U.S. Steel. General Motors. AT&T. Exxon Mobil.

Small potatoes.

Apple on Thursday reached a milestone that these icons of capitalism never dreamed of: a market value of more than $1 trillion.

That landmark is the result of an extraordinary corporate success story. In a span of just 21 years, a near-bankrupt computer maker evolved into the most valuable publicly traded company in the United States, pushing the tech industry away from big, bulky machines and producing some of the world’s most popular consumer products, like the iMac, the iPod and the iPhone. Apple’s products have reshaped swaths of everyday life.

Apple’s new 13-figure valuation highlights how a group of enormous companies has come to dominate the United States economy. Today, a smaller cluster of American companies commands a larger share of total corporate profits than since at least the 1970s.

The impact of this phenomenon has been clear in the stock markets, where a band of household-name companies — led by Apple, Amazon, Facebook and Google — has fueled the nine-year bull market, the second-longest behind the rally that ended in 2000. Their successes also are propelling the broader economy, which is on track for its fastest growth rate in a decade.