When did humans become brainy?

The so-called breakthrough in human evolution could be traced to a single gene, says a new study

By Kiran N. Kumar

The long-standing question before the anthropologists for long was about when humans evolved into a different species from other Homo species or their own ancestors – Neanderthals.

A new study has found that the so-called breakthrough in human evolution that happened about 500,000 years ago could be traced to a single gene.

Neuroscientists Anneline Pinson and Wieland Huttner at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, identified the gene as “TKTL1” – that encodes a protein made when a fetus’ brain is in its initial days of developing.

Read: No, human brains didn’t shrink just 3,000 years ago: study (August 8, 2022)

This single genetic mutation in the human version of TKTL1 changed one amino acid, resulting in a protein that is different from those found in hominin ancestors, Neanderthals and non-human primates, says the study published in Science.

Till recently, the biological significance of similar sized brains in both Neanderthals and modern humans kept the question unresolved, implying something else in neurons as the differentiating factor.

Neanderthal brain in lab with CRISPR
Ever since researchers first fully sequenced a Neanderthal genome in 2010 and identified 96 amino acids or the building blocks that make up proteins, studies have been going on to identify which of these tweaked amino acids or genetic tweaks helped modern humans to outshine Neanderthals and other hominins.

Anneline Pinson, the lead author and her colleagues introduced either the modern human or the Neandertal variant of TKTL1 into the neocortex of mouse embryos and observed that basal radial glial cells increased with the modern human variant of TKTL1 but not with the Neandertal variant.

They also found that the brains of mouse embryos with the modern human TKTL1 contained more neurons.
“We found that with the Neandertal-type of amino acid in TKTL1, fewer basal radial glial cells were produced than with the modern human-type and, as a consequence, also fewer neurons,” says Pinson.

“This shows us that even though we do not know how many neurons the Neandertal brain had, we can assume that modern humans have more neurons in the frontal lobe of the brain, where TKTL1 activity is highest, than Neandertals.”

So, the Dresdon researchers have zeroed in on a single mutation in TKTL1 gene that might have led the evolution of modern humans with a greater neuron growth in brains, probably giving them a cognitive advantage over their Neanderthal cousins.

Arnold Kriegstein, a neurologist at the University of California, San Francisco, is confident that the new finding may prove crucial. “This is a surprisingly important gene,” says Kriegstein. “I think it sheds a whole new light on human evolution.”

However, neuroscientist Alysson Muotri at the University of California, San Diego, is more sceptical. He points out that different cell lines behave differently when made into organoids.

He wanted to see the ancestral version of TKTL1 tested in more human cells as the original Neanderthal genome should also be compared with modern European populations in other parts of the world.

In fact, a few months ago, another study revealed that the cohabitation of Neanderthals and modern humans continued for a long period, based on traces of Neanderthal DNA in the genome of modern humans.

“Ancient DNA caused a revolution in how we think about human evolution,” says Prof Steven Churchill of Duke University and co-author of the study published in Biology. “We know there was interbreeding.”

However, one question remained strange for scientists still. Modern Asian populations seem to have more Neanderthal DNA than modern European populations, strange since Neanderthals lived in what is now Europe.

Read: When did we become fully human? What fossils and DNA tell us about the evolution of modern intelligence (September 9, 2022)

It could have been that Neanderthals interbred with what are now modern humans as our prehistoric ancestors left Africa, but before spreading to Asia, they said.

The evidence also shows us that the Near East was an important crossroads, both geographically and in the context of human evolution, said the study based on craniofacial measurements of 13 Neanderthals, 233 prehistoric Homo sapiens, and 83 modern humans.

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