Comparative Anatomy Discussion Question 1.
The Aquatic Ape Theory, first proposed by Alister Hardy from
bipedalism,
loss of body hair
Decended larynx
subcutaneous fat
ventro-ventral copulation
inferiorly projected external nares (nostrils)
extensive sweating and salt excretion
the diving response (slowed pulse with immersion under water)
voluntary control of breathing
hydrodynamically positioned remaining body hair and the finding of hominid fossils near bodies of water.
Do you agree with the
Aquatic Ape Theory? What comparative anatomical evidence would you use to
support or refute some
(or all) of the above lines of evidence?
Aquatic Ape
Theory Arguments
Bipedalism
loss of body hair
Descended larynx
subcutaneous fat
ventro-ventral copulation
inferiorly projected external nares (nostrils)
extensive sweating and salt excretion
the diving response (slowed pulse with immersion under water)
voluntary control of breathing
hydrodynamically positioned remaining body hair and the finding of hominid fossils near
bodies of water.
Bipedalism-
Aquatic Ape Theory-
Compared to Quadrupedal
Posture and Gait, Bipedalism causes
Lower back muscle pain
Slipped discs
Locomotor
constraints on pregnant women
Excessive acetabular
wear (hip socket)
Grinding and loss of the
femur head
Knee problems
Ankle problems
Sore Feet
Unstable and unbalanced
Slower gait than quadrupeds
Increased risk of herniation of abdominal region
Costs are too great
Other primates assume bipedal
locomotion in water
Elongated lower limbs increase
swimming effectiveness
Arguments
against Savanna Theory:
Humans forced to walk simulating joint
angles of chimpanzees, orangutangs, and gorillas have
energy expenditures of at least twice that of a normal gait
Chimps that walk with a bipedal
posture exert nearly twice as much energy as if they
knuckle walk
Slower gait would increase predation
risk
No other savanna primate has adopted a
bipedal type of locomotion
Primary
Adaptive Advantage-
Decreased chance of drowning
Standing on bottom uses less energy
than swimming
Allows greater exploitation of fish
and other protein resources to feed a large brain
Bipedalism
Savanna Theory
No evidence that immediate ancestors of hominids were quadrupeds.
Therefore, any energetic comparisons
between quadrupedal mammals (primates) and humans is
a dubious one.
Our shoulder rotation is an arboreal
adaptation as is forearm length.
Citing the number of quadrupedal primates on the
Babboons all have quadrupedal locomotion because
their immediate ancestor had quadrupedal locomotion.
Thus any baboon exhibits this locomotory pattern for the same reason. Morgan
transposes an adaptive explanation for each quadrupedal
pattern in babboons with a decidedly more
parsimonious explanation---an evolutionary one.
The
proper measure of energetic efficiency for bipedalism
has not been done--and may not be able to be done.
Examine
costs of bipedal locomotion in water versus swimming (walking in water takes
more energy than swimming if the human is in waist deep water or higher.
There
are several examples of bipedal animals that are much faster than quadrupedal counterparts. Kangaroos and Ostriches are
extraordinarily fast.
Hominids
All fossil evidence suggests that our
most recent pre-hominid ancestors were arboreal or semi-arboreal brachiators and bole-climbers.
Brachiating as a form of locomotion is
three to four times more energetically expensive than bipedal locomotion (and
also explains why your average chimp or orangutang
can rip a human limb out of it's socket with little
effort)
Limb
length is directly related to locomotor efficiency in
bipeds as well as quadrupeds in terrestrial environments.
Although
hind limb length does facilitate increased swimming speeds, it does little
without modifications of the propulsive appendages (i.e. fins, feet with
webbing etc.).
Although
Morgan pokes at the fact that only humans have taken up a bipedal existence on
the savannah. On purely logical grounds, one may make the same claim against
the aquatic ape theory. Why have only humans gone bipedal of those mammals that
have become semi-aquatic?
Advantage
Reduced
Heat load
Ground temperature may be 20-40
degrees warmer than air just 3-4 feet above the ground
Compatible with body hair reduction
Compatible with increased amount of
sweat glands
But--Bipedalism may have preceded living on a savannah.
