In 1871, English naturalist Charles Darwin published The Descent of Man, and Selection in Relation to Sex as a follow up to his renowned 1859 work on evolutionary theory, The Origin of Species. In the introduction of The Descent of Man, Darwin states that “the homological structure, embryological development, and rudimentary organs of a species, whether it be man or any other animal, to which our attention may be directed, remain to be considered; but these great classes of facts afford, as it appears to me, ample and conclusive evidence in favor of the principle of gradual evolution” (1). Although at the time Darwin’s reinforcement of the idea of human evolution from a primate lineage was a controversial stance, over the past century the model of natural selection and gradual evolutionary theory has been solidified as a basis for evolutionary biology and physical anthropology. More recently, important aspects of human-specific evolution have been revealed. Here, four interesting landmarks in the descent of Homo sapiens are outlined and their origins investigated. These include Homo sapiens sapiens speciation, the FOXP2 gene, lactose tolerance, and childhood. Although these topics are still being explored, these recent research breakthroughs have provided further insight into the topic of human uniqueness.
Models for H. sapiens sapiens Speciation
For many years, it was assumed that extinct hominid species H. sapiens neanderthalensis, or “Neanderthal man,” had traveled a separate evolutionary path from anatomically modern humans. The extinction of Neanderthals was attributed to simple natural selection, an idea supporting the replacement evolutionary model of H. sapiens speciation, which states that modern humans originated in Africa and dispersed, replacing any other hominid species that existed in the world. The multiregional evolutionary model of H. sapiens speciation supports the hypothesis that the ancestral species H. erectus first dispersed out of Africa to different areas of the world, so that different regional populations developed down their own evolutionary path.
Yet, a more prudent model has since been developed in light of genetic evidence in the modern human population. The assimilation model provides a middle ground between the multiregional and replacement models, explaining that there may have been several dispersions out of Africa, and that regional populations most likely interbred with other existing hominid species. This hypothesis is supported by several pieces of morphological evidence, such as the discovery of a skeleton displaying both Neanderthal and anatomically modern human physical traits, the existence of the ancestral shove-shaped incisors trait in some modern Asia populations, and the overall pattern of gradual reduction in Neanderthal anatomical traits in ancestral European human populations. The most parsimonious explanation would be that Neanderthals, as well as early H. sapiens known as “Cro-Magnon Man,” shared the same territory as anatomically modern humans, and that there was considerable interbreeding between these early human populations. This would lend to a large amount gene flow, and eventually these morphologically distinct groups would become one species sharing the same gene pool. Genetic evidence, such as a high degree of regional DNA markers in the human genome, and the shared FOXP2 gene in Neanderthals and H. sapien sapiens, also paint of picture of assimilation, rather than abrupt distinction of our Neanderthal relatives (3).
Recent archeological evidence has shown that Neanderthals and humans share the same form of the FOXP2 gene.
Language and the FOXP2 Gene
Animal communication systems, such as signaling or sonar, are used by many non-human social animals. Yet, only humans have the ability to think abstractly, thus transforming signals into symbols. Grammar and syntax, which allows for the creation of an infinite array of new thoughts in the form of sentences composed of a finite vocabulary, is unique only in human language. The human capacity for language is still a major area of unknowns for evolutionary biologists. In 1996, a study of a language developmental disorder in a UK family lead to the discovery of a highly conserved human version of the gene called forkhead box P2 or FOXP2 (5). In this family and in others who suffered from the same rare disorder, scientists found that a genetic mutation in the FOXP2 gene caused severe language disabilities, both in the production and the comprehension of sentences. With comparative genetic analysis, it is known that the FOXP2 human genotype is a trait unique to humans. Although there is only a discrepancy of two amino acids in the human FOXP2 gene and the chimpanzee FOXP2 gene, this amino acid sequence has been highly conserved. It is evident that this trait developed fairly recently, within the last 200,000 years (5). However, recent archeological DNA analysis has shown that Neanderthals also possessed the human form of the FOXP2 gene. The FOXP2 gene is also associated with human morphology, mainly the lowering of the larynx and directing fine muscle development in the tongue and palate, two biological features necessary for producing the broad range of sounds associated with human speech (4). This could be key evidence for uncovering the selective pressures that lead to the conservation of the human-specific FOXP2 gene.
