ARGUING THAT LIFE EVOLVED WITHOUT INTELLIGENT GUIDANCE, say the people who believe in “intelligent design theory,” is akin to arguing that a tornado hitting a junkyard could create a Boeing 747. So, I ask them, why does the 747 have an upstairs cabin? Because Boeing planned it that way, they answer. They are quite wrong, but the example of the 747 is useful because it points to some important general rules about how airplanes come to be the way they are.

Behind the scenes at the Farnborough airshow in England in the summer of 2000, Boeing was keen to share its opinion that Airbus’ weight numbers for the A380 dual-deck jumbo, which is to enter the fleet in 2006, were optimistic. Retired 747 chief engineer Joe Sutter was there just to remind people that Boeing had studied a dual-deck airplane as long ago as 1965 and had rejected it as too heavy.

Boeing was competing for a supersonic transport contract in 1965, at about the same time the 747 was conceived, and Pan Am founder and chairman Juan Trippe believed that the big subsonic jets would end up as freighters and that the SST would replace the 747 on passenger routes. Trippe was one of Boeing’s best customers and usually the first to order new models, so Boeing put the flight deck of the 747 above the passenger cabin to give the aircraft a hinged nose for a front-loading cargo door.

The first design for the cockpit enclosure was a hemispherical hump atop the fuselage. This produced too much drag, so Boeing extended the aft portion of the hump to form a teardrop. Then, in a deliberate echo of the below-deck lounge on the model 377 Stratocruiser, Boeing’s 1940s flagship, Trippe and his colleagues persuaded Boeing to turn the extra space behind the cockpit into a bar and lounge.

The party days ended with the 1973 fuel crisis, when virtually every 747 operator got rid of the lounge and replaced it with more seats for paying passengers. In announcing the change, a British Airways press release noted that the upstairs area was “currently used for a first-class lunge”—the spelling was probably appropriate more often than not.

The décor in the 747 lounge is a vaguely horrible memory, but burnt-orange sectional sofas weren’t the only aesthetic transgression to come out of Seattle during that era. There was the 747SP. This short-body, long-range version of the 747 was as economical as it was elegant (that is to say, not at all), but when Boeing sawed 47 feet out of the fuselage to create the SP, the engineers had to look at what would happen when the rear of the cockpit hump lined up with the front of the wing. There were no ill effects, as it turned out, so Boeing designed the new 747-300 with a longer upper deck to seat up to 70 passengers.

Today’s 747 has a large upstairs cabin because Trippe thought it would be a freighter, because a round hump produced too much drag, because Pan Am bosses had fond memories of the Stratocruiser’s bar (and all-night parties held by the light of the Pratt & Whitney R-4360s’ flaming exhausts), and because fuel prices went through the roof. Nobody set out to design the airplane with an upper deck. It came about because it was possible and because it adapted the 747 to a changing environment. It evolved.

The history of the airplane is remindful of biological evolution. New species emerge through mutation and then survive or perish. In detail, every part of an airplane is designed deliberately and meticulously for its job. Nine times out of ten, though, the broad layout of the airplane is evolutionary.

Trace the line back and you will find an airplane that looks new and revolutionary—only to find that its own design is usually the happy combination of many strands of previously existing DNA, with results that the designers themselves never quite foresaw until the airplane was complete.

Most large commercial jet aircraft—even those designed by arch-rivals—look like members of the same family. Their engines hang on pylons, in front of and below their swept-back wings. The landing gear is in exactly the same place: hinged behind the rear wing spar and folding inward to stow behind the wing. The wings all have lift-boosting devices—flaps—across their leading and trailing edges.

All these airplanes carry the DNA of a common ancestor: the Boeing 367-80, also known as “Dash 80.” That Boeing 707 prototype, in turn, emerged from a tumultuous nine-year process as the third in a sequence of significantly different airplanes, the first two being the B-47 and B-52 jet bombers.