Few
Loss of Body Hair
Aquatic Ape Theory
Reduced body hair is also present in
most marine mammals as well as in hippos and other semi-aquatic mammals.
No other primate has as great a loss
in body hair as humans and physical anthropologists have been slow to offer an
explanation
Streamlined advantage-decreases
surface friction while swimming.
Naked animals in the
savannah more prone to sunburn and skin cancers than hairy counterparts.
Sweating
allows for excretion of excess salts from the body (marine environment)?
Humans
can quickly dehydrate from excessive salt and water excretion through sweating.
Must have abundant access to water.
Examples
of hairlessness
Loss of Body Hair
Hypothesis 1.
Hair loss is a general trend with increasing body size
(doesn't
explain why gorillas are hairier than humans then?)
Doesn't explain fur seals,
otters, beavers, or muskrats either does it?)
Non-adaptive hypothesis-
Doesn't explain micro-hairs
in humans
Sweating is an effective
means of cooling. Humans have lots of sweat glands relative to most other
mammals. Sweating is not efficient and is minimally, superfluous in aquatic environments.
Also, our sweating is determined by heat load and so is directly related to
body temperature.
Why do humans, like most other
terrestrial mammals, crave salt if we were in a marine/aquatic environment? We
should never crave salt if we live near or in marine environments.
Descended Larynx-
Both theories--
Most
non-mammals and many mammals have nearly completely separate tubes from the
nose to the back of the throat with little areas for overlap between the two.
A
descended larynx is intensely maladaptive for many reasons.
1. Increased choking hazard
while eating
2. Increased incidence of crib
death (SIDS) during the period when the larynx descends during infancy
3. Increased chance to
asphyxiate on vomit if in prone position
Results
of Descended larynx
Better control and regulation between
mouth breathing and nose breathing
Better ability to modulate
vocalizations (increased number of sounds possible)
Also, more ability to pass
air during exhalation (
Unfortunately,
these same anatomical and physiological adaptations work for two main
adaptations-which was the primary adaptation and which was the pre-adaptation?
Aquatic breath-holding
Taking deep breaths through the mouth
and with high volume
Expelling air quickly through the
mouth
Voluntary control and timing breaths
for diving adaptive
Speech
Human are the only animals that time
exhalations through the mouth by sentence length.
Expelling air quickly through the mouth allows an
increased opportunity to create more diversified sounds by manipulating buccal sphincter, buccal cavity,
and tongue during exhalation
Descended larynx allows one to modulate pitch (high
and low) which also offers diversification and precise control of sounds--a
prerequisite for language.
Also, our larynx position is more like that of
chimpanzees than any aquatic animal. In fact, given the extreme relocation of
the nasopharynx--on top of the head and concommittant extreme specialization of the ethmoid bone, conchae, turbinates, and soft palate, one can make little direct
comparison.
Bipedal locomotion ALLOWS for breath control.
Respiratory
valves-
Aquatic Theory
The
human soft palate can elevate and close off the nasopharynx,
unlike most other animals. This would be adaptive in keeping water out via the
nose. Also young infants and some older
humans can seal their nostrils externally, unlike other primates
The
soft palate can only be seal off the nasopharynx when
the tongue is pressed on the soft palate (as when making a nasal N sound). The
principle muscles that could be be
used for sealing the nose--which they do poorly, are mostly used extensively
for controlling facial expression and thus serve primarily in communication.
Subcutaneous Fat
Aquatic Theory
Human babies start with much more subcutaneous fat than other primates,
but similar to patterns seen in more aquatic animals. Offers
insulation in water and buoyancy.
Subcutaneous fat is usually bypassed
by blood vessels when the body is trying to shed internally produced heat. This
phenomenon can be most readily observed by feeling areas of your body with high
subcutaneous fat during heavy exercise and notice that they often feel cool to
the touch.
The overall pattern of subcutaneous
fat is the same in humans as it is for any terrestrial mammal especially
primates. Only humans have more body fat at birth and maintain it for a year
due to fat-rich breast milk.
Fat is an energy store and protective
covering not just a form of insulation. One must consider ALL the functions of
fat. Abdominal fat protects underlying viscera. In an aquatic environment, it
should be more diffuse over the body than it is.
