Freethought & Rationalism Archive

scigirl · 11-05-2002, 03:39 PM

Well I learned a few intriguing things in class today in regards to human anatomy and evolution:

1. We have a connection between the venous blood of our nasal cavity and our brain. This is usually not a good thing, since it allows infections of our nose/eyes/mouth to spread up into our brains and get menigitis.

So why do we have this connection? Well, animals that have to run a long ways (such as antelope) use this connection (called the cavernous sinus) to cool their blood so their brain doesn't fry every time they sprint away from a predator.

So our blood supply in our head is sub-optimal because we evolved from such creatures.

From my Moore's anatomy book, Clinically Oriented Anatomy:

Quote:

The facial veins make clinically important connections with the cavernous sinus through the superior othamlic veins. Cavernous sinues thrombosis usually resuts from infections in the orbit, nasal sinuses, and superior part of the face. In patients with thrombophlebitis of the facial vein, pieces of an infected clot may extend into the cavernous sinus, producing thrombophlebitis of the cavernous sinus...Septic thrombosis of the cavernous sinus often results in the develpment of acute meningitis.

2. Our maxillary sinuses drain into the superior aspect of our middle meatus (in our nose). This is an inefficent way to drain the head if you are an upright primate. However, if you are one of our ancestors, that walked on four legs and had a different head orientation, this drainage system is optimal.

Why is this important? Again, from Moore's:

Quote:

The maxillary sinuses are the most commonly infected, probably because their ostia are located high on their superomedial walls, a poor location for natural drainage of the sinus. When the mucous membrane of hte sinus is congested, the maxillary ostia are often obstructed. Becuase of the high location of the ostia, when the head is erect it is impossible for the sinues to drain until their are full. Because the ostia of the right and left sinuses lie on the medial sides (i.e., are directed toward each other), only the uppoer ostium drains when lying on one's side...The maxillary sinuses can be cannulated and drained by passing a cannula from the nostril through the maxillary ostium into the sinus.

Sounds like adaptation from a 4-legged animal, with a different head orientation to me.

3. Our ability to talk, which is clearly an evolutionary advantage for our species (at least it used to be, before we invented talk shows

) is not designed as optimally as it could be. In order for us to have a "talking" larynx, our respiratory system had to be compromised.

Chimpanzees (as well as human babies) can breathe and swallow simultaneously, because the epiglottis extends all the way to the soft palate. As the larynx grows longer, however, (which it has to do to allow us to talk), the epiglottis moves inferior. Thus, we can't breathe and swallow at the same time. In addition, this jury-rigged design makes it easy for us to aspirate or choke, and die.

scigirl

scigirl · 11-05-2002, 04:10 PM

Here are some relevant pubmed articles:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=160531 9&dopt=Abstract" target="_blank">Homology and evolution of the orbitotemporal venous sinuses of humans.</a>

Quote:

The orbitotemporal venous sinuses accompany the intracranial branches of the stapedial artery. These sinuses are large in primitive primates and drain the extensive territories supplied by the stapedial artery as well as the brain. The orbit is drained by a wide cranio-orbital sinus which empties into the postglenoid emissary vein. Also emptying into the postglenoid vein is the petrosquamous sinus. The latter diverts cerebral blood from the transverse sinus and also drains the temporalis muscle. Emptying into both the cranio-orbital and petrosquamous sinuses are meningeal tributaries, which drain the cranial side wall and the dura mater. The relatively small sinus communicans runs in the angle between the petrosal bone and the cranial side wall. It commences at the postglenoid vein and connects the distal end of the petrosquamous sinus to the pterygoid venous plexus. In humans, the orbitotemporal sinus system is greatly modified. Its remnants persist for the most part as "middle meningeal veins." The system no longer drains the orbit, the temporal fossa, or the brain. The petrosquamous sinus becomes attenuated or obliterated along part or all of its length. The postglenoid vein vanishes. The cranio-orbital sinus is reduced in diameter and its connection to the orbit is feeble or absent. During development, the posterior end of the cranio-orbital sinus migrates inferiorly along the sinus communicans. In most individuals, this migration ceases at the foramen spinosum, site of the emissary vein of the sinus communicans. Meningeal tributaries are relatively large in humans, and drain principally into the cranio-orbital sinus or sphenoparietal sinus. The sphenoparietal sinus is an evolutionary novelty restricted to hominoids and is frequently developed in only Homo and Pongo.

