How have your skills developed over time

The evolution of the human brain

Research Report 2014 - Max Planck Institute for Evolutionary Anthropology

Max Planck Institute for Evolutionary Anthropology, Leipzig; Department of Human Evolution
The evolution of the human lineage is inextricably linked with the evolution of the brain. In a project at the Max Planck Institute for Evolutionary Anthropology, researchers are comparing the skull bones of modern humans with those of their closest living and fossil relatives. The aim is to gain knowledge about the evolutionary changes in brain development.

The brain volume of people living today is about three times that of chimpanzees. The brain volumes of our fossil ancestors, such as the species Australopithecus afarensis (known by her most famous representative "Lucy"), were comparable to those of chimpanzees living today (Fig. 1). In the last two million years in particular, there has been a dramatic increase in the size of the human brain. Discussions about the cognitive abilities of our fossil ancestors or relatives therefore mostly revolve around archaeological finds and skull volumes. However, the volume alone cannot adequately explain the outstanding capabilities of the human brain. The internal structure of the brain is more important than its size for cognitive abilities. This networking of the brain is created in the first years of life. Recent research therefore emphasizes the importance of the growth pattern in the course of child development. How and when the brain grows is crucial to the development of cognitive skills.

The first step and its consequences

To better understand the human brain, one has to look back six million years, to the time when the chimpanzee lineage separated from the lineage of human ancestors, the so-called hominins. The first stops on this journey through time to Africa have nothing to do with the brain, but with legs and hips. About six million years ago, an unusual form of locomotion for primates developed within the hominin lineage: the upright gait. Since there are only a few fossil fragments from this period, many details about this crucial step are still unclear and controversial. What is certain is that the representatives of the genus Australopithecus could walk upright 3.6 million years ago. This point in time is considered certain because the fossilized footprints of upright hominins were found in Tanzania in the 1970s. These footprints have individuals of the species Australopithecus afarensis left behind in a layer of moist volcanic ash that can be dated to exactly 3.6 million years. The evolution of the upright gait thus preceded the dramatic evolutionary expansion of the brain volume by up to four million years. This chronology of events is important because the evolutionary adaptations to walking upright have dramatically changed the skeleton. Among other things, the pelvis became narrower and thus the birth canal of the bony pelvis became smaller [1]. In the course of the evolution of the upright hominins, a baby with an increasingly larger head had to pass through the already narrowed bony birth canal at birth. The birth became an ever greater risk for mother and child and thus also an evolutionary risk for the entire species. The evolutionary solution to this dilemma was a change of strategy with dramatic consequences.

The solution to an evolutionary dilemma

There are basically two strategies not only in the case of birds, but in the entire animal kingdom: fleeing nest and stool. Nesters are dependent on the attention of their parents for different periods of time and can neither move nor feed on their own. Primates typically flee nests and are very independent after a short time. Human children, on the other hand, are nestled and thus deviate from the strategy of the primates. Even at birth, the brain of a human baby (Fig. 2A) with about 400 ml about the size of an adult chimpanzee brain. The species differences are already clear prenatally (Fig 3A): As early as the 22nd week of pregnancy, the growth rate of the brain in chimpanzees decreases [2]. In humans, the volume of the brain triples in the first few years of life (Fig 3B). Chimpanzees and other great apes also have brain growth after birth, but in humans a greater proportion of brain growth and development occurs after birth [3]. Compared to great apes, the human brain increases in volume significantly faster in the course of child development and grows over a somewhat longer period of time. In relative terms, however, this means a slowdown in human brain development. Human brains are characterized by particularly high plasticity and they mature more slowly than those of chimpanzees, for example [4, 5].

In humans, all of the nerve cells are already in place at the time of birth, but they are hardly linked to one another. The first years of life are crucial for the networking of the brain. Clinical studies have shown that in early childhood, even minor deviations in the pattern of brain development affect the structure of the brain and thus cognition and behavior. This dynamic network is the substrate for cognition and develops especially in humans under the influence of the stimuli outside the womb. The connections between different brain regions that are made in the first years of life are important for modern people for social, emotional and communication skills.

Petrified brain prints

Since brains do not petrify, with fossils one can only examine the internal imprint of the brain and its surrounding structures in the skull. First, high-resolution three-dimensional x-ray images of the skull are recorded using computed tomography (CT). Then a virtual imprint of the brain skull is created on the computer (a so-called endocast). These impressions of the inner skull capsule provide information about the size and shape of the brain (Fig. 2C). With the most modern measurement and analysis methods, it is possible to compare the changes in shape of the endocast between living and extinct species in the course of child development. This allows additional insights into the evolution of the human brain.

Brain development in Neanderthals

Whether there were differences in intellectual and social abilities between Neanderthals and modern humans is one of the big controversial topics in anthropology and archeology. Since Neanderthals and modern humans had brains of similar size, some researchers assume that the cognitive abilities of these species must also have been similar. However, some archaeological findings indicate differences in behavior between modern humans and Neanderthals. Scientists were able to show [7] that the pattern of endocranial shape changes directly after birth differs between Neanderthals and modern humans. The most important evidence for this was the fossil fragments of the skulls of two Neanderthals who died at birth or shortly afterwards. As early as 1914, a team of French archaeologists discovered the skeleton of a Neanderthal baby in the Dordogne. The petrified child bones were hardly noticed and finally forgotten. It was not until ninety years later that the lost bones were rediscovered in the warehouse of the Museum of Les Eyzies-de-Tayac-Sireuil in France. The fragile fragments were then scanned with a high-resolution ┬ÁCT device and then reconstructed on computers at the Max Planck Institute for Evolutionary Anthropology in Leipzig. The researchers applied the same procedure to the fragments of the Neanderthal baby from Mezmaiskaya in the Caucasus (Fig. 4) at [7]. At the time of birth, the face of a Neanderthal is already larger than that of a modern human baby. The well-documented differences in brain shape [8] between adult modern humans and Neanderthals do not develop until after birth. Both Neanderthals and homo sapiens have elongated skulls at birth (Fig. 2A) with brains of roughly the same size. Only in the course of the first year of life does the characteristic round skull shape develop in modern humans. Shortly after birth, the skull bones are very thin and the bony seams are still wide open (clearly visible, for example, on the fontanel). Since the bony brain capsule adapts to the expanding brain, this means that the brains of modern humans and Neanderthals grow differently from birth until the first milk teeth break through [7]. Neanderthals and modern humans thus achieve similar brain volumes in adulthood along different developmental patterns.

Modern humans differ from Neanderthals at an early stage of brain development. As soon as the milk teeth have erupted, however, the growth patterns of these two groups of people no longer differ. These developmental differences immediately after birth could have an impact on the neuronal and synaptic organization of the brain. Only recently, genetic studies have shown that modern humans differ from Neanderthals in a number of genes that are important for brain development [9, 10]. The results of the gestalt analysis could therefore help to understand the function of those genes that set us apart from Neanderthals.