Human
babies have much higher metabolic needs than similar sized mammals because of
brain size which is very expensive metabolically to maintain. Fat serves as a
local energy depot.
ventro-ventral copulation
Aquatic
Theory-
Common in aquatic mammals. Not common in terrestrial ones except humans and bonobos.
Note
that vaginal depth is greater in marine mammals as protection from saltwater.
Male copulatory organs in humans is disproportionately large
relative to other primates.
Aquatic
waterfowl and reptiles have intromittent organs
lacking in their terrestrial relatives.
Ventro-ventral copulation is easily facilitated by being bipedal. This is not
the proximate cause in aquatic mammals.
Human
increased vaginal depth is a secondary consequence of being by bipedal (i.e.
the distance from the pelvic brim to the external genitalia in a female is
necessarily greater in a bipedal stance.
inferiorly projected external nares (nostrils)
Humans
have unique noses
Aquatic
nose is adaptive for keeping water from moving up it unlike the laterally
placed or anteriorly directed nares
of other primates.
The
Proboscis monkey has a large similarly positioned nose and is also
semi-aquatic. (Also true of the semi-aquatic tapir).
Like
many of Morgan's ideas, she fails to take into account either sex-based or
ethnically based differences in traits. The human nostrils are variously
positioned. Her perception only accounts for Western European noses.
Our
faces have become flattened muzzles relative to most other terrestrial animals.
Consequently the usual function of a muzzle other than olfaction,
is compromised---warming and moistening air, capturing foreign particles.
Having a projecting nose allows compensation.
The
proboscis monkey nose is likely to be a sexually selected trait. Males have
disproportionately large noses and females chose males based, in part, on the
size of their nose.
Jimmy
Durante and Karl Malden would be sex symbols if this
were true with humans though.
Activity of Sebaceous Glands
Aquatic Theory
Oil
glands are prevalent in aquatic animals to protect the fur, insulate, and
waterproof, but have little function in terrestrial animals.
Function
in pheromone production, defensive glands but these would be washed away in an
aquatic environment.
the diving response (slowed pulse with immersion under water)
Various
marine mammals have been found to have adapted special abilities which help in
their respiratory processes, enabling them to remain down at great depths for
long periods of time. The Weddell seal posseses some
amazing abilities. It only stores 5% of its oxygen in its lungs, and keeps the
remaining 70% of its oxygen circulating throughout the blood stream. Humans are
only able to keep a small 51% of their oxygen circulating throughout the blood
stream, while 36% of the oxygen is stored in the lungs. The explanation for
this is that the Weddell seal has approximately twice the volume of blood per
kilogram as humans. As well, the Weddell seal's spleen has the ability to store
up to 24L of blood. It is believed that when the seal dives the spleen contracts
causing the stored oxygen enriched blood to enter the blood stream. Also, these
seals have a higher concentration of a certain protein found within the muscles
known as myoglobin, which stores oxygen. The Weddell seal contains 25% of its oxygen in the muscles,
while humans only keep about 12% of their oxygen within the muscles.
The
diving reflex is one of the life saving reflexes that vertebrates invoke when
their oxygen supply is being jeopardized. It usually features 1)
breath-holding, 2) slowing of the heartrate, 3)
decrease of blood supply to the extremities and 4) gradual rise in mean
arterial blood pressure [1,3]. Such defenses against
asphyxia are seen in many different vertebrates under various circumstances.
For instance fish taken out of the water, newborn kittens and human baby's with birth anoxia [2].
A
classical diving reflex is found in water mammals such as seals,
that display a decrease in heart-rate of 80-85% during a deep dive
[reference in 5].
Human
beings also display a diving reflex. Under laboratory conditions it can be
triggered by cold stimulation of the face. In exercise, the decrease in
heart-rate can be as high as 40% [7]. Sir Alistar
Hardy (quote) c[ould] not believe that [the diving reflex] could
have been evolved by natural selection unless man had taken to diving under
water some considerable period of his past history[11]. However, also other
terrestrial animals have a diving reflex:
Tchobroutsy et al. [8] found in rabbits
that head immersion (with free access to air) in water of different
temperatures (6-40 C) caused breath arrest and slowing of heart-rate. Newborn
lambs reacted with breath arrest and a slowing of the heart rate of 59%. Adult
sheep reacted variable, usually with partial breath arrest and a mean drop in heart
rate of about 39%.
