The world's changed a lot in the last 20 years, and that's certainly true of the business world.

Once upon a time, the American economy was dominated by banks, oil companies, and manufacturers. These days, the tech sector reigns supreme, as the "Magnificent Seven" makes clear.

However, it can be informative to take a ride in the wayback machine and see what's changed. Doing so is a reminder that nothing lasts forever in business and it's a useful way of studying what worked and what hasn't.

On that note, the list below shows the five biggest companies in the U.S. by revenue in 2004.

Source: Fortune 500

If you're familiar with the stock market and the economy from 20 years ago, that list might not be a big surprise. Walmart dominated the retail sector, and Exxon was the leading oil major.

However, what's notable about that list is how all of those companies except for Walmart have faltered since then. In fact, all five of these stocks have underperformed the S&P 500 over the past 20 years. General Motors filed for bankruptcy protection in 2009, and Ford nearly followed it. General Electric has been dismantled after years of mismanagement. Exxon hasn't endured a major crisis, but the oil sector has fallen out of favor with electric vehicles (EVs) on the rise.

Now let's look at the five largest companies by revenue today.

Source: Fortune 500

Impressively, Walmart has retained its top spot, though Amazon is likely to surpass it in the next few years.

The other companies tell a story about how the economy has changed. Amazon and Apple, at Nos. 2 and 3, are consumer-driven tech companies, while healthcare makes an appearance at No. 4 in the form of insurance. Warren Buffett's Berkshire Hathaway rounds out the list, and Berkshire's primary business is insurance. It also holds a large stake in Apple, though that doesn't count toward its revenue.

On the other hand, oil and gas and manufacturing have declined as witnessed by the absence of companies like ExxonMobil, GM, Ford, and GE.

Will tech, healthcare, and insurance remain ascendant or will other industries displace them? It's worth keeping your eye on the companies atop the Fortune 500 as the list continues to evolve.

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John Mackey, former CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Jeremy Bowman has positions in Amazon. The Motley Fool has positions in and recommends Amazon, Apple, Berkshire Hathaway, and Walmart. The Motley Fool recommends General Motors and UnitedHealth Group and recommends the following options: long January 2025 $25 calls on General Motors. The Motley Fool has a disclosure policy.

These Were the 5 Biggest Companies in 2004, and Here Are the 5 Biggest Companies Now was originally published by The Motley Fool