more derived brain type present in later Homo, so there was a dispersal from Africa before the brain’s evolutionary transition.

“The earliest populations of Homo had much more primitive brains than we had thought – similar to those of their Australopithecus ancestors. But these populations showed complex behaviors and dispersed from Africa into Eurasia, so we should not underestimate the capabilities of their brains,” Ponce de León and Zollikofer explained by email.

The more human-like organization of the frontal lobe probably evolved in Africa between 1.7 and 1.5 million years ago. This also means there was a second dispersal out of Africa for the Homo genus, because the “modern-brained” Homo populations found in Java are around 1.5 million years old.

Why it matters — This in-depth study upends what was previously thought about the evolution of the modern human brain and when it happened in the earliest species in our genus Homo. For the first time, anthropologists are able to pinpoint when the “language-ready brain” originated, likely between 1.7 and 1.5 million years ago.

While there are still questions about why the frontal lobe of early Homo reorganized to form the brains we all use today and exactly how language originated, this period marks a critical transition in the evolution of early humans — and of us.

Abstract: The brains of modern humans differ from those of great apes in size, shape, and cortical organization, notably in frontal lobe areas involved in complex cognitive tasks, such as social cognition, tool use, and language. When these differences arose during human evolution is a question of ongoing debate. Here, we show that the brains of early Homo from Africa and Western Asia (Dmanisi) retained a primitive, great ape–like organization of the frontal lobe. By contrast, African Homo younger than 1.5 million years ago, as well as all Southeast Asian Homo erectus, exhibited a more derived, humanlike brain organization. Frontal lobe reorganization, once considered a hallmark of earliest Homo in Africa, thus evolved comparatively late, and long after Homo first dispersed from Africa.
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