Speech, a vital form of communication, could be the precursor for the development of language. This paper will be used to assess different theories of how speech developed. This paper is not written to say that only one theory is correct, but rather should be used to provide an analysis of many papers containing theories that all could have contributed to the development of speech in humans. Speech is a multifaceted development in humans that is influenced by many anatomical and genetic factors.
A study conducted by Dr. Krause et al. demonstrated that FOXP2, a gene that is implicated in speech and language development, is present in Neanderthals. Both of the individuals that were analyzed were found in northern Spain. This study demonstrated that Neanderthals carried a FOXP2 protein that is the same in present day humans and is two nucleotides different from humans to chimpanzees which resulted in to amino acid substitutions (Krause et al., 2007). Whether or not these amino acid changes resulted in speech acquisition scientists are still investigating to help determine the functions of these amino acid changes. What these scientists speculate is that late Neanderthals along with early humans could have developed the ability to speak due to these amino acid changes. Some of the limitations of this paper is that Neanderthal DNA is hard to isolate and frequently contaminated by human DNA and lab reagents. If a sample if contaminated by human DNA, the sequence is so similar that scientists would not be able to tell if it was human DNA. Multiple controls were developed to combat these issues. It is possible that other Neanderthal-specific mutations are in FOXP2 that are not identified by scientists yet. The presence of FOXP2 in Neanderthals suggests that they may have been the first species to acquire speech.
A study conducted by Zhang et al. suggests that FOXP2 is a gene that was responsible for a hominid-specific acceleration in selection. They suggested three possible driving forces behind the accelerated evolution of human FOXP2: enhanced mutation rate, relaxed purifying selection, and positive selection. They were able to eliminate enhanced mutation rate and relaxed purifying selection which led them to state that positive selection is the most likely cause (Zhang, Webb, & Podlaha). In addition, the scientists analyzed introns 6 and 7, adjacent to exon 7, where the two amino acid substitutions occur in humans. Noncoding regions and found that FOXP2 introns had the lowest level of polymorphism out of all of the introns examined. If the lower-than-expected nucleotide diversity in FOXP2 introns is a result of a relatively recent selective sweep, then the sweep is likely to have occurred no more than 0.5 N generations ago. With N being the effective population size of humans of about 10,000 people, scientists concluded that this sweep could have occurred no more than 5,000 generations ago (about 100,000 years ago), and is within the time frame that speech likely developed, about 40,000 years ago (Zhang et al., 2002). A limitation of this study is that a different speech related gene could have mutated and been selected for in humans while FOXP2 remained unchanged. Because scientists do not know the identity of this other speech-related gene, it will be difficult to identify it/them until the entire human genome is sequenced (occured in April 2003). FOXP2 is a gene that was positively selected for and this led to the development of speech.
A study conducted by Dr. MacLarnon and Dr. Hewitt at the Roehampton Institute of London demonstrated that an increase in thoracic innervation and breathing control allowed for the development of speech in humans. Narrowest height of the vertebral canal through each vertebrae and the narrowest width of the vertebral canal through each vertebra, was measured and these two measurements were used to calculate the narrowest cross-sectional area of the vertebral canal through each vertebra. These measurements were used to see if there were any separate groupings of canal dimensions that would represent an increased or decreased level of innervation. The groups whose thoracic vertebrae were analyzed were: extant primates less humans, extant primates less hominids, and apes. The results demonstrated that the vertebral canal of early hominids was a similar size to the ones of extant nonhuman primates but were significantly smaller than modern humans (MacLarnon & Hewitt, 1999). The increased size of the thoracic canal demonstrates that local innervation was increased in Neanderthals, early modern humans, and apes. Although there are many hypotheses that could account increased innervation, the best supported is for speech. The most important muscles involved in speech are the intercostal muscles and a set of abdominal muscles. They must be finely controlled to produce the subtleties of speech, which calls for increased thoracic innervation. In addition, the control of metabolic respiration which uses the pons and medulla is functionally distinct from the control of breathing for phonation which utilizes forebrain and midbrain pathways (MacLarnon & Hewitt, 1999). A limitation of this study is that increased thoracic vertebral canals could have resulted from speech or a different human attribute, like bipedalism, or both. It is unfair to history and science to postulate that only one thing can cause another. Dr. MacLarnon and Dr. Hewitt argue that the increased neural control of breathing for phonation and increased thoracic vertebral canal size could be factors that led to the development of speech.
