Billionaire Microsoft co-founder Bill Gates says his favorite class is "Big History." And you can take it online for free.

Historian David Christian created the self-guided, online social studies course, which covers almost 14 billion years of history, starting with the big bang.

Gates says the class is "my favorite course of all time," in a May post on his blog.

"Big History tells the story of the universe from the big bang to the first signs of life to today's complex societies. It shows how everything is connected to everything else, weaving together insights and evidence from across disciplines into a single, understandable narrative," says Gates.

Christian turned his online course into a book, "Origin Story," and Gates recommends the book, too. ("Origin Story" was published on May 22.)

.@BigHistoryPro is my favorite course of all time. Its creator, @davidgchristian, has condensed the big history story into a terrific new book.

"If you haven't taken Big History, Origin Story introduces you to its concepts in a straightforward, understandable way. David is a very good writer, and he has a way of making complicated subjects fun," says Gates. "If you're already a Big Historian, Origin Story is a great refresher. It does a fantastic job distilling the latest thinking about the origins of the universe."

Gates, like his billionaire buddy Warren Buffett, has said his most important quality is his curiosity.

Curiosity is "an amazing thing," said Gates, on stage with Buffett at Columbia University in 2017, "where you try to predict what is going to happen, and then, when it doesn't, you think: Well, that drug didn't get invented, that stock didn't go up, that approach wasn't popular. What is it about my model of the world that's wrong? Who could I talk to? What could I read?"

Gates said he often gets carried away reading.

"This is a phenomenal time to be a curious person," said Gates, who currently has an estimated net worth of almost $93 billion, according to Forbes. "The information that is out there! My biggest problem is that I stay up too late because I am reading and then I am a little bit tired the next day."

Gates is also famously optimistic, and he points out there's one point in Christian's view of the future with which he disagrees.

Progress is sometimes hard to see…but when you look at the facts, things are looking up:

"David is more pessimistic about the future than I am. He gets a little stuck on the current economic and political malaise happening in the West, and I wish he talked more about the role innovation will play in preventing the worst effects of climate change," says Gates, in his blog post about the course and the book.

"Understanding where humanity comes from is crucial to shaping where we go next. Origin Story is an up-to-date history of everything that will leave you with a greater appreciation of our place in the universe."

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