Grogaard et al. [9] placed an ice-cold towel against a lamb's snout that
resulted in a mean drop in heart-rate of 24%, quite comparable to the heart
rate drop that is usually seen in human beings in rest (15-30%)[6].
Also
dogs show a slowing of the heart rate when their larynx is stimulated by water
or electrical pulses [4], although this may rather represent a laryngeal
chemo-reflex (LCR), an anti-choke reflex, than a trigeminal diving reflex
(TDR).
In
conclusion, the diving reflex argument for the AAT hypothesis can be ignored
until comparative studies are done between humans and their nearest relatives.
A diving related reflex however, does provide an argument against the AAT: most diving animals exhale reflexly
on submergion. Man inhales [10].
1.
Background: what is a diving response?
The diving response
The diving response is well known from diving mammals
such as seals and is considered to prolong diving by restricting oxygen
consumption in tissues resistant to asphyxia. Thereby, oxygen is reserved for
the most sensitive organs, the heart and brain. The response has two major
features: a restricted blood flow to certain organs and a lowered heart rate.
Our studies have focused on the following questions. Do humans have a diving
response? How is this response triggered? How does it work? What is the
adaptive value of the response? How does it compare with the response of other
mammals? Thus, the general field of interest within our group is cardiovascular
and respiratory regulation and function.
Do tropical divers have a
diving response?
In several studies, it has been found that the temperature of the water is an
important determinant of the magnitude of the diving response triggered. Water
holding a temperature of about 10 degrees Centigrade has often been found most
efficient. This has lead to the conclusion that the diving response will not be
efficiently triggered in a tropical diver. Yet human diving is most likely to
occur in warm water!
In experiments using subjects acclimated to different ambient temperatures, we
have found that both water temperature and ambient air temperature have
significant, but opposite, effects on the magnitude of bradycardia
developed during apneic face immersion. These results
indicate that the diving response is negatively correlated to water temperature
within a range that is determined by the ambient air temperature. This means
that, in the range of temperatures most likely to be encountered by breath hold divers, the warmer the ambient air and the
colder the water, the stronger the diving response will be.
Thus, within a range of temperatures, the difference in temperature between air
and water seems to be more important than the temperatures per se. This information should encourage the diver to keep his
body warm and, thereby, keep the peripheral blood flow high between dives
permitting a fast recovery of the face skin temperature. The mechanisms most
likely to be responsible for this temperature effect were identified as 1) the
relative vasoconstriction in cold ambient air and 2) the involvement of the
dynamic cold receptor response in initiating the diving response.
3.
Does the human diving response conserve oxygen?
There
are several possible mechanisms for reducing the rate of oxygen consumption
during apnea, e.g. selective vasoconstriction, causing local reduction in
oxygen consumption in tolerant tissues, and the bradycardia
in itself, leading to a reduction of the demand of oxygen by cardiac muscle.
The oxygen conserving effect of the human diving response is a matter of
debate. Some studies do assign such an effect to the response, while others do
not.
We
investigated the diving response and apneic time in
nine groups of divers and non-divers. A positive correlation was found between the
diving response and apneic time, with the trained
divers showing the most pronounced diving response and the longest apneic time. Apneas were done both in air and with face
immersion. Trained divers with a powerful diving response were found to prolong
their maximal apneic time in water, when their diving
response was most efficiently triggered.
We
also compared apneas with a given duration, with and without face immersion,
with respect to arterial hemoglobin oxygen saturation. We found that the
arterial hemoglobin was more saturated after apneas with face immersion, when
the diving response was more pronounced. During apneas without face immersion,
when the diving response was not fully developed, more oxygen had apparently
been used. Thus, our data favor the view that an efficient, oxygen conserving
diving response is present in man, as in diving mammals.
4.
Training studies
Short term training by
repeated apneas
In both diving mammals and naturally diving humans,
dives are performed in series with the duration of apneas and surface intervals
adapted to the intended working depth. In humans, repeated apneas with short
(less than 10 min) intervals have been shown to prolong apneic
time. The mechanisms causing this "short term training effect" are
poorly understood. Previously, it has been suggested that the increased apneic time with repeated apneas is caused by an increased
diving response, a progressive hyperventilation or an increased inspired lung
volume throughout the series.
In our studies, we have measured the duration of the period before the
physiological breaking point. At the physiological breaking point, involuntary
breathing movements are triggered by a high arterial carbon dioxide tension. We
have found that both physiological and psychological factors contribute to the
prolongation of apneas during short term training. Physiological factors
contribute during the first three apneas, whereas psychological tolerance
contributes until apneas 5 to 7. In our studies, neither an increased diving response,
nor an increased inspired lung volume, could explain the physiological
contribution to the increased apneic time during
repeated apneas. Moreover, hyperventilation alone was not found to be
responsible. In ongoing projects, the physiological factors involved in the
short term training effect are further studied.
Long term training
Why do trained divers have such a powerful diving
response? Is this trait genetically determined or an effect of their daily
training? If the trait is caused by training, what part of the training is
responsible? To answer these questions, several studies concerning the effects
of general and specific training regimes on the diving response were made in
our laboratory. Longitudinal effects of general physical training and apnea
training were studied in different groups of subjects. We found that physical
training, leading to an increased maximal oxygen uptake and decreased resting
heart rate, does not increase the diving bradycardia during simulated diving. The time period before
the physiological breaking point, e.g. before involuntary breathing movements
are triggered, was also unchanged after physical training. However, the apneic duration was prolonged by an increased duration of
the phase after the physiological breaking point. This indicates that the
psychological tolerance to apnea appears to be enhanced after physical
training.
Apnea training, on the other hand, was found to increase the diving response
and prolong apneic time by postponing the
physiological breaking point. This indicates that factors associated with the
production or accumulation of carbon dioxide are
affected by apnea training or that the response to a given stimulus by the chemoreceptors is altered. The results suggest that apnea
training may be an essential factor in breath-hold diving training. After apneic training, the dive becomes not only longer, but also
easier!
5.
The "diving response" in pigs
Knowledge
of the diving response in mammals has, to a large extent, been obtained from
studies of diving mammals, mostly seals. It is interesting to evaluate the
human diving response in a mammalian perspective, especially compared with that
of terrestrial mammals. The response has been shown to be present also in dogs
and rats. Studies in our laboratory have shown that it is possible to train
pigs to voluntarily perform apneas and snout immersion, and that they respond
with a diving response involving a reduction of their heart rate and skin blood
flow. In humans, the magnitude of the diving bradycardia
varies between 15-30 % for untrained persons, which corresponds to the response
in these trained pigs, and between 30-50 % for trained divers, which is
comparable to the response found in many semi-aquatic mammals. Thus, among
humans, non-divers react like pigs, while apneic
divers respond like beavers!
The finding of hominid
fossils near bodies of water.
Comparative
Anatomical Evidence is equivocal, but fossil evidence may ultimately answer the
question.
Australopithecines
were said to be the ape that returned to land by Elaine Morgan.
This
means that they should share all the features of an aquatic ancestry, but
instead, many of their anatomical details are even more closely related to that
of chimpanzees than to ourselves.
This
means that an aquatic phase must have been more recent somewhere between 2 and
4 million years ago. At this time, we find Homo fossils everywhere--not just
around lakes or aquatic habitats.
The
Anyone
who dredges a century of hypotheses can find many to ridicule---in any science.
But
many of her arguments have already been rejected.
The
savannah setting has been largely disregarded over the last 10 years in favor
of a woodland or mosaic habitat for early australopithecines.
The
strongest hypotheses have predictive value--
Surmising
an aquatic phase to primate ancestors does not predict the evolution of the
human form.
Does
not explain inconsistencies any better than the
(e.g. The Savannah model doesn't account for why only humans
have reduced body hair among
Logical
Fallacies
Valid Deduction
Aquatic
mammals have reduced body hair.
Humans
are aquatic mammals.
Humans
have reduced body hair.
Invalid Deduction
Aquatic
mammals have reduced body hair.
Humans
have reduced body hair.
Humans
are aquatic mammals.
Other fallacies
If
one scientific hypothesis is demonstrated to be false, then it strengthens the
validity of an alternative hypothesis.