hearing

I'm doing a little literature review this week on Middle Pleisocene postcrania. On a somewhat tangential topic, the description of the Sima de los Huesos cervical vertebrae, by Gómez-Olivencia and colleagues (2007), includes a nice summary of the current knowledge of the thoracic vertebral canal of KNM-WT 15000 and other early Homo specimens.

Much attention has been devoted to vertebral-canal size and its relationship to spoken language. One factor in the evolution of human language that would be reflected in vertebral-canal morphology is increased breath control (MacLarnon, 1993, MacLarnon and Hewitt, 2004). Modern humans have an enlarged thoracic vertebral canal, reflecting a larger amount of gray matter. Based on the morphology of the KNM-WT 15000 individual, a narrower thoracic canal has been proposed for Homo ergaster, indicating that this species may only have been capable of short, unmodulated utterances, such as those used by extant nonhuman primates (MacLarnon and Hewitt, 1999). However, significant abnormalities have been found in the KNM-WT 15000 individual (Latimer and Ohman, 2001), which could indicate some form of axial dysplasia, and so the small canal may be a reflection of a neural-canal stenosis associated with the pathology. In contrast, Schiess et al. (2006) argued that the diagnosis of a congenital dysplasia is not supported, indicating that the pathological lesions in the KNM-WT 15000 individual may not be as severe as previously reported. Moreover, the Dmanisi vertebrae (Meyer, 2005 and Meyer et al., 2006), which are the oldest known for the genus Homo, follow the modern human pattern in all regions, as the raw and relative sizes of the vertebral canals fall well within the human range, indicating that these hominins may have had fine control of the respiratory muscles involved in spoken language (Meyer, 2005 and Meyer et al., 2006).

Arsuaga et al. (1997a) showed that the mean cranial capacity of SH's three most complete crania (1245 cm³) (Arsuaga et al., 1993 and Arsuaga et al., 1997c) is slightly less than that of two comparative samples from the Hamann-Todd Osteological Collection. However, given the large body-weight estimates for these hominins, their encephalization quotients are below both modern human or Neandertal values (Arsuaga et al., 1999). In Neandertals, higher encephalization quotients are reached by expansion of the cranial capacity, while in modern humans it is mainly achieved by a reduction in body mass (Arsuaga et al., 1999 and Carretero et al., 2004). In addition to the parallel trends in encephalization in these two lineages, the absolute size of the bony vertebral canal in the upper cervical spine reached modern human values by the middle Pleistocene. Preliminary studies (Carretero et al., 1999, Gómez et al., 2004 and Gómez-Olivencia, 2005) have shown that the SH lower cervical spine's canal had a similar size compared to modern humans, but a full assessment of this anatomical region will not be possible until larger sets of cervical and thoracic vertebrae are associated. In any case, as demonstrated by Martínez et al. (2004), the SH hominins had the skeletal characteristics of the outer and middle ear that support the perception of spoken language (Gómez-Olivencia et al. 2007:22).

The Meyer references are to Marc Meyer's dissertation on the Dmanisi vertebral remains and a subsequent conference presentation. I think those are more than sufficient to say that this particular piece of anatomy isn't evidence for restricted breathing control in early Homo. I don't have much more to say, just though these two paragraphs sum up a lot of information in a useful way.

References:

Gómez-Olivencia A, Carretero JM, Arsuaga JL, Rodríguez-García L, García-González R, Martínez I. 2007. Metric and morphological study of the upper cervical spine from the Sima de los Huesos site (Sierra de Atapuerca, Burgos, Spain). J Hum Evol 53:6-25. doi:10.1016/j.jhevol.2006.12.006

A couple of months ago, the Washington Post ran an article by Michael Chorost, who has written a book about his experience with a cochlear implant. I meant to link it at the time, but got it lost on a different computer. The book is titled, Rebuilt, with alternate subtitles in paperback and hardcover editions. The Post article is titled, "Confessions of a Bionic Man":

My implants don't aid my hearing. They create my hearing.

What I hear is, quite literally, a computer simulation of real sound. The day my first implant was activated in 2001, voices sounded bizarre; the radio might as well have been in Esperanto. That was because the software couldn't reproduce all the aspects of a normal auditory system. Still, I learned how to recognize consonants and vowels again by listening to books on tape. Now I can turn on the radio and hear it all but effortlessly.

In 2005, I got new software that made music sound brighter and clearer. The software's improved frequency resolution enabled me to distinguish between tones that had sounded identical before. It was a simple upload; no surgery was necessary.

Chorost also maintains a blog, discussing the themes of the book and his experiences promoting it. He provides an interesting account of his experiences conversing and interacting (sometimes uncomfortably) with deaf advocates of signing:

One burly fellow with enormous wrists introduced himself to me as having been in the classroom during a two-hour debate I had at Gallaudet last year with Dirksen Bauman’s students. That debate had the feel of history, of titanic forces clashing: the passion of the deaf community colliding with a technology that penetrates and transforms everything it meets. I’d spoken with candor. I’d said, Look, ninety-six percent of the deaf children born in this country are born to hearing parents. Offered a technology that lets their child hear, what do you think they’re going to choose? But I’d also said that sign language and the community sustained by it are precious, and that their disappearance would be a tragedy. I offered no easy answers, because I had none. Everyone was unsettled. Nothing was settled. At the end of the debate I felt worn out and anxious. Anxious, because I wondered if I had alienated them. I had wanted to build bridges, and I wondered if I had.

I happen to be reading Ray Kurzweil's book, and this article (and blog) make a more tangible example than many of the speculations that Kurzweil provides. It is sort of a best-case example, considering that a cochlear implant is intended to exploit brain areas that already exist and are tuned for interpreting auditory information. But the "upgrade" that Chorost describes is an incredible example of the way that technology can be improved once it is enabled.