Our brain is a greedy organ: it devours a large part of the body's fuel on its own. Yet, the first members of our lineage managed with a chimpanzee-sized brain! How did it grow so much, why, and what does it cost us from birth? Explanations.
Our brain is a greedy organ: it devours a large part of the body's fuel on its own. Yet, the first members of our lineage managed with a chimpanzee-sized brain! How did it grow so much, why, and what does it cost us from birth? Explanations.
Place your hand on your forehead: just behind it lies a brain averaging about 1,350 cm³. This figure is far from obvious: for most of prehistory, the skulls of the human lineage contained much less.
On the scale, it weighs almost nothing: about 2% of an adult's weight. When it comes to energy expenditure, it's a whole different story! At rest, it consumes nearly one-fifth of what the body uses. Much more than its share!
Why did evolution bet on such a greedy organ? To understand this, paleoanthropologists (the specialists in human origins) measure the interiors of fossil skulls, scrutinize teeth and broken bones, examine chipped stones, and even read ancient DNA.
Here, we recount the major stages of this growth, the explanations put forward by researchers, and how it has changed our way of being born and growing up.
DNA allows us to trace back far: comparing our genes with those of chimpanzees indicates that our paths diverged at least seven million years ago. The very first representatives of our branch (specialists call them hominins) all emerged from African soil: Sahelanthropus comes from Chad, Orrorin from Kenya, and Ardipithecus from Ethiopia.
What connects them to us? A more or less pronounced bipedalism and reduced canines. However, beneath the skull, nothing new: about 360 cm³ in Sahelanthropus and slightly less in Ardipithecus. These volumes would not surprise anyone in a modern chimpanzee.
Australopithecines do not change the game immediately. Take anamensis, the oldest known australopithecine by a skull: it arrives long after Sahelanthropus, yet its cranial capacity is hardly more spacious! The real gain comes later with afarensis and africanus: their brains are about a third larger than that of an ordinary chimpanzee. The same goes for their cousins with enormous molars, the Paranthropes.
This increase does not change the architecture: the cortex of an australopithecine is organized like that of a chimpanzee, much to the dismay of the early readings of the famous child of Taung. These distant relatives move upright with legs extended but cannot run for long. They still climb trees and primarily feed on plants. A small sign of the future: the hand of africanus was capable of a strong pinch between the thumb and index finger, an ideal grip for holding a tool.
Did you know that stone was being shaped even before the appearance of the Homo genus? The stones thus crafted belong to the Lomekwian and date back 3.3 million years. Their site is located in Kenya, on the western shore of Lake Turkana.
Their production remains rudimentary: a fist-sized block of hard rock was struck against a stone anvil to chip off pieces. Who were these artisans? No one knows: likely an australopithecine or a yet-unknown close cousin.
Next comes the Oldowan, whose earliest traces, about 2.6 million years old, have been found in Ethiopia, in the great Afar depression. The toolmakers there produced sharp flakes from quartzite or volcanic rock. Between these two dates, an incomplete lower jaw dating back 2.8 million years represents the first known evidence of the Homo genus.
Be careful not to jump to conclusions too quickly: these tools alone do not prove that the brain grew because of them. However, they provide access to new food sources. And this is where the story becomes interesting.
About three million years ago, temperatures dropped and rainfall became scarce. Hominins had to adapt, and two strategies emerged: Paranthropes relied on tough plants that they ground with gigantic molars, while the genus Homo turned to meat.
Eating animal flesh is not unusual among primates: baboons consume it, and chimpanzees even hunt colobus monkeys in groups. The novelty on the human side? Targeting large prey and bringing the carcasses back to a specific location.
Initially, our ancestors were probably more scavengers than hunters. They fed on carcasses left by the big cats of the savannah and then broke the bones with a stone to extract the marrow: a concentrate of fat, hence energy. Our bodies even bear the trace of this relationship: our tapeworms (the flatworms) are most closely related to those that infest hyenas and big cats!
The bones marked by stone tools confirm this scenario. Those from Dikika in Ethiopia, dated to 3.4 million years ago, are believed to be the oldest. However, it must be established that these marks were indeed made by a hand, which not everyone agrees on. Until about two million years ago, these marked bones remained extremely rare: on a scale across all of Africa, the collection would barely fill a shoebox! After that, they multiplied, to the point that a single Kenyan site (Kanjera) yielded hundreds.
Let's jump back two million years before us. The African continent is home to several types of humans at the same time, which specialists categorize under the names habilis, rudolfensis, and ergaster. Their brains range from 500 to 900 cm³ according to fossils. So forget the image of a line of descent where a monkey gradually stands upright to become a man: our history resembles more of a bush, most of whose branches have gone extinct.
It is also during this time that Homo erectus appears. This newcomer is the first human truly comfortable in open terrain, far from forests, and he soon leaves his native continent.
