Human Neoteny Revisited : The Case of Synaptic Plasticity

The process of learning requires morphological changes in the neuronal connections and the formation of new synapses. The key mechanism by which different forms of memory are encoded, processed and stored in the brain is modulated depending on the activity, strength and structure of specific synaptic connections. Long-term memory requires changes in genetic expression, which induce the growth of new synaptic connections.

Due to the importance of memory and learning in our species, some authors had suggested that the synaptic plasticity in a number of association areas is higher in the human brain than in the other primates.

Cortical neurons in mammals are characterized by higher metabolism and synaptic plasticity and activity during development and the juvenile stage than in the adult.

In Homo sapiens, brain development is retarded compared with other primates, especially in some association areas, and the rate of brain growth in humans in the postnatal period is much higher than the rate of cerebral growth in non human primates, including chimpanzees. The prolongation of cerebral growth and fetal growth rates during infancy and childhood explains the large brain size in Homo sapiens.

Some association areas in the human brain are characterized by the presence of neurons, which remain structurally immature throughout their lifespan and show an increase in the expression of the genes which deal with metabolism and the activity and synaptic plasticity in the adulthood. High degrees of synaptic plasticity and activity in association areas of the adult human brain seem to have been retained through an increase in the expression of certain genes, many of which are related to cerebral development and, to a lesser extent, through the positive selection of certain genetic variants. Adult human beings are similar to juvenile chimpanzees in terms of their cerebral genetic expression profiles. Neotenic changes only affected a limited group of genes expressed in the brain, which suggests mosaic evolution.

Prefrontal cortex

Aerobic glycolisis in the adult human brain, which seems to be related to an increase in synaptic activity and plasticity, is significantly elevated in certain areas of the cortex of association which have undergone considerable modification during the evolution of human species, as the dorsolateral prefrontal cortex, which is associated with working memory, and a group of areas that make up the brain’s default mode network, which display elevated activity when the individual is at rest and is related to autobiographical memory, planning, and functions related to social interaction and navigation, such as theory of the mind and moral decision making.

Neurons belonging to certain areas of the human cerebral cortex exhibit a higher metabolism and a higher degree of synaptic plasticity and activity in adulthood than the cortical neurons of other mammals. So it appears that human neurons belonging to particular association areas retain juvenile characteristics throughout adulthood, which suggests that a neuronal neoteny has occurred in Homo sapiens, which allows the human brain to function, to a certain degree, like a juvenile brain during adult life.

The increase in the aerobic metabolism in these neurons may lead, however, to higher levels of oxidative stress, therefore favouring the development of neurodegenerative diseases which are exclusive, or almost exclusive, to humans, such as frontal dementia and Alzheimer’s disease, which may be, in part, the result of the retention of juvenile characteristics in adulthood in neurons associated with learning, memory and other complex cognitive functions, and the price our species pay for our elevated cognitive capacity, our longevity and our advanced social intelligence.

For further information:
Bufill, E., Agustí, J., Blesa, R., 2011. "Human Neoteny Revisited: The Case of Synaptic Plasticity". American Journal of Human Biology. 23: 729-739-284.

Contact
Enric Bufill
ebufill@telefonica.net

Continuity of the first human occupation in the Iberian Peninsula: Closing the archaeological gap

The recent discovery of a site at Vallparadís (Terrassa), dated to the upper boundary of the Jaramillo sub-chron (0.98 Ma), allows us to close the archaeological gap of the Late Lower Pleistocene in the Iberian Peninsula—between Orce and Sima del Elefante (1.4-1.3 to 1.2 Ma), on the one hand, and TD6 (0.78 Ma), on the other—and to propose the hypothesis that Western Mediterranean Europe may have been continuously inhabited by humans from 1.4-1.2 Ma until the early Middle Pleistocene. The Iberian Peninsula was home to a stable human settlement throughout the time range in question. This hypothesis would seem to be supported by the number of sites already discovered, their chronology, their ecosystem diversity and, especially, the similar technological and adaptive characteristics they represent.

Fig. 1. Geographic location of the Lower Pleistocene archaeological sites of the Iberian Peninsula and summary of the lithostratigraphy and chronology of the Vallparadís site. IPHES

Taking into account that the lithic technology, adaptive strategies and even biological characteristics of the hominids (in the case of Atapuerca) have certain characteristics in common at Sima del Elefante, Orce, Vallparadís and TD6, we conclude that the first groups of hominids in Europe were capable of successfully withstanding the different climatic conditions that they encountered, thereby ensuring the continuity of human settlement in Europe throughout the Lower Pleistocene. They were able to do this thanks to certain adaptive strategies based on a Mode 1 technological capacity and group social cohesion. These strategies enabled them to eat meat by gaining primary access to herbivore carcasses and successfully compete with large carnivores (e.g. Panthera gombaszoegensis and Pachycrocuta brevirostris at Vallparadís). These first hominids succeeded in hoisting themselves to the top of the food chain, and in doing so guaranteed the continuity of human settlement.

