The Logic of the Human Body Plan

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Do the outlined reasons which led to modern man help to give us ideas about extraterrestrials? It is very difficult to predict how an extraterrestrial intelligent being might look. Nature is extremely inventive and imaginative, as the many life forms on Earth testify. There are, however, a number of physical and biological restrictions that limit pure fantasy, which would also apply to extraterrestrials at a similar stage of biological development as us (prior to the onset of conscious manipulation of DNA and the arrival of a completely artificial nonorganic existence, discussed in Chap. 9). Looking more closely at our own anatomy can reveal much about those parts that are there for an overwhelming internal reason, and those that exist due to mere chance.

Consider the picture of a hypothetical alien in Fig. 7.36. Like a snail, it has its eyes on stalks. Such eyes might be considered advantageous, allowing the alien to effortlessly survey its surroundings without much need to turn the head. The existence of such aliens, however, can largely be discounted. One of our most important abilities, inherited from our tree-dwelling ancestors, is stereoscopic vision, which remains absolutely essential for judging distances in our day-to-day existence. The physical basis of stereoscopic distance determination is to measure angles to an object from two different viewpoints (our two eyes). However, these angles can only be converted to a distance scale if the basis-length (the distance between the eyes) is kept meticulously constant. With eyes on freely moving stalks this could never be assured, which shows that stalk-eyed aliens are unrealistic.

Let us consider what parts of the human anatomy are there by necessity and dictated by broader physical and biological laws:

1. Eyes and ears: on the surface of our planet, light and sound are easily available as media that can be used by living organisms to recognize the environment. On a typical land surface with a complex geometry, stereoscopic vision and directional hearing constitute a decisive advantage for survival. Eyes and ears are thus set in a solid structure, the head, to ensure a constant basis-length for stereoscopic distance determination. Keeping the head on a movable neck has the added advantage of providing a turn-able range-finder.

2. Upright walk and hands: an upright posture has many advantages, including freeing the hands for transport, manufacture, and tool use.

3. Mouth and anus: this is the old alimentary body tube, dating back to the gastrula stage of simple multicellular organisms.

4. Brain: learning, thinking, rational acting, and quick adaption all constitute intelligence, which gives man a decisive selective advantage.

5. Language: intelligent beings communicate, which on planets (in atmospheres and oceans as well as solids) is most easily accomplished by sound. This is because sound can be easily generated and controlled by muscle movements.

6. Economy: unnecessary expenditure of energy, or to grow nonessential body parts, is disadvantageous in the competition for survival. Thus more than two eyes or more than four extremities are superfluous, as they require greater brain control and produce extra body mass without providing decisive additional advantages. In the relentless struggle for survival, natural selection always opts for the more efficient and economical course.

This rudimentary list of essential biological and physical constraints suggests that extraterrestrial beings might not differ greatly from us in their general bodily appearance, since on other planets the same basic biological and phys ical laws will operate if a similar environment is present on a habitable planet. How the environment on Earth determined the architecture of our body is intensely investigated in developmental biology. For instance, would a planet completely covered by oceans allow the appearance of the above listed traits?

Clearly the gut developed in the oceans. The same can be said about the eyes and ears, where for instance the eyes have developed independently in many animal phyla (nematodes, mollusks, arthropods, chordates, see Arendt and Wittbrodt 2001). The ocean environment also led to the development of specialized appendages (hands) for gripping and manipulation by invertebrates such as mollusks (octopus) and arthropods (crabs) but the development of such organs in vertebrates (e.g. velociraptor, squirrel, otter, monkey, ape) occurred on land.

What about the brain development? As discussed above in Sects. 7.8 and 7.11, the mutually beneficial relationship with mammals and birds of the angiosperms, the flowering and fruit plants, was probably instrumental in increasing the primate and bird brains. In monkeys and apes, brain development, moreover, was greatly accelerated by the evolution of hands to multipurpose organs where walking was no longer the main function. But only after the complete freeing of the hands in hominids did our type of brain develop, made possible by the upright walk based on two legs when our ancestors left the rain forest. It is therefore very likely that the development of the human type of intelligence is intimately tied to the existence of continents, their plant cover and to the evolution of angiosperm plants.

If intelligence is so important for survival, why did it not appear at earlier stages in the history of the Earth? Clearly, for a high level of intelligence a sufficiently complex brain with specialized centers must be available, and such a brain could only develop over many steps from earlier stages. But why are there so many successful organisms with low intelligence? Here we see a similarity to the situation in Sect. 7.5, where the success of bacteria with relatively small genomes was compared to that of multicellular organisms having large genomes: clever behavior, highly specialized know-how, superb adaption, and specialization are very successful. Cockroaches are such examples, with body plans and behaviors that have changed little since the Devonian times. Yet it is clear that in the evolution from fish to amphibians, from reptiles to primates and man, one sees a progressive increase in intelligence: predatory dinosaurs are believed to have been more intelligent than amphibians or fish, the primates more so than reptiles and other mammals, the great apes more so than monkeys, and humans more intelligent than them all.

Finally, the innate logic of the human body plan can help us to hypothesize what might have happened to the development of intelligence if the K/T boundary event had not occurred. The high demand on mental capability required for survival in the angiosperm rain forest may well have triggered the appearance of primate-like intelligent tree-dwelling dinosaurs. The anatomical appearance of human-like creatures eventually based on them might not have differed much from our body plan in general layout. Future computer simulations might possibly give more answers here, and also provide clues to the appearance of extraterrestrial beings under alternative planetary parameters, including different gravity and other land to sea area ratios.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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