Some interesting evolutionary theories about paranasal sinuses:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=156437 8&dopt=Abstract" target="_blank">The paranasal sinuses and other enigmas: an aquatic evolutionary theory.</a>

Quote:

The functional role of the paranasal sinuses in man has long been in dispute and as yet no satisfactory explanation has been offered for these 'unwanted' spaces. An answer may be found by study of the comparative evolutionary development of the sinuses in man and other higher primates. Several unique physical characteristics of man not seen elsewhere in the ape family, or indeed in other terrestrial mammals, including some relating to the upper aerodigestive tract, are not satisfactorily explained by the traditionally held theory of evolutionary development of early man directly from the arboreal ape. It is argued that these developmental differences are much more logically explained by a period of aquatic adaptation at a crucial period in the evolution of pre-hominid man. A new theory is proposed which might explain the importance of the sinus air cavities as buoyancy aids for protection of the upper airway tract in such an aquatic environment. Further evidence is offered relating to a pathological condition of the external ear canal which supports this theory that man at some stage in his early development acquired an affinity for an aquatic environment. Explanation of these unique hominid characteristics in terms of an aquatic evolutionary theory may help to resolve some of the enigmatic inconsistencies between man and other higher primates, and may account for man's eventual emergence as the dominant extant species, and perhaps an explanation for the 'missing link'.

And one about the evolution of the pituitary gland (refuting Behe's common claim that our bodies are "irreducibly complex")

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=881682 3&dopt=Abstract" target="_blank">Two new forms of gonadotropin-releasing hormone in a protochordate and the evolutionary implications.</a>

Quote:

The neuropeptide gonadotropin-releasing hormone (GnRH) is the major regulator of reproduction in vertebrates. Our goal was to determine whether GnRH could be isolated and identified by primary structure in a protochordate and to examine its location by immunocytochemistry. The primary structure of two novel decapeptides from the tunicate Chelyosoma productum (class Ascidiacea) was determined. Both show significant identity with vertebrate GnRH. Tunicate GnRH-I (pGlu-His-Trp-Ser-Asp-Tyr-Phe-Lys-Pro-Gly-NH2) has 60% of its residues conserved, compared with mammalian GnRH, whereas tunicate GnRH-II (pGlu-His-Trp-Ser-Leu-Cys-His-Ala-Pro-Gly-NH2) is unusual in that it was isolated as a disulfide-linked dimer. Numerous immunoreactive GnRH neurons lie within blood sinuses close to the gonoducts and gonads in both juveniles and adults, implying that the neuropeptide is released into the bloodstream. It is suggested that in ancestral chordates, before the evolution of the pituitary, the hormone was released into the bloodstream and acted directly on the gonads.

Stuff on the larynx:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=841246 2&dopt=Abstract" target="_blank">The vertebrate larynx: adaptations and aberrations.</a>

Quote:

The complex anatomy of the vertebrate larynx shows a steady progression from the simple slit on the floor of the lungfish's pharynx to the fine-tuned mechanism of the human vocal apparatus. The frog's larynx acts as a check valve to prevent collapse of the lungs during a dive, since the animal has no rib cage. The crocodile's laryngeal framework has acquired an epiglottic analogue which fits snugly into the nasopharynx and protects the lower respiratory tract from inundation while the animal drowns its prey. The snake's larynx lies intraorally and can be extended beyond the lower teeth while the reptile leisurely swallows its prey intact. The mammal has acquired a cricothyroid joint, allowing its membranous vocal folds to be stretched during phonation. In Homo sapiens, vocal performance has reached its highest degree of versatility, with a vocal fold capable of adjustment in length, tension and shape. In the course of organic evolution, man appears to have chosen the ability to speak and sing over the security that an intranarial epiglottis would have given him.

