The Coexistence of Neanderthal and Human Populations: The Reasoning for Human-Neanderthal Hybrid DNA

The exact time period when the modern human species (Homo sapiens) replaced the hominid Neanderthal species (Homo neanderthalensis) has been under significant scrutiny for quite some time. Recently however, researchers have found that the Neanderthal extinction was not a rapid one, and did not happen all at once. In fact, for a period of 2600-5400 years, Neanderthals and modern humans may have lived side by side. In fact, other studies suggest that there existed some interbreeding between the two groups, as 1.5-2.1% of modern non-African DNA originates from Neanderthals.

Professor Thomas Higham of the University of Oxford led the research by obtaining new radiocarbon dates for samples of bone, charcoal, and shell from 40 European archaeological sites believed to have been linked with the Neanderthal tool making industry (Mousterian) or transitional sites where tools belonged either to Neanderthals or early humans. Higham and his team of researchers used ¹⁴C accelerator mass spectrometry techniques to construct chronologies of these archaeological sites. Prior to Higham’s work, research regarding the coexistence of Homo sapiens and Neanderthal populations on Earth was incorrect be several millennia due to the difficulty of contaminated samples. These contaminated samples would contain ¹⁴C that was far younger than the sample itself. Because of this, the mass spectrometry results would give information that was not at all accurate. Higham was able to produce far more accurate results by using previously established archaeological statistical methods to confirm the accuracy of his carbon samples. When the carbon samples he used appeared to be contaminated with younger carbon, he would discard those samples and instead used samples that were statistically likely to show older carbon.

The data Higham collected confirmed prior hypothesis, which estimated the end of the Mousterian time period throughout most of Europe around the same time. The data also confirmed that the extinction of Neanderthals in the Iberian Peninsula was far later than anywhere else. However, the data was unable to confirm exactly how much later Neanderthals went extinct in the Iberian Peninsula as compared to the rest of Europe. Higham also found a connection between the appearance of modern humans to the great innovation Neanderthals experienced during the end of the Mousterian time period and shortly before the extinction of the population. Higham believes that this overlap may have led to some interaction that was responsible for such innovation, but remarks that it is not possible to confirm this at this time.

Therefore, Higham was able to definitively establish a time period in which both populations of Neanderthals and populations of modern humans existed. He was also able to confirm prior hypotheses regarding the extinction of Neanderthal populations. Further research may prove some of his other assertions, which at this time are merely speculation.

Citation: Higham, et. al. 2014. The timing and spatiotemporal patterning of Neanderthal disappearance. Nature 512, 306–309

Evolution of Humans: High Metabolic Costs of Brain Development and Slow Childhood Growth

Date Published: September 9, 2014

Source: Proceedings of the National Academy of Sciences of the United States of America

Institutions Involved: Northwestern University, Children's Hospital of Michigan, Wayne University School of Medicine, Harvard University, George Washington University, Icahn School of Medicine, and University of Illinois

Summary: A study reveals that the rate of glucose uptake by the brain peaks during childhood, not infancy as previously believed. This new finding has implications regarding the evolution of a slowed period of growth.

     The human brain is proportionally large compared to the brains of other animals. Developing and maintaining a large brain is energy intensive; previous research suggests that brain development in infancy accounts for 87 percent of the baby’s resting metabolic rate. Consequently, scientists previously believed that the rate of glucose uptake by the brain was greatest at infancy.

     A new study claims that the absolute and relative rates of glucose uptake actually peak during childhood, the age when neural metabolic processes are maximal. Researchers measured the rate of glucose uptake by using Positron Emission Tomography (PET Scan). Researchers accounted for variables by calculating the brain’s rate of glucose uptake as different percentages of daily energy requirements and resting metabolic rate. These percentages show how much of daily energy needs could be met by breaking down the glucose that the brain takes up.

     The results showed that at birth, the glucose uptake is 52.5 percent of resting metabolic rate in males and 59.8 of resting metabolic rate in females. However, at around age four-and-a-half, the rate of glucose uptake by the brain is 66.3 percent of resting metabolic rate in boys and 65.0 percent of resting metabolic rate in females. After childhood, glucose uptake rates begin to decrease. From this data, researchers suggest that peak uptake of glucose happens in childhood. The measurements reveal that the peak rate of glucose uptake by the brain happens specifically around year five of human development.

     The second idea in the study deals with human growth; in this study, human growth is measured in terms of change in height and weight. Humans grow quickly during infancy, slow down during childhood, speed up again after they reach puberty, and slow down as they exit puberty. The primary interest is childhood, and why it experiences a slowed physical growth rate.

   The data collected in this study support the hypothesis that slowed physical growth offsets the metabolic costs of increased brain development. Researchers explain that slowing childhood growth and increased brain metabolism probably coevolved. The study makes the point that the evolution of a slow childhood growth rate probably required cultural influences such as transitioning into a hunter-gatherer society and having social networks to care for young children. Brain development is an energy intensive process that is vulnerable to shortfall of resources. The transition into the hunter-gatherer society provided the calorically rich diet and the social networks helped buffer shortfall. Researchers state that the period of slowed physical growth allows the brain to develop the mental processes that give humans their cognitive ability.

     Jean Piaget, a developmental psychologist explains that mental development occurs in five stages. This research article maintains that peak glucose uptake and maximal neural metabolic processes occur at age five. According to Piaget, age five marks the midpoint of the preoperational stage. During the preoperational phase, children learn how to think symbolically, and they develop memory and imagination. These basic mental processes are the foundations for the abstract thinking and rationality that makes humans unique. Researchers conclude that slowed childhood growth evolved in order to allow the brain to develop these cognitive fundamentals.

   This research provides insight into the development of the human brain and into the evolution of human life history. By learning more about how human life history evolved, scientists can better understand the evolution and speciation of other primates and mammals.

Citation:

Kuzawa, C. K., H. T. Chugani, L. I. Grossman, L. Lipovich, O. Muzik, P. R. Hof, D. E. Wildman, C. C. Sherwood, W. R. Leonard, and N. Lange. 2014. Metabolic Costs and Evolutionary Implications of Human Brain Development. Proceedings of the National Academy of Sciences of the United States of America 111:13010-13015