His great asset? Endurance. His bare skin and abundant sweating allow him to run for long periods in the heat without overheating. The San hunters of southern Africa still give us an idea of this today: they can follow an antelope until it collapses, overwhelmed by heatstroke.
Researchers have even calculated for the present-day Kalahari: a human can track their prey for hours, without a sip of water, before dehydration becomes critical. A formidable weapon... for a primate that has neither claws nor fangs.
This runner also equips himself with more refined tools. Around 1.8 million years ago, a new way of shaping (the Acheulean) gave rise to more elaborate tools, crafted on both sides: bifaces and hand axes, quite handy for butchering an animal. Their shape then evolves with astonishing slowness. And wood? It does not withstand the test of time well. The oldest spears that have come down to us (about 300,000 years old, from Schöningen in Germany) surely do not mark their invention: our ancestors likely wielded wooden weapons much earlier.
Why would the brain have grown in this context? According to some specialists, by becoming a predator deprived of natural weapons, humans had to compensate with intelligence, tool-making, cooperation... and sharing meat.
The comparison with chimpanzees supports this idea. Observations conducted at Gombe show this well: for them, hunting remains primarily a male affair, meat is never essential, and it circulates little. In hunter-gatherer societies, it is the opposite: the distribution of game follows a precise code, and no one could do without it to survive.
The question of task division remains. Among the Hadza (a people of hunter-gatherers still living in Tanzania), work is clearly divided: it is mostly women who dig up tubers while men go out to hunt game. But does this division date back to prehistory? Specialists are still debating this.
At first, there is nothing extraordinary about erectus: its skull remains modest and the surface of its brain retains an archaic pattern. However, in the last representatives of the species, the volume exceeds 1,200 cm³. But initially, this increase mainly accompanies that of the body: a larger organism logically carries a bigger head.
When the Paranthropes go extinct in turn, our family has only one representative left on the planet (for a time at least). Its descendants then split into several lineages at the pace of glaciations, not excluding interbreeding between them. Their lineage bears a name derived from Heidelberg, a German city where an old jawbone was unearthed: Homo heidelbergensis. From this trunk emerge two well-known branches: Neanderthal on one side and on the other the Denisovans, Asian cousins known from simple fragments found in the Altai and on the Tibetan plateau.
Over the last 500,000 years, these humans reach record volumes: some late Neanderthals exceed 1,700 cm³, well beyond our current average! However, their bodies have not grown. The brain thus occupies an increasing space relative to body mass (this is what the encephalization quotient measures) and the energy expenditure rises with it.
History, however, does not always go in the same direction! Let's head to Indonesia, on the island of Flores. Deep in a cave, excavators uncovered a miniature human dubbed Homo floresiensis. Its cranial capacity was only 380 cm3, roughly the same as that of the very first hominins. Initially, tools and hearths found on site were attributed to it, but that was a mistake: once its remains were better dated, this link collapsed. In fact, the earliest datings made it much too young: its last traces actually date back over 50,000 years.
The most likely explanation has a name: insular dwarfism. When food becomes scarce, as on a piece of land isolated by the sea, animals eventually change in size. Thus, we have seen the emergence of dwarf elephants, stegodons (relatives of elephants) reduced to the height of a small horse, or conversely, giant rats on islands. In such an environment, a large brain becomes an unattainable luxury. In the Philippines, another small human named Homo luzonensis may have followed the same path.
Homo naledi, in South Africa, is even more intriguing: dated to about 300,000 years ago, it combines a small brain, a modest size, climbing hands, and very human feet. Some researchers believe it occupied a unique ecological niche, somewhat like the okapi (a relative of the giraffe that remained unknown to naturalists for a long time).
Let's make way for our own species, which appeared on the scene about 300,000 years ago. Its elders were unearthed in Morocco. The site is called Jebel Irhoud, and the faces found there would almost seem familiar to us. Since then, the volume of the brain has curiously changed little. Sapiens took a different, more economical path: reorganizing rather than enlarging.
Specifically, our brain has become rounder (this is referred to as globularization) with pronounced bumps on the top and sides of the skull: the parietal bumps. As for the cerebellum, this small folded mass nestled at the back of the skull, it occupies an unmatched relative space among all hominins. However, it does not only manage the coordination of movements: it is involved in speech (which can produce up to four sounds per second), attention, reward circuits, and our relationships with others.
This rounded shaping occurs in infants during the first twelve months. The little Neanderthal did not go through this. He too had gained volume at the front of the brain, but his skull retained an elongated, wide, and low silhouette.
Genes retain the trace of these differences. The FOXP2 gene, linked to language, carries the same mutation in us as in Neanderthals: the difference would therefore lie less in the gene itself than in how it is regulated. Certain portions of our DNA have even been cleared of any Neanderthal heritage (as if natural selection had tidied up), particularly around genes that serve the brain. And Neanderthal variants located near the UBR4 and PHLPP1 genes make the brain... less round!
A human newborn enters the world with about a quarter of the brain volume it will have when fully grown. This is very little: a baby chimpanzee starts with about 40% of its brain volume, and a newborn macaque has already made most of the journey. For us, most of the work happens after birth.