For further information:

Garcia, J., Martínez, K., Carbonell, E., 2011. "Continuity of the first human occupation in the Iberian Peninsula: Closing the archaeological gap". C.R. Palevol. 10 (4), 279-284.

Contact

Joan Garcia
jgarcia@prehistoria.urv.cat

Kenneth Martínez
kmartinez@prehistoria.urv.cat

More than 500,000 years of right-handedness in Europe

Considerable research supports the high frequency of right-handedness in living Homo sapiens, with worldwide rates of approximately nine rights-for every one left- hander. Right-handedness appears to be a uniquely human trait, as no other primate species, no matter how proficient in tool use, shows frequencies even close to the strong right bias typical of humans.

Here we review our research on human fossils from Sima de los Huesos (Atapuerca, Spain) and their likely descendants, the European Neandertals. We document hand preference in fossils by scratch patterns that occur on the labial (lip) face of incisors and canines, and contend that these patterns provide a reliable means for identifying predominant hand use in these samples. Manipulatory marks on the anterior teeth show a persistent pattern of right-handedness actions, implying that the modern human pattern of dominant right-handedness extends deep into the European past.

Right-handedness has long thought to be associated with left cerebral dominance and language. Usually, language is left lateralized in brains of right-and left-handers and in both sexes. Many neurologist and palaeoneurologists accept the relationship between language, lateralization, and handedness (Falk, 1987; Holloway, 1976; Knecht et al., 2000). Discovery of the FOXP2 gene sequence in two male El Sidron Neandertals dated before 50,000 years ago (Krause et al., 2007) is consistent with the existence of language between neandertals. And the most likely evolutionary scenario is that the common ancestor of living humans and neandertals hared the modern sequence and had the elemental protein associated with language production. If correlations between handedness and laterality hold, evidence from Sima de los Huesos now pushes modern language capacity to greater than 500,000 years ago.

Article

Frayer, D. W.; Lozano, M.; Bermúdez de Castro, J.M.; Carbonell, E.; Arsuaga, J.L.: Radovcic, J.; Fiore, I.; Bondioli, L. (2011) “More than 500,000 years of right-handedness in Europe. Laterality: Asymmetries of Body, Brain and Cognition”. Laterality, First published on: 14 April 2011 (iFirst)

Contact

Marina Lozano

mlozano@prehistoria.urv.cat

What novice knappers have to learn to become expert stone toolmakers

The development of human cognitive capabilities is one of the most interesting and complex topics in human evolution. Cognitive archaeology focuses on stone tools to give insight into it. Stone tools are linked to cognition because they are the product of a conscious goal-oriented action: stone knapping. In this sense, the aim of our research was to determine what technical gestures must be learned to produce stone tools. Knowing what the technical requirements of stone knapping are, we believe it is a first step towards understanding what its cognitive requirements are and, so, what the cognitive abilities of our ancestors were. To contribute to this first step, in this article we focused in the essential technical gestures of stone knapping.

Mean values (number of percussions) for the angle of percussion displayed by experts and novices - IPHES

Stone tools are the protagonist of cognitive archaeology, but they only retain information about the last stage of their production and use. To understand the cognitive aspects of stone tool production, we need to study the entire knapping process and, so we must turn to modern-day knappers. That is why we designed an experimental study to compare the performance of expert knappers and complete novices. We hypothesised that the differences between novices and experts would indicate which technical gestures experts had learned to successfully produce stone tools.

Mean values (number of percussions) for the percussion support displayed by experts and novices - IPHES

Nine experts and nine novices participated in our experiment. They were video recorded while attempting to produce a crude handaxe on a brick. The production of a handaxe was chosen because its reduction sequence is long and diverse enough to generate rich-knapping behaviours, while being easy enough for novices to achieve. Concerning the bricks, three reasons justified this choice: they have the same mechanical properties (conchoidal fracture) as stones; they provide a homogeneous raw material and a standardised core form for all subjects; and they are a safe material for complete novices to handle. Homogeneity of the raw material was important for the aim of the experiment because we wanted to determine the knapping variables regardless of raw material type. Through the use of bricks we neutralised the effects of raw material variability.

Mean values (number of percussions) for the position of the blank displayed by experts and novices - IPHES

Concerning the methodology of analysis, the video recordings were analysed with observational methods, which are used in Psychology and Ethology to study human and animal behaviour, respectively. In fact, the innovation of our study, apart from the results, is the application of the observational methods to archaeology. Observational methods allow segmenting any behaviour or action into units and, so, identifying which are the atomic elements of this action. In our research, the atomic elements are the technical gestures of stone knapping.

