PARIS — In 2017, paleoanthropologist Jean-Jacques Hublin, now a professor at the Collège de France, shook up his discipline by pushing back our species’ origins by 100,000 years.
While the emergence of Homo sapiens had previously been dated at around 200,000 years B.P. (Before Present), Hublin and his team discovered 300,000-year-old Homo sapiens fossils at Jebel Irhoud, in present-day Morocco: our first known direct ancestors.
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Back at that time, no fewer than six different forms of humanity shared the Earth. Across the vastness of Eurasia, Neanderthals ranged from Western Europe to Central Asia, and Denisovans from the Altai Mountains in Mongolia to the far reaches of Asia. Neanderthals and Denisovans, along with the archaic Homo sapiens of North Africa, formed the three dominant forms, both in terms of geographic range and population.
The other three, confined to particular ecological niches, were reduced models of humans: the tiny Homo naledi, confined to the extreme south of Africa, and two equally lilliputian island species, Flores Man, in the Indonesian archipelago, and Luzon Man, in the Philippines.
Of these six, only Homo sapiens has survived to the present day.
But while the Homo sapiens of Jebel Irhoud were undoubtedly related to our species in certain key respects (dentition, facial morphology), their skulls, and subsequently their brains, still differed greatly from our own. It’s this evolution that Hublin recounts in his exciting latest book, The Tyranny of the Brain.
“Of all human organs, the brain has evolved the most over the past 300,000 years,” say the paleoanthropologist in his office at the Collège de France, which has a breathtaking view of the newly restored Notre Dame Cathedral. Hublin explains that Neanderthal, Denisova and Homo Sapiens, the three great terminal forms of humanity between which the evolutionary match was to be played, were all big-brained.
Back 300,000 years ago, the Hominins — the term used to describe all species related to man since the separation of this branch from that of the chimpanzees — had already been engaged in a slow process of increasing brain size.
Selection pressure
“Selection pressure was exerted in the same way everywhere: As social organization and technology both became more complex, greater cognitive capacities became necessary, enabling even greater social and technological complexity as a feedback effect,” Hublin explains. “Up until 300,000 years B.P., increasing the overall volume of the brain was the main way evolution met this adaptive challenge.”
Housing an ever-larger brain in an ever-larger cranium comes at a cost.
But with the appearance of the first Homo sapiens, a new direction was taken. Transforming rather than continuing to grow — that’s how it could be summed up. From an evolutionary point of view, housing an ever-larger brain in an ever-larger cranium comes at a cost. And an enormous one at that, making the risk-benefit trade-off uncertain.
With an encephalization quotient of 7.5 (which means a brain 7.5 times larger than that of average mammals such as dogs and cats, where this quotient is close to 1), modern man is flirting with the limits. Our brain, which represents only 2% of our body weight, consumes 20% of our total basal metabolic rate, or 10 times more than the average for the rest of our organs.
And the cranial cavity required to house such a monster is not without serious problems either. “Just think that, in 18th-century France, one birth in 20 ended in the mother’s death,” Hublin says.
A bifurcation
A bifurcation occurred, and the Homo sapiens brain became increasingly distinct from that of their Neanderthal cousins, which was longer, wider and proportionately smaller. The Homo sapiens brain reconfigured itself, changing shape to that of a globe.
It’s an evolutionary solution to the energy challenge of a very large brain.
Homo sapiens babies are born with brains that do not differ significantly in shape from those of neanderthals. But during the first year of life, they (and only they) undergo the “globularization” typical of our species. This is the first stage in a cerebral development process that began in utero but continues long afterwards.
It takes between six and seven years after birth for the brain of a small Homo sapiens, quadrupling in volume, to reach adult size, compared with three to four years for a small chimpanzee — an evolutionary solution to the energy challenge of a very large brain.
But above all, once adult size has been reached, the work is anything but finished. Windows of plasticity will continue to open periodically until adolescence, and it won’t be until the age of around 20 that a human brain will have completed its very long maturation process.
Cerebellum focus
If, beyond this generalization, we look in detail at which regions or structures of the Homo sapiens brain have been most transformed by the changes of the last 300 millennia, we see that three areas have been the main focus: the prefrontal areas, the parietal areas and, less expected, the cerebellum.
We now know that this “little brain” at the back of the big one contributes to the fine coordination and synchronization of movements. But aren’t the latter an indispensable prerequisite in a species, such as ours, in which speech has taken on such importance?
This circuit and neurotransmitter play a key role in social interactions.
“Articulate language, in which we produce up to four different sound elements every second, requires extreme agility and perfect coordination of the lips, tongue and vocal cords,” Hublin says. Another prerequisite is a low larynx, which was the case in both Neanderthal and Homo sapiens, but not in our distant australopithecine ancestors.
Furthermore, it has been demonstrated that the cerebellum is connected to numerous cortical areas, in particular those managing the reward circuit and dopamine production. A circuit and a neurotransmitter that play a key role in social interactions: when we interact with others, it’s the production of dopamine that makes us feel pleasure in this interaction.
And if there is one and only one “human characteristic,” it certainly lies in this word: cooperation.
In this respect, Hominins are a cut above all non-human primates, and Homo sapiens has taken this tendency to the extreme: It’s natural for us to cooperate with (and feel solidarity with) fellow creatures we’ve never seen and never will — sometimes for the better, sometimes for the worse.
