New Study Changes Our Understanding of Human Brain Evolution

We thought an advanced frontal lobe was a basal human condition, but the earliest archaic humans to leave Africa and reach Europe had a frontal cortex like chimps

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Skulls of early Homo from Dmanisi, left, and Homo erectus from Sangiran, Indonesia. Virtual filling of their braincases permits inferences on brain organization
Skulls of early Homo from Dmanisi, left, and Homo erectus from Sangiran, Indonesia. Virtual filling of their braincases permits inferences on brain organizationCredit: M. Ponce de Leon and Ch. Zollikofer, University of Zurich

We humans take vast pride in our brain, not that it’s helped us understand it. Now a new report in Science reveals that the unique structures of the human brain evolved later than thought – certainly after our species began venturing out of Africa 2 million years ago.

Contrasting our fond imaginings, the modern structure of the frontal cortex where we do our advanced-human things such as language hasn’t been with us since our evolution began after all.

We know this because it turns out that the first members of the Homo line to leave Africa – the diminutive primitive specimens found at Dmanisi, Georgia, dating to 1.8 million years ago – had frontal lobe structures like great apes, not like humans, according to a new study published in Science by Marcia Ponce de León of the University of Zurich and colleagues.

On the other hand, hominins younger than 1.5 million years in Africa, and Homo erectus in southeast Asia from that time, did have human-type frontal lobe structures.

In other words, human-type cognitive complexity wasn’t necessary for us to leave Africa in the first place, trekking some 6,000 kilometers (3,700 miles) from sub-Saharan Africa to Georgia.

And in yet other words, there must have been two out-of-Africa dispersals of early Homo: the first of primitive-brained people, probably as early as 2 million years ago (since they had reached Dmanisi by around 1.8 million to 1.85 million years ago), the second of advanced-brain populations, who are the ones we find in Southeast Asia after 1.5 million years.

“It’s interesting to speculate what happened when the second-dispersal people encountered in Eurasia the descendants of the first-dispersal people. Probably something similar to what happened when our own species encountered Neanderthals and Denisovans while spreading into Eurasia,” lead author Ponce de León told Haaretz.

Dmanisi cranium mounted for synchroton tomographyCredit: Paul Tafforeau / ESRF
The Dmanisi skulls, with internal braincase structures revealed by tomography Credit: M. Ponce de Leon and Ch. Zollikofer, University of Zurich

Whatever else they did, when anatomically modern humans met Neanderthals and Denisovans, they had sex.

What actually is the difference between the human brain and that of a chimp or gorilla? Our brain is bigger, and structurally different, particularly in the frontal lobe areas involved with complex cognitive tasks such as tool-making and language.

It is true that chimps make tools. The famed primatologist Jane Goodall recognized actual tool manufacture in chimps, who stripped leaves from twigs to fish ants out of anthills, for example. She even believes to have observed diarrhea-suffering chimps making toilet paper out of masticated leaves. A bonobo in a zoo fashioned a spear, no less, to stab an irritating scientist.

But even the earliest human tool manufacture, some 3.3 million years ago, was at a higher level of complexity altogether than what apes can produce, and the same can be said of language. Whatever communication other animals engage in, from birds to cats to Koko the gorilla, the human command of complexity of language is apparently unique.

By the way, the identity of the Dmanisi hominin is, as usual in hominin archaeological circles, disputed. They were small in size, about 1.5 meters tall, and in brain, which was about a third to a half the size of ours.

The classic Homo erectus was a foot taller and had a bigger brain. Dmanisi Person may have been the type of hominin that erectus evolved from, or a precursor to erectus, or a subspecies – it isn’t clear. Ponce de León counsels that in any case, they “are Homo erectus (that’s that only species designation that really fits this population of early Homo).”

Early Homo skulls found in Dmanisi, from 1.8 million years agoCredit: M. Ponce de Leon and Ch. Zollikofer, University of Zurich

Cast of the past

Whoever the Dmanisi hominins were, how do we know what their brains were like 1.8 million years ago, and those of the 1.5-million-year-old African hominins and erectuses in Southeast Asia?

Fossils of hominin brain tissue are basically nonexistent. Ponce de León’s team could, however, analyze the impressions the brain’s folds and blood vessels created on the interior of the skull.

Dmanisi, happily, provided posterity with no less than five hominin crania that were beautifully preserved, as well as other clues to the behavior of these enigmatic hominins such as stone tools and faunal remains, all of which speak to subsistence strategies, social organization and cognitive capabilities.

Since it had been assumed that the advanced organization of the frontal and parieto-occipital lobes had been characteristic of the genus Homo from its beginnings, it would have been nice to know what our brain was like shortly after we split from chimps 7 million years ago, or 4 million years ago and so on. But the endocranial fossil evidence is only available from about 1.8 million years ago, and is incredibly rare, the authors explain.

Long story short, without going into the minutiae of analyzing frontal lobe organization in endocasts of fossils, the team concluded that the Dmanisi crowd had ape-like structures where it counts, and all the Homo erectus from southeast Asia had a more modern-like structure.

Ergo the advanced structure had to arise after the Dmanisi residents left Africa. However, these structural innovations in the cerebral regions, thought to allow for many of humans’ unique behaviors and abilities, were probably in place by 1.5 million to 1.7 million years ago, the authors say.

The secrets of Broca’s cap

In an accompanying article in Science, Amélie Beaudet of the University of Cambridge explains a nuance about brain evolution and the part of the frontal lobe known as Broca’s area or Broca’s cap. It generally can’t be seen in casts but its condition can be inferred by other brain structures, Beaudet says.

This Broca area is involved in language processing, though in keeping with the enigma that is our brain, it isn’t the only part of the brain involved in language processing. In fact, studies of brain tumors indicate that if the brain is impaired, it still manages; language processing just gets handled by other areas. As hinted above, there is much we don’t understand about the brain.

Early Homo skull found in DmanisiCredit: M. Ponce de Leon and Ch. Zollikofer, University of Zurich

But it can be said that our Broca’s cap and that of our cousins the chimps and bonobos are different. (Signing gorillas cannot reasonably be said to have acquired language.) “Chimpanzees and bonobos have one distinct furrow in that region …. This is absent in humans, who instead have two vertical furrows,” Beaudet writes.

Ergo if a brain cast indicates a chimp-like Broca’s cap, the assumption is that the brain is primitive: The “single-furrow condition” is interpreted as representing the ancestral condition, she writes.

The Dmanisi specimens show the unique furrow, which is the primitive condition, Beaudet told Haaretz. The Homo erectus, based on the inference of the casts, had a modern organization of the lobe.

Also, more “recent” human species such as Homo naledi also show the human-like configuration, the double furrow, of the Broca’s cap, she adds.

She also notes that the Broca area is involved in tool-making, and that all this begs the question: What kind of selection pressure may have been responsible for the reorganization of the human frontal lobes? Good question.

A set of studies at Tel Aviv University provide food for thought. Work by Prof. Ran Barkai, Miki Ben-Dor and others posits that in contrast to whatever gauzy notions we entertain about the evolution of our appetite, from about 2 million years ago – note the timeline well – the Homo line was a super-predator. We hunted mega-fauna to death, crucially because we craved their fat for calorie content – we can only digest so much lean meat before poisoning ourselves, they contend.

And as we drove our dinner extinct, we had to develop capabilities and technologies to hunt down smaller, fleeter animals. We may also have needed increasingly to communicate in order to strategize the hunt for fast food. Right now this is speculative, but the truth is, it adds up.