A few months ago, as I walked down narrow Dubois Road, in the Central Business District of Nairobi, Kenya, I came to a small shop selling cell phones. There are thousands of these stores across the city. In some places, they line both sides of the street.

When I got to the counter, I asked the young man, whose name was Paul, about getting a SIM card for my phone so I could make calls in Kenya. I handed the phone to him. He took it, looked at it, then shook his head in pity.

“This is a very old phone. It is a phone from zamani!” he said. The Swahili word he used means “a long time ago,” but it can also mean ancient times, prehistory. I felt a little like Richard Leakey bearing some fossilized tool I’d just dug up in the Olduvai Gorge.

The phone was not that old. I bought it in 2005 in Nigeria, and it works so well, so reliably, that I can’t see any reason to replace it. The battery lasts forever. It has survived innumerable impacts. And it gets amazing reception. But then, Paul was not that old either – no more than 20. For half of his time on this planet, cell phones have been a fact of life.

I have been around twice that long and can remember zamani, and for me, the change has been amazing to see. The last time I’d been in Nairobi was 1997, and I don’t remember seeing any cell phones. Even in the United States at that time, few people had them. The notion that someday almost everyone in Kenya would have one would have seemed laughable.

In his book, Africa: Altered States, Ordinary Miracles, British journalist Richard Dowden marvels at the speed and scope of this development. A longtime correspondent on the continent, he recalls that when he lived in Uganda in the early 1970s, he used a telephone only once. In the 1980s, he had to drive seven hours across the country to call in a story to his editors. Back then, the most common sound on an African phone was a voice saying, “The number you require is unavailable. Please try later.”

But around 2000, the mobile phone revolution began to sweep across Africa. By 2001, there were 24 million subscribers. As of 2011, there were half a billion, with 15 million of them using smartphones. Paul told me that the shop in Nairobi sells 20 to 50 phones a day.

It’s a story that has repeated itself across Africa and around the world. One day I was standing in line at the Minneapolis airport when the woman next to me took a call from Somalia as casually as if it were coming from across town. That country doesn’t have a government, but it has six huge telecom companies serving at least two million subscribers with some of the cheapest calling rates in Africa.

Text messaging has become another ultra-cheap way to communicate. Once, in the middle of Nigeria, I met a woman who was instant messaging with her sister, who was having a baby in England. The woman in Nigeria was thrilled that, in some way, she could be there.

The potential benefits of this technology are being tapped in creative ways. A program in Tanzania called SMS for Life helps track supplies of malaria medications to ensure availability in remote areas. In some countries, patients far from hospitals or clinics can text or call health care providers for advice. In South Africa, an AIDS mobile hotline is helping fight the disease. Crisis-reporting software for cell phones, such as FrontlineSMS and Ushahidi, has helped create real-time pictures of events as they unfold during wars, disasters, and elections, