Your thoughts, your knowledge and understanding, your emotions, your hopes and dreams, and the feeling you get when your lover kisses you gently on the lips, are all managed by a 3 pound organ in your head. Your brain is you, and you are your brain. We know this from neuroscience, from brain injuries, and even certain experiments - but it was not always so. Ancient civilizations dissected corpses, and understood the functions of most of the organs: the heart beats, the lungs breathe, the stomach digests, and the intestines process and excrete. But the liver and the brain were large enigmatic organs with no discernable purpose. Most cultures, Babylonians, Assyrians, Etruscans, Greeks, Romans, and Hebrews, guessed wrong, and assumed the brain was fairly useless, a conduit for four of the five senses at best. The liver however - that particularly large organ must be the seat of the soul, the center of thought and emotion. I suppose an ancient Greek, raised with this mindset, would picture his consciousness in his lower right abdomen, the way we imagine our thoughts as living in our heads. This is one of those beliefs that is harmless; it could be 100% wrong without affecting survival. Sometimes I try to forget everything I know, and imagine my consciousness located in my liver. One day, for a moment, I was almost there, I was thinking from my liver, but then my thoughts snapped back into my head, where they "belong".
If your entire experience here on earth is mediated by a 3 pound organ, what is the other 167 pounds of stuff for? Why aren't we just heads walking around on little tiny feet? As it turns out, such a design is unsustainable, but beyond this, the question isn't even wellfounded. It implies a purpose, as though you and your thoughts are the culmination of life on earth, and the body is there to serve. In fact, your brain, and your thoughts, and your consciousness (whatever that is), is just another organ that helps us survive, just another tool in evolution's bag of tricks. It is adaptive, like the monkey's tail, or the eagle's feathers, or the turtle's shell, or the lobster's claw. We are able to unravel the mysteries of gravity, and enjoy a symphony, but that is merely a happy side effect of an organ that is honed to help us survive on the plains of Africa. Everything we are, and everything we feel, is just another organ, just another trait that has proved useful in the survival of our species. Darwin's reality conflicts with a proclamation that seems self-evident: cogito ergo sum, I think therefore I am. In reality, the mighty redwood, that thinks not at all, is, as surely as I am, and as surely as you are, and with equal validity. Yes, we may clear the redwood forest into extinction some day, but that does not imply superiority or destiny. After all, lions could have eaten us into extinction 1.3 million years ago, and it's largely a matter of luck that they didn't. Well they didn't, so let's press on.
Setting your brain aside, what do your other organs do? The heart has a clear purpose, pumping blood around the body. Why is that necessary? Tissues die without oxygen and nutrients, and when glucose is burned for energy, carbon dioxide must be carried away. The same problem exists on the International Space Station. With no gravity, there are no natural air currents, and your breath simply hangs around your face. Sit motionless for an hour, and you could suffocate. Thus fans are a vital part of the station, fans in every corner of every room. Wherever you sit, or stand, or float, or sleep, air must move gently past your body. In the same way, blood must carry oxygen to all your tissues. Any long-term disruption of blood flow can be catastrophic. A diabetic, for instance, often has poor circulation in his legs and feet. The slightest wound can lead to infection, tissue death, and even amputation. And yet, hands and feet are fairly tolerant when it comes to oxygen deprivation. Other organs are more sensitive, and damage can occur within an hour. The brain, for instance, is dead in 5 minutes without oxygen.
Now change perspective, and consider a single cell. It doesn't have a beating heart, because it doesn't need one. A typical human cell is 10 micrometers across, which is one sixth the thickness of a human hair. When oxygen enters a cell through the cell wall, it diffuses throughout the interior almost immediately. There is no need for circulation; the space is simply too small. In the same way, carbon dioxide, generated by metabolic processes inside the cell, diffuses instantly throughout the interior of the cell, and leaks out the cell wall and into the environment. Of course the cell wall exists to keep certain compounds in, and other compounds out - but if the cell wall is permeable to a particular compound, such as oxygen, then the concentration of oxygen is the same inside and out. There is no need for a beating heart, because oxygen anywhere inside the cell, or at its surface, implies oxygen throughout. In the same way, you wouldn't expect a cell, or even the center of a cell, to be cooler or warmer than its surroundings. A cell is instantly the temperature of the water that contains it.
Now increase the cell's diameter to a centimeter, about the size of a cherry. The radius is multiplied by a thousand, the surface area by a million, and the volume by a billion. In general, the surface area increases as r2, and the volume increases as r3. The oxygen that can pass through the cell wall increases as r2, but the oxygen required for life increases as r3. If there is no active mechanism to bring oxygen in and send carbon dioxid out, like a pair of lungs, the cell runs out of oxygen and dies. Thus the largest cell is about 100 micrometers across, ten times the average, and barely visible to the naked eye under ideal conditions.
An insect seems small to us, yet it is large enough to require a beating heart, albeit a simple one. Muscles in the abdomen contract, and force hemolymph (an early form of blood) through a tube that runs along the dorsal wall of the insect and towards the head. At this point the hemolymph exits the tube and flows through the body cavities and back towards the heart. This is an open circulatory system, since the hemolymph is not confined to tubes all the way around. The heart beats 20 to 25 times per minute, and continues to beat even if the head is removed. In most insects, hemolymph is not used for oxygen transport. It has no hemoglobin, and is not red like our blood. Rather, it transports sugars, proteins, and electrolytes. These larger molecules would pool and collect, if not distributed by the beating heart. Oxygen, however, still distributes by diffusion, provided it can pass through the insect's hard exoskeleton. Spiracles, small holes in the insect's body, allow air to flow in and out. Once again, air moves through the spiracles by diffusion, though in some cases air can be pumped in and out by activity, particularly wing beats, which is when oxygen is in high demand. This tracheal system isn't as efficient as lungs, not by a long shot, and thus the size of insects is limited. No worries about Mothra - oxygen would never reach the interior of his body. In the late Paleozoic, when oxygen levels were higher, insects were larger, some having 2 foot wing spans. However, even in a pure oxygen environment, insects could not get much larger than that. The enormous arthropods in Stephen King's The Myst, coming from another dimension, must have lungs, and a closed circulatory system, and red blood capable of carrying oxygen throughout their massive bodies. Of course this isn't a treatise on exobiology, but rather, an exercise in sheer terror, and an absolute must-read.
So, how many miles of veins and arteries do you need to distribute oxygen to every part of your body?
"Miles?" you might exclaim.
Yes, miles. If all the veins and arteries and capillaries of an adult were laid out end to end, the line would be 100,000 miles long, wrapping four times around the earth. Cut these veins and arteries open and lay them flat, and find a lot of surface area. This is the effective surface of your body, relative to oxygen, carbon dioxide, nutrients, and waste products such as urea. The entire circulatory system is a surface area expander, bathing each cell in oxygen and carrying away carbon dioxide.
Blood distributes oxygen throughout, but how does oxygen get inside in the first place? Some diffuses through your skin, but not nearly enough, thanks to the volume to surface area power law. Even if you had spiracles on your chest, it would not be enough. Lungs pull oxygen into the body and pass it into the blood, where it is transported throughout the body. In the other direction, blood releases carbon dioxide into the lungs, which is then blown out into the atmosphere. Gas exchange takes place in small air sacks called alveoli, which are rich in blood vessels. If these were cut open and spread out, they would cover an area of at least 50 square meters, the first floor of a good sized house. Human lungs are efficient surface area expanders for O2 CO2 transport.
Move on to the alimentary canal, where the small intestine, if stretched out, would reach 23 feet. This tube presents 2 to 3 square feet of area, to absorb nutrients into the blood stream. The large intestine, which pulls water out of the waste product, is 5 feet long, and represents yet another surface area expander.
Finally, the kidneys extract water soluble waste from the blood, and redirect it to the bladder for excretion as urine. Like the alveoli in the lungs, a kidney contains nephrons, over a million nephrons, where blood is filtered. Each nephron has internal structures, cuboidal epithelium, that increase surface area even further. Thus our blood passes, continuously, through millions of filters, to remove the waste products of metabolism.
In summary, most of our organs from neck to waist are surface area expanders, mandated by our large size. If you need at least a pound of brain to attain some level of consciousness, and perhaps three pounds of brain to chip away at Fermat's Last Theorem, then you need another 50 pounds of surface area expanders to support the metabolism that goes on in that hungry brain. Compare, by analogy, the CPU with the whole of the computer. Supporting structures often include fans, and a heat sink connected directly to the CPU. This system extracts waste heat, rather then metabolic byproducts, but it is still a surface area expander, as though the cpu had an effective area hundreds of times its actual size. A modern CPU must shed heat through its tiny surface at a rate comparable to the surface of the sun. It would quickly fry itself to death if it did not have an active cooling system. And of course it needs power, electrical rather than chemical, and communication lines with the outside world etc.
Life evolved on earth almost as soon as it was cool enough to do so - but then it took 3 billion years for macroscopic animals to evolve. Why is that? Nobody knows of course. Cells working together for the greater good is, perhaps, a tough nut to crack, but beyond this, surface area expanders, which are absolutely necessary, are, I suppose, more than just a few mutations away. You have to pull that lever many many times to hit the jackpot. This leads to a pessimistic projection, not at all compatible with popular science fiction. Over the next million years we explore our sector of the galaxy using unmanned probes, and discover 342 planets with life. In each case the biochemistry is similar, yet distinct in subtle ways, assuring each has evolved independently of the others. 342 planets with life, all bacterial life. Not an animal in sight. Maybe we are the only animals in the universe. No Vulcans, no Krell, no Cylons, no Chirpsithra, just bacteria here and there. Maybe that's how our universe works. Well I hope, on at least one extraterrestrial planet, we find another animal, and another evolutionary solution to the surface area / volume problem. It must be solved, if animals are going to run, and hunt, and love, and ponder their own existence.