It was not mainly about gold, it was about pepper. Europeans used a lot of it, and went to enormous efforts to get their hands on it. Shake or grind it with due respect: If the asteroid belt were known to have anything we want as badly as some of our ancestors wanted pepper, we might already be there.
Gold did figure in at the start, no surprise, and so did Prester John. Persisting rumor or legend placed a Christian kingdom somewhere in India, then a thoroughly vague term meaning 'way off east somewhere.' A Portuguese royal younger son became interested in Prester John, in the rather grubbier African gold trade, and - alas, but you knew it was coming - in the grubbiest African trade of them all.
The age of exploration was on, and one of the first things discovered was original sin.
Henry the Navigator's establishment at Sagres was not a 15th century NASA, and the Portuguese caravel was not a nautical revolution (though it was developed amid an ongoing revolution in nautical technology). In fact the lateen-rigged caravel turned out to be a bit of a technological dead end that faded away in the 16th century.
But what Henry and his shipwrights did was still remarkable; they took a handy, seaworthy type used mainly for deep sea fishing and modified it specifically for exploration. And Henry's captains pushed systematically along the coast of Africa, into waters then unknown to Europeans. They (re-) discovered Madiera in 1420, and the Azores in 1427.
They kept going after Prince Henry died. At last in 1488 Bartolomeu Dias rounded Cape Horn, and the way to the East was open. Ten years later da Gama reached India. The fabled wealth of the East, and especially pepper, was at last in direct reach.
And the world was changed. No need to beat the point to death, but good old Christopher Columbus was just trying to go one better on the Portuguese, armed with a gross underestimate for the size of the Earth and sublime ignorance that there was a continent in the way. Lucky for him there was. But it was the Portuguese who got the ball rolling.
No one, so far as I know, had ever explored before in the systematic way they did. There had been explorers: Pytheas, Hanno the Navigator, and the unnamed, semilegendary Phoenician whose circumnavigation of Africa was reported by Herodotus even though he did not believe it. But those were all one-offs.
The Chinese were doing it just as systematically, and at the same time. Zheng He's last expedition ended in 1433; the Portuguese had already reached the Azores and in 1434 they rounded Cape Bojador, pushing into waters unknown to Europeans since Hanno. It was a remarkable historical coincidence.
Unlike the great treasure junks the caravels were small, perhaps 20 meters long and 50 tons capacity - about a third the length and a thirtieth the displacement of their Chinese contemporaries. Handy and well suited to inshore work, to our eye they seem far better suited to exploration. Columbus thought the Santa Maria was too big and clumsy.
But the comparison is misleading. The Chinese treasure fleets had small handy ships, and more to the point the Chinese fleets were not sailing into entirely unknown waters. They were not so much survey missions as 'commercial exploration' and flag showing. When it comes to cargo capacity and sheer coolness factor, the great treasure junks could give value for money.
And most of all to the point, China had perhaps 50 times the resources of Portugal. A timely and interesting link from Winch of Atomic Rockets gives estimated GDPs in 1600. Portugal is not listed, but Spain and China are: Ming China, $96 billion; Philip II's Spain, $7.4 billion. Portugal a century earlier would have been at most perhaps $2 billion. Sending out a few hundred men aboard a squadron of caravels was as heavy a burden on Portugal as sending 30,000 men aboard a treasure fleet was for China.
So why did Portugal's program of exploration succeed, while China's was cancelled and forgotten? The reason can't be things like decentralization versus centralization or government versus private initiative. Both were more or less the same on those counts, pushed by government factions and supported by the merchants, shipbuilders, and such who benefited.
There are no doubt a host of other factors. The Chinese court eunuchs had domestic rivals who wanted to cut them down to size, while Henry the Navigator was a royal younger son who seemed out to make none of the trouble that royal younger sons can make.
But I believe the more important reason is that the Chinese treasure fleets had absolutely nothing to do with broader Chinese concerns of the time. The Indian Ocean produced nothing that the Chinese wanted the way Europeans wanted pepper, and it was irrelevant to the empire's security concerns. (Whereas central Asia was all too relevant.) Seafaring itself was incidental to most of China's population.
It was much different with Portugal, a small country with a substantial fishing population that would readily go to sea for anything more profitable. More important, Portugal shared the reconquista heritage and crusading enthusiasm of neighboring Castile, and shared with much of Western Europe a late medieval fascination with knighthood and quests that gave us Sir Thomas Malory's Le Mort d'Arthur.
Gold, spice, landed estates, adventures in exotic lands, and fighting Muslims (in a pinch, any 'paynims' would do) all got swirled together in the late medieval European imagination, and it was a powerful high. In literature it would produce Amadis of Gaul and his endless successors, the fantasy quest adventures of the 16th century. In retrospect it was embarrassing and often worse, but it had big consequences. It explored the world and conquered a fair part of it.
Pepper would help. But ultimately we will be propelled into space by the power of cool.
Related posts: Zheng He, Leif Ericson, and - from the early days of this blog - the roots of fantasy fiction.