Looking at results, they showed considerable differences between experts and novices in three technical gestures: the type of percussion support, the position of the blank and the angle of percussion. Regarding the percussion support, experts used some part of their body, such as their tight, to support the blank while knapping, whereas novices put the blank on the ground or on an anvil. With regard to the position of the blank, experts tilted the blank while knapping it, whereas novices had it on a vertical position. Finally, experts knapped with an angle of percussion lower than 90º; on the contrary, novices knapped with an angle of percussion around 180º, that is to say, they hit the whole surface of the hammerstone against the blank.

Therefore, the technical gestures that have to be learned to successfully knap stone are the angle of percussion, the type of percussion support and the position of the blank. As explained by many experienced knappers, the angle of percussion must be acute (i.e., less than 90º), the blank must be supported on the body and finally, the blank must be more or less tilted. What is critical to learn is not each technical gesture individually, but the interaction between the three of them. Although on some occasions novices used an acute angle of percussion, the support on the body technique and the titled position, they did not manage to combine them.

In conclusion, these three technical gestures seem to be important variables in understanding stone knapping, both at the level of successful flaking and shaping. In this sense, if we reveal the challenges and difficulties a modern apprentice faces in acquiring, combining and mastering the three of them, we may begin to understand how stone knapping is acquired and what were the cognitive abilities of both the first stone knappers and the early bifacial stone toolmakers.

For further information

Article “What novice knappers have to learn to become expert stone toolmakers”. Geribàs, N.; Mosquera, M.; Vergès, J.M. Journal of Archaeological Science, 37: 2857-2870, November 2010

Contact

Núria Geribàs

nuria.geribas@gmail.com

Dietary change and horse evolution in North America

Paleontologists have long believed that evolutionary changes in horse teeth were caused by natural selection due to changes in diet that were caused by climate change and the appearance of these habitats. The evolution of horse dentitions remains one of the major hypotheses linking the evolution of animals to climate change. To test the hypothesis that dental evolution in horses was driven by changing diets, we investigated the wear patterns on the molars of fossil horses throughout their entire history, by recording the shape of worn molar cusps in than 6,500 fossil horses representing 222 different populations representing over 70 extinct horse species. Different diets cause the teeth of horses and other herbivores to wear differently, making it possible to reconstruct diets in extinct species by examining patterns of wear on fossil teeth. This methodology, known as dental mesowear analysis, was first developed by Nikos Solounias and Mikael Fortelius to interpret the diets of extinct species. Measuring the mesowear (sharpness of worn cusps on fossil teeth) is a way of interpreting the abrasive nature of the diets of extinct horses.

According to the dental wear patterns, we were able to verify that horse diets did indeed change along with changes in vegetation through the 55 Ma sequence.

Trend in the dietary evolution of horses in the context of climatic changes and the phylogeny of Equidae - AAAS

Grasslands began to spread in North America in the early part of the Miocene because abundant grass phytoliths are found in ancient soils from that time. We observed a shift in the mesowear patterns of horses at the same time and low abrasion diets temporarily disappeared between 22–18 Ma, suggesting that horses were pioneering these new grasslands, and adopted abrasive grassy diets. We also found that evolutionary changes in tooth anatomy lag behind the dietary changes by a million years or more. For instance, about 18 million years ago, horses with taller and more complex molars begin to appear, several million years after diets began to shift. The lag time in the evolution of horse teeth in comparison to dietary changes is critical in supporting the classic hypothesis that horse teeth evolved due to natural selection caused by changing diets. The observation that dental changes follow dietary changes is consistent with evolution due to adaptation.

Abrasion

The study revealed that most species of horses showed tremendous variation in their diets, with many populations having relatively non-abrasive diets. This discovery suggests that natural selection for dental change may have been weak or non-existent much of the time, but with punctuated intervals of intense natural selection caused by extreme dietary abrasion. The mesowear data also show that low abrasion diets began to return, after a temporary absence, about 17 Ma, and that diets of horses greatly diversified after this time, the same interval of time during which the number of horses species reached their maximum. Many horses with archaic dentitions began to gradually disappear in the late Miocene along with evidence of low-abrasion diets and by 2 Ma not only most horses had gone extinct, but the mesowear indicates that dietary diversity of this group had been greatly diminished. By 2 Ma, the only remaining horses in North America exclusively utilized high-abrasion grazing diets, like the living horses or zebras. This study shows that not only has the number of horse species been greatly reduced in the last few million years, but the diets of horses have been narrowly restricted as well. The mesowear data indicate that the living horses are anything but typical examples of the dietary ecology of this group of mammals.

For further information

Article "Dietary change and evolution of horses in North America" Mihlbachler; M.C; Rivals, F. et al. Science, 331:1178-1181, March 2011

Contact

Florent Rivals

florent.rivals@icrea.es