The ontogeny of postnatal hyoid and larynx descent, relative to the palatal plate and mandible, as a unique feature in humans that allowed for the anatomical development of quantal speech. According to the quantal theory of speech, the horizontal and vertical supralaryngeal vocal tracts (SVT) are advantageous to produce a wide range of acoustically differentiable sounds recognized as quantal speech when their ratio is 1.0 (Lieberman, McCarthy, Hiiemae & Palmer). Hyo-laryngeal descent is important in swallowing but it could also increase the chance of aspirating food or developing dysphagia. The gonion (the angle at the back of your lower jaw), hyoid, and larynx all descended gradually relative to the palatal plate and each other in an expected facial growth trajectory with the most rapid changes occurring between 9 months and 2.75 years of age (Lieberman et al., 2001). In addition, these scientists observed two phases of growth of the supralaryngeal vocal tracts, one from 0 to age 6 years of age where the SVTH/SVTV ratios changed by about 50% and in phase II, in subjects from ages 6-8 years, the ratio of SVTH to SVTV remains approximately 1:1 (Lieberman et al., 2001). This allows subjects to produce quantal speech from about the age of 6. However, better quality data is needed on the relative positioning of all structures involved in pharyngeal growth during the adolescent growth spurt after 13.75 years. Sexual dimorphism and individual growth spurt rates will have to be accounted for in order to accurately document the relative positioning of all relevant structures involved in pharyngeal growth. Postnatal descent of the hyoid and larynx and specific supralaryngeal vocal tract growth are traits unique to humans and are anatomical prerequisites for quantal speech.
Dr. Matsuo and Dr. Palmer at Johns Hopkins University studied the kinematic linkage of the tongue, jaw, and hyoid during eating and speech. They observed that the tongue and jaw movement are temporarily linked in eating but the tongue can also move independently from the jaw in eating and speaking due to its intrinsic muscles (Matsuo & Palmer, 2010). However, any movement of the lower jaw does have significant influence on tongue movement because the tongue is connected to the floor of the mouth through physical muscle connection. The hyoid bone’s role on the tongue during speech remains unclear because the spatial domain of hyoid movement was significantly different between eating and speech (Matsuo & Palmer, 2010). The hyoid bone has less influence on the tongue than during eating due to the hyoid bone’s reduced movement. The hyoid bone serves different functions during eating and speech which is shown by their significantly different spatial domains. The sample size of this study was small, sixteen subjects were tested in speech but only ten recordings were obtained due the anterior or posterior tongue markers on the tongue falling off. In addition to having better adhesive for the markers, a larger sample size may have provided stronger support for their hypothesis. The tongue, jaw, and hyoid bone all serve different roles in the production of speech, some more clearly mapped out than others.
A further research topic that would aid in documenting the variations present in FOXP2 would be to determine the function of the amino acid changes through using in vitro assays. This would allow scientists to determine the function of the amino acid and determine what function FOXP2 has without its two human mutations. Another topic that would be advantageous to research in the future would be to create a model of tongue surface motion that takes into account the impact of jaw and hyoid movements during speech and eating.
Speech is a multifaceted development in humans that is influenced by many anatomical and genetic factors. Speech is the stepping stone to language, a vital component of all of our lives. No matter how or what language we speak, we can thank our ancestors for the modifications they passed down to us that allows this characteristic to occur.