As a result, once out of the womb, our brain quadruples in volume while that of the chimpanzee grows less and stops earlier. It's a real construction site!
Biologists have a term for this: humans are a secondary altricial species. Translation: a litter most often reduced to a single baby, who can see and hear from its first hours but will remain unable to fend for itself for a long time. Compare this to a young wildebeest, which can run on its first day... The human infant, on the other hand, will need many years of attention before it can fly on its own.
The first explanation is mechanical. The anthropologist Washburn summarized it under the name of the obstetrical dilemma: to walk well on two legs, a narrow pelvis is needed, but to give birth to a baby with a large head, the opposite would be required. Current specialists nuance this: what primarily limits the passage is bipedalism and the pelvic floor (the muscles that support the organs of the lower abdomen).
Consequence: a complicated childbirth. To emerge, the baby must flex its head and rotate a quarter turn. This ordeal has long been formidable: in 18th century France, about one in twenty births cost the mother her life, and cesarean sections remained almost always fatal until the end of the following century.
The second explanation is energetic: no mother could carry in her womb a fetus whose brain was already nearly mature. The human placenta, however, does its utmost. Very invasive, it draws without restraint to nourish the developing brain. The downside: delivery exposes the mother to hemorrhage.
In humans, growth spans about twenty years and follows a specific order: the brain first, then the baby teeth replaced by permanent teeth, and finally the body. The body grows slowly during childhood before experiencing a significant growth spurt in adolescence.
Why such a spread? Probably because the developing brain takes priority in the energy budget. Right after birth, it already consumes a large portion of the baby's basal metabolism, and this share continues to rise: by around five years old, it approaches 70%! The rest of the body has to wait its turn.
Some specialists compare this strategy to a mortgage: repayments are spread out over a very long time. But to sustain this duration, a stable environment and a group capable of producing more energy than it consumes are necessary to support these children who are dependent for so long.
In toddlers, it is primarily the white matter that causes the brain to swell. This corresponds to the nerve fibers surrounded by myelin, a fatty sheath that accelerates the transmission of nerve impulses (somewhat like the insulation around an electrical cable).
At the same time, the baby produces an abundance of connections (synapses, which link neurons together), and then the brain sorts through them and eliminates those that are less useful. Hence the windows of plasticity: vision adjusts during a period that closes around the age of seven, and a foreign language can be easily learned until adolescence.
In chimpanzees, myelination stops around the age of twelve, whereas in humans it continues into young adulthood. This creates a loop: a long childhood allows for more complex thinking, which in turn enables the formation of groups capable of raising children who are dependent for a long time. And to establish this timeline, a few mutations were sufficient, particularly in the genes that regulate the duration and speed of brain development.
How can we know about the childhood of an individual who died hundreds of thousands of years ago? Thanks to their teeth! Unlike bone, enamel does not renew itself: it retains a record of the early years of life.
The principle is quite simple. Enamel is deposited in layers marked by fine lines (the Retzius striae) that appear roughly every eight or nine days in hominins (with a slightly tighter rhythm in great apes). In the first molar, a more pronounced line indicates the shock of birth: from this marker, the striae are counted like the rings of a tree to date each stage of growth.
The results are telling. In two afarensis infants, maturation is already lagging, even though their brains remained below that of a chimpanzee of their age. In the early erectus, the tempo is still halfway between that of anthropoid apes and ours: the skeleton of Nariokotome (Kenya) had adolescent bones while its teeth indicate a death around eight years old.
At the Mousterian site in France, a young Neanderthal died while his last molars were erupting, indicating a shorter adolescence than ours. As for the little sapiens from Jebel Irhoud, his teeth grew at the same pace as those of a current European schoolchild.
Our brain did not come out of nowhere. It was built piece by piece over an immense period of time, and every cubic centimeter gained came at an energetic cost. To summarize, several ingredients would have played together:
a richer diet, with meat and marrow obtained through stone toolsendurance running and hunting, which made us clawless predatorscooperation and sharing, essential to nourish such an expensive organan early birth followed by a very long childhood, during which the brain completes its constructionfinally, in Homo sapiens, a reorganization rather than an enlargement.
We also carry within us the memory of our extinct cousins: Eurasians carry just over 2% Neanderthal DNA, and some populations in Oceania have up to 6% Denisovan DNA. According to some researchers, what distinguished us from them was primarily social behavior, much more than technology.
Want to go further? During your next visit to a natural history museum, compare the casts of skulls: you will quickly notice the round shape of ours compared to the elongated skull of Neanderthal. And if you are a parent, see the slowness of childhood for what it is: the price of our intelligence. Take the opportunity to introduce your children to foreign languages before adolescence, while their brains are so well suited for it!
Author Audrey on 19 September 2022
Health and Beauty : Medicine
Author Audrey on 11 May 2023
Health and Beauty : Medicine