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Best Size 



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 Number of 



Human 

 Body Parts 



We function well with one kidney, 

 so why do we have two? 



by Tared Diamond 



When a routine medical test performed 

 on me recently detected an unsuspected 

 kidney cancer, my first thought was how 

 to spend my final year of life, in case that 

 proved to be all the time left to me. But 

 after my diseased kidney had been re- 

 moved and the other one proclaimed 

 healthy, I grew reaccustomed to the expec- 

 tation of a normal life span. I began to 

 wonder instead how my life style would 

 be affected, now that I had only one of 

 what was originally a pair of vital organs. 



Gradually, the answer emerged: There 

 seems to be no effect. Having just returned 

 from an even more demanding than usual 

 New Guinea expedition, I can't detect any 

 hmitation on my capacity for exercise or 

 for digesting food. As a physiologist and 

 evolutionary biologist, I am left to wonder: 

 Why did we evolve to have at least double 

 the necessary mass of kidney, which 

 ounce-for-ounce is the most energy-guz- 

 zling organ of our body to operate? 



Actually, people can survive on only 

 part of a single kidney, and our combined 

 kidney mass has to drop by more than two 

 thirds before it affects our life style. Hence 

 we have a surfeit of kidney tissue, at least 

 three times what we need. The outcomes 

 of surgery on other organs show that, sim- 

 ilarly, our intestine is approximately dou- 

 ble and our paricreas a remarkable ten 

 times the necessaiy size. As a result of that 

 enormous excess of pancreatic tissue, one 

 friend of mine who had the misfortune to 



develop pancreatic cancer felt no symp- 

 toms until 90 percent of his pancreas had 

 been destroyed, by which time he was 

 within a few months of death. 



Why have we evolved to build and 

 maintain such excesses in vital organs? 

 Or — to reverse the question — if some ex- 

 cess is good, why don't we maintain even 

 more? Fifty pounds of kidney would, of 

 course, be too heavy, would fill too much 

 space, and would require too much energy. 

 But why are our kidneys the particular size 

 that they are, 3 times, rather than 50 or 1 . 1 

 times, their necessary size? 



This question is part of a broader prob- 

 lem in biological design. In addition to the 

 puzzle of "how big," there is an analogous 

 puzzle of "how many." For example, why 

 are we endowed with two breasts, rather 

 than with one or sixteen? (Some mammal 

 species do have sixteen breasts). At the 

 molecular level, why does each of our en- 

 zymes exist in its particular number of 

 copies, rather than in some higher or lower 

 number? Like clamshells and spider webs, 

 our bones pose a third obvious, analogous 

 puzzle of "how strong." Why didn't evo- 

 lution result in our having stronger bones 

 that would break less often? 



Of course, you'll say, the answer has 

 something to do with natural selection, 

 which adapts each species to its particular 

 hfe style and environment. For example, 

 grass-eating cows, but not meat-eating 

 tigers or humans, evolved a big rumen to 

 digest cellulose. Similarly, Northern Euro- 

 peans dependent for millenniums on 

 drinking fresh milk as aduhs evolved to 

 retain the milk-digesting enzyme lactase 

 beyond childhood, but most peoples in the 

 rest of the world did not. 



Alas, most evolutionary reasoning re- 

 mains at that qualitative, gee-whiz level 

 and hasn't progressed since Darwin's day. 

 (As a frequent author of such qualitative 

 accounts myself, I'm not blaming other 

 scientists for failings of which I claim to 

 be innocent.) Rarely do biologists tackle 

 the problem of adaptation quantitatively. 

 We still lack a quantitative theory of bio- 

 logical design to predict numerical out- 

 comes and to explain their variation in na- 

 ture. We have yet to identify the selective 

 pressures that keep our kidneys, breasts, 

 and bones the size, number, and strength 

 they actually are. 



Exacdy the same questions arise in con- 



nection with structures that we ourselves 

 engineer Such questions are now much on 

 my mind and on those of my fellow earth- 

 quake-shocked Angelenos as we try to un- 

 derstand why some of our houses and free- 

 ways fell down while others didn't. 

 Engineers analyze such questions by 

 means of a well-developed framework 

 that could serve as a model for biologists. 



Like biologists, engineers have to deal 

 with such questions as: How big? How 

 many? How strong? Typical questions for 

 them include: How strong should this 

 house or bridge be built? How big should 

 a hot-water heater be for a house expected 

 to hold six occupants? How many emer- 

 gency exits should be designed for a 12- 

 passenger commuter prop plane or for a 

 500-passenger jumbo jet? 



Engineered structures are qualitatively 

 adapted to their "hfe styles," as are biolog- 

 ical structures. For example, a bridge in- 

 tended to bear the traffic of Sherman tanks 

 is built more strongly than a bridge in- 

 tended only for pedestrian traffic. But en- 

 gineers go further than these qualitative 

 analyses by calculating a "safety factor," 

 that is, the ratio of a structure's capacity to 

 its actual expected load. The cable sup- 

 porting a fast passenger elevator, for ex- 

 ample, is built with a safety factor of 11, 

 meaning that the cable could actually sup- 

 port eleven times the maximum legal pay- 

 load specified in the sign posted inside the 

 elevator. Safety factors differ among engi- 

 neered structures: for instance, 7 for slow 

 freight elevators, but only 5 for hotel food 

 elevators (dumbwaiters). 



Why do engineers build with safety fac- 

 tors exceeding 1.0? Obviously, the answer 

 is that actual capacities, as well as loads, 

 are somewhat uncertain or variable, so 

 that elevator cables with a safety factor of 

 exactly 1.0 would often snap. Capacities 

 vary because even batches of steel or con- 

 crete manufactured from the same mold 

 differ in strength, and because strength de- 

 teriorates depending on age and use. 

 Loads also vary unpredictably because 

 one cannot be sure how many sumo 

 wrestlers will try to crowd into an elevator 

 at once or how many big trucks will si- 

 multaneously be driven across a bridge. 



Actual safety factors are set depending 

 on the expected magnitude of variation in 

 capacities and loads, as well as on the 

 costs and benefits of excess capacity. 



78 Natural History 6/94 