It is postulated that this genetic change was driven by morphological adaptation. As humans stood upright, the descent of the larynx was crucial to avoid choking during food consumption. Although the direct correlation between FOXP2 and the development of human language is still unclear, many have postulated that the genetic history of this gene is key to determining a timeline for the use of spoken language among humans. The most intriguing aspect is that this small genetic change may be associated with the large human population growth about 50,000 years ago. Agriculture and subsequent urbanization may have only been possible after the development of human language. Since our Neanderthal ancestors also possessed the same FOXP2 genotype as modern humans, this could mean that this subspecies also had the capacity for human language.
Lactose Tolerance
The ability for certain human populations to digest lactose, the sugar found in milk, is one example of the interaction between cultural and biological evolution in human genetics. Although the consumption of dairy products is an accepted norm in Western societies, there are many individuals in the world who are lactose intolerant, because they do not possess the genetic trait that triggers cells in the small intestine to produce a high level of the enzyme lactase. The earliest humans, however, did not have the genetic makeup for lactose tolerance.
With the intensification of agriculture came a spread in the domestication of animals. In ancient Europe, livestock rearing became a staple of settled civilizations, and so dairy-farming practices took hold (4). In a response to the heightened level of lactose introduced to the diets of these populations, selection favored a higher production of the lactase enzyme. Today, regional distribution for lactose tolerance in different genetic populations is correlated with the presence or absence of ancestral dairy farming. For example, individuals descendent from peoples of Britain, Germany, and Scandinavia have high lactose tolerance, possibly linked to a cultural history of drinking unprocessed milk, whereas those of southern Europe, such as in Rome, where lactose-free dairy products such as cheese were more often consumed, lactose intolerance is more prevalent (2).
The Human Childhood
The developmental cycle of an organism is a fundamental determinant in natural selection in a species. The important differences between the human life cycle and the developmental pattern of other mammals and primates serve as a template for understanding how human evolution is unique. For example, the childhood stage is an aspect uniquely present in the human life cycle. The juvenile stage, the time between infancy and sexual maturity, is longer in primates than in any other social species. However, humans also possess an additional three to four years of relatively slow physical growth that extends the juvenile period even longer (3). Human childhood, which typically lasts from ages three to seven, is a period of stagnant morphological development, but provides humans a unique opportunity for rapid mental growth. The extension of this pre-pubescent period may have allowed for greater intellectual growth during the elastic stage of development. The childhood stage gives humans a longer learning period before autonomy is reached, so that cognitive abilities such as language, reasoning, and problem-solving skills can be expanded while simple biological needs, such as food, are still being provided by parents.
Conclusion
Humans share 99.6 percent of their DNA sequence with their closest living species, the chimpanzee. Yet, it is obvious that since the speciation of H. sapiens, humans have traveled down a distinct evolutionary path. Outlined here are just several key aspects that define human uniqueness. However, it is important to note that pure genetics cannot solely account for these evolutionary landmarks. Rather, analyzing the bio-cultural forces in the course of human pre-history is the best way to understand human evolution and continue the investigation Charles Darwin began over a century ago.
References
1. C. Darwin, The Descent of Man and Selection in Relation to Sex (D. Appleton and Company, New York, 1871), vol. 1, pp. 1-8.
2. J. H. Relethford, The Human Species (McGraw-Hill, Boston, MA, ed. 7, 2008), pp. 434-435.
3. S. Stinson et al., Human Biology: an evolutionary and biocultural perspective (Wiley, New York, 2000), pp. 310-312.
4. W. Haviland et al., Evolution and Prehistory: the human challenge (Wadsworth/Thomson Learning, Belmon, CA, ed. 8, 2008), pp. 134-136.
5. J. Itzhaki, The FOXP2 story: A tale of genes, language, and human origins (The Human Genome Project, 2003; http://genome.wellcome.ac.uk/doc_wtd020797.html).