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=115066 79&dopt=Abstract" target="_blank">The descended larynx is not uniquely human.</a>

Quote:

Morphological modifications of vocal anatomy are widespread among vertebrates, and the investigation of the physiological mechanisms and adaptive functions of such variants is an important focus of research into the evolution of communication. The "descended larynx" of adult humans has traditionally been considered unique to our species, representing an adaptation for articulate speech, and debate concerning the position of the larynx in extinct hominids assumes that a lowered larynx is diagnostic of speech and language. Here, we use bioacoustic analyses of vocalizing animals, together with anatomical analyses of functional morphology, to document descended larynges in red and fallow deer. The resting position of the larynx in males of these species is similar to that in humans, and, during roaring, red-deer stags lower the larynx even further, to the sternum. These findings indicate that laryngeal descent is not uniquely human and has evolved at least twice in independent lineages. We suggest that laryngeal descent serves to elongate the vocal tract, allowing callers to exaggerate their perceived body size by decreasing vocal-tract resonant frequencies. Vocal-tract elongation is common in birds and is probably present in additional mammals. Size exaggeration provides a non-linguistic alternative hypothesis for the descent of the larynx in human evolution.

And a clincher for the sub-optimal design argument for our larynx:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=942261 5&dopt=Abstract" target="_blank">The human aerodigestive tract and gastroesophageal reflux: an evolutionary perspective.</a>

Quote:

In order to appreciate fully the nature of supraesophageal complications of gastroesophageal reflux in humans, it is essential to view the problem within an evolutionary framework. Examination of the aerodigestive tract anatomy of our mammalian relatives shows that this region in humans is highly derived as compared to other mammals. Among the specializations that adult humans exhibit is a caudal position of the larynx, which results in a permanently expanded oropharynx. These anatomical features underlie our distinctive breathing and swallowing patterns and provide the substrate that allows for the production of articulate speech. While the selection factors that have shaped human evolution obviously favored our derived aerodigestive tract, aspects of this anatomy appear particularly unsuited to accommodate gastroesophageal reflux. Indeed, our unique aerodigestive tract morphology may predispose us to an array of supraesophageal complications of gastroesophageal reflux.

scigirl

[ November 05, 2002: Message edited by: scigirl ]</p>

Bubba · 11-05-2002, 06:05 PM

Excellent. What other scientific theory has as much supporting evidence as evolution?

Bubba <img src="graemlins/notworthy.gif" border="0" alt="[Not Worthy]" /> <img src="graemlins/notworthy.gif" border="0" alt="[Not Worthy]" />

lpetrich · 11-05-2002, 06:05 PM

Some of that jargon can be difficult to follow; I gather from that last one that our larynx and throat architecture is not well-adapted for efficient vomiting.

As to size exaggeration, could that be a function of the manes of lions, certain deer, and some other species?

Doubting Didymus · 11-05-2002, 06:27 PM

Quote:

What other scientific theory has as much supporting evidence as evolution?

Certainly not gravity!

You physicists should be ashamed. Biologists have a pretty good idea about the scientific explanation for life itself, and physicists haven't even properly worked out why people fall over yet.

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11-05-2002, 06:05 PM	#3
Bubba Veteran Member Join Date: Mar 2001 Location: Orient, OH USA Posts: 1,501	Excellent. What other scientific theory has as much supporting evidence as evolution? Bubba <img src="graemlins/notworthy.gif" border="0" alt="[Not Worthy]" /> <img src="graemlins/notworthy.gif" border="0" alt="[Not Worthy]" />

11-05-2002, 06:05 PM	#4
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Some of that jargon can be difficult to follow; I gather from that last one that our larynx and throat architecture is not well-adapted for efficient vomiting. As to size exaggeration, could that be a function of the manes of lions, certain deer, and some other species?

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