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Challenging Biological Normality
I. THE PROBLEM: NATURAL NORMALITY
Is biological normality real? Is it a scientific discovery about the natural world? This question is both complex and important.
Complexity: Interconnection of Function and Environment
The ability for an individual to function depends
not only on its anatomy, but also on the environment it finds itself in. This
is true whether or not the anatomy is judged to be
· Humans build their own environments. The policy question arises: what environments should we build? Whose anatomies should we build environments to suit?
Importance: Choosing a Design Policy
Policy A: Design for Normals. Build the
environment to suit the
· Policy B: Build the environment so that everyone can function well in it.
· Policy B seems better to me, but A has many supporters because it’s cheaper and simpler. A supporter of Policy A probably believes that Normality is REAL. If Normality is not real, then Policy A is arbitrary.
Species Typical Function: A Defense of Natural Normality
· Philosopher Christopher Boorse proposed a theory of Natural Normality called Species Typical Function in 1977. Many bioethicists use this theory. It has two premises.
· Functional Uniformity: Almost all members of a species function very similarly.
· Statistical Centering of Function: The more typical an individual is, the better that individual functions. Typical body parts function well, atypical body parts function poorly.
Both premises will be challenged below using biological evidence. Part II will challenge Functional Uniformity, and Part III will challenge the Centering of Function.
II. THE FIRST CHALLENGE: FUNCTIONAL DIVERSITY
The Boorsian doctrine requires that species members be extremely similar in their functional anatomy. They are not.
Example 1: Variations in Aorta configurations.
Species members just aren’t as functionally similar as Boorse requires. Peoples’ bodies are functionally organized in different ways. We have many different modes of function. Humans (like other species) vary enormously from individual to individual.
Nevertheless, our bodies are functionally integrated – the parts fit together well. This is because our embryological development makes sure of functional hookups even when the organization is atypical.
No matter how atypical the configuration of your aorta, it’s bound to hook up to your heart on one end, and the rest of your circulatory system on the other.
The online Illustrated Encyclopedia of Human Anatomic Variation lists thousands of variants of body parts in humans.
Example 2: Bones
How many bones in the typical human body? The answer is indeterminate. Forty two separate bones are variable in their presence: they exist in some people and not in others.
Example 3: Hands
Lumbricals are the muscle-tendon connections that move the fingers. 20% of the population have an “abnormal” configuration of lumbricals in their hands. The Encyclopedia of Human Variation describes about 30 variations.
III. THE SECOND CHALLENGE:
PLASTICITY AND ACCOMMODATION
Where does all this variation come from? Many sources, including:
· NATURE:Genetic variation within a population
· NURTURE: Environmentally mediated variation, causing several kinds of
Kinds of “nurture” – non-genetic variation in body form and function
· The first kind is “mere” variation – differences between individuals because of accidents of their environment (including their maternal environment – the womb that they developed in).
· The second kind is passive adaptive variation – variation in an organism’s body that automatically adjusts it to the environment that it is living in. (Mammals grow thicker fur when they live in colder climates.)
· The third kind is active adaptive variation – variation in an animal’s body that suits it to how it acts in the world. If you exercise your muscles, they will grow.
· The point is that there are lots of causes of functional variation, and many of them lead to increased functional adaptation.
Example 1: Slijper’s Bipedal Goat:
E.J. Slijper, a Dutch morphologist, raised a goat who was born without front legs. The goat learned to hop on its hind legs. No big deal. The important thing was the amazing extent of the active adaptive modification in the goat’s body – both its skeleton and its musculature.
Slijper’s goat had one single, large “abnormality” (atypicality) – two legs instead of four. That condition, together with the goat’s persistent activity (it ran around the laboratory yard ceaselessly), led to many more atypicalities.
Slijper’s Goat Fig. 1: typical goat, Slijper’s goat, kangaroo, human
In the first picture we can see that Slijper’s goat’s hind legs were shaped more like a kangaroos’ than like a typical goat’s legs. Its spine also developed the S-shaped curve that many bipedal mammals have (and that produces back pain in many adult humans).
Slijper’s Goat Fig. 2: typical goat, Slijper’s goat, orangutan, human
The second picture shows cross sections of the thorax of four animals, with the sternum towards the bottom of the page. Notice that Slijper’s goat’s thorax is shaped much more like the other two bipeds (orangutan and human) than like the typical goat.
Slijper’s goat showed many other traits that were typical of bipeds. For example, the gluteus muscle was much better developed, and fastened to its back skeleton and muscles with a strong band of ligaments that typical goats do not possess.
Slijper’s goat unfortunately died at the age of one year old because of a freak accident, but its body was carefully studied by anatomists.
What can we learn from Slijper’s Goat?
1. One atypicality leads to another, and function improves.
2. These atypicalities are adaptive, and they were developed because of the goat’s bipedal behavior.
3. After the first one, the more atypicalities, the better the function.
4. The goat’s body was not dictated by its genes. It would have had the same atypical body whether its bipedality was genetic or environmentally caused.
Example 2: How we shape our bones
Most people think that bones are stiff and inflexible. They are not. They take their shape from how we use them. In fact our bones are shaped in the womb by our kicking and pushing. If we didn’t move our bones, they would have different shapes.
Above: The femurs (upper leg bones) of a human, a fossil hominid, and a chimpanzee. The human and hominid are both bipeds. You can tell by the angle of the femur with respect to the tibia (the shin bone). The outward bend allows the upper end of the femur to seat in the hip socket while the lower end of the femur sits flat, directly on top of the tibia. (See the next picture for how flat the femur sits.)
The chimp is a knuckle-walker, and so the bend is not needed. But how did the bend get there? Is it in our genes? No.
The bend grew at the end of the bone. It did so as we learned to walk, as infants. Proof is in the picture below:
Above: This is a series of femurs of humans of different ages. The bend in the femur increases as the human ages. That’s because she’s walking upright. The 8-11 degree angle of the femur for adult humans is adaptive for an upright walking gait.
There’s an important example here regarding disability.
How about humans who do not walk upright? Consider three examples:
1. typical upright walkers,
2. people who are paraplegic from birth and do not walk upright, and
3. people with a moderate degree of cerebral palsy, who have a distinctive gait with their legs spread wider than upright walkers. What happens?
1. the typical upright walkers develop the typical 8-11 degree femur angle.
2. the paraplegics keep the straight femur angle.
3. the people with a CP gait develop a distinct angle of the femur that is optimal for their gait – different from both paraplegics and upright walkers. The crucial point is this: The femur angle for the CP walkers is exactly the angle that would be most stable for people with their gait!
Example #3: How fishes change their fins.
Typical fishes have a top (dorsal) fin, a bottom (anal) fin, and caudal (tail) fin. Some fish lose their caudal fin or are born without one. The caudal fin is functionally important for forward motion. So when it’s missing (for whatever reason) fish tend to make do by modifying the functions of their upper and lower fins. Here are two examples, one accidental and the other congenital. Both of these fish were healthy when caught, so their abnormalities had not cost them their lives.
First, comparison of a typical and an injured croaker: its tail was probably bitten off.
Notice how the croaker’s upper and lower fin have grown around the back of its body to take on the position and function of a caudal fin. It has not only survived the loss of the tail, but adapted to it.
Second, a typical and a congenitally caudalless filefish. The caudalless filefish is less than half of the length of the typical fish, but it is still fat and healthy. Its top and bottom fins have adapted to do the job of the caudal fin.
The filefish example is exciting for another reason. It has evolutionary implications. Here is a picture of an ocean sunfish, or molamola. It’s a very unusual fish – large (up to 9 feet long) and slow, a jellyfish eater with very few predators.
The molamola is related to the filefish. Typical molas have no caudal fin at all. Instead they have a structure that seems to be derived from the upper and lower fins. Where did the molamola get its anatomy? It may well have evolved from ancestors which, for some reason, reduced their caudal fins, and accommodated for that functional problem by modifying their dorsal and anal fins to compensate.
Take-home message: adaptive accommodation contributes to evolutionary change.
IV. MORAL OF THE STORY:
NORMALITY IS A MYTH, NOT A NATURAL PROPERTY
· Lots of functional variation exists in any natural species.
· Typicality does not closely match up with well-functioning.
· Atypical anatomies often function perfectly well.
But one point is very important to recognize. In many cases, atypicality makes necessary more atypicality. We call this accommodation. (See above on adaptive accommodation.)
· An atypical person can perform at her/his best by using modes of function that are even more atypical. This is true in nature just as much as in human circumstances. Adaptive technology is exactly this phenomenon. It happens in nature also.
Conclusion: Organisms are too variable to be captured in the dichotomy of normal versus abnormal.
NOW, BACK TO POLITICS.
The intentional creation of environments that privilege the “normal” while penalizing the “abnormal” can receive no support from the facts of biology.
Topics: Fish: the Tyler and Gunter articles.
Levator Claviculae: Bergman.
Species typical function: Boorse.
Human femurs: Shefelbine.
Bergman, Ronald A.,
Adel K. Afifi, and Ryosuke Miyauchi. 1999. Illustrated Encyclopedia of
Human Anatomic Variation.
Boorse, Christopher. 1977. Health as a theoretical concept. Philosophy of Science 44: 542-73. The founder of “species typical function.”
Allen E., Dan W. Brock, Norman Daniels ,
and Daniel Wikler. 2000. From
Chance to Choice: Genetics and Justice.
Gunter, Gordon, and J. M. Ward. 1961. Some fishes that survive extreme injuries, and some aspects of tenacity of life. Copeia 4: 456-62.This was my source for the image of the caudalless croaker fish. The typical croaker was found somewhere on the internet.
Sheffelbine, S. J., C. Tardieu, and D. R. Carter . 2002. Development of the femoral bicondylar angle in hominid bipedalism. Bone 30: 765-70. This was my source for the two images about the angle of the human femur.
Slijper, E. J. 1942. Biologic-anatomical investigations on the bipedal gait and upright posture in mammals, with special reference to a little goat, born without forelegs. Proc. Koninklijke Nederlandse Akademie Van Wetenschappen 45: 288-95, 407-15. This was my source on Slijper’s goat.
James C. 1970. Abnormal fin and vertebral
growth structures in Plectognath fishes. Proceedings of the
West-Eberhard, Mary Jane. 2003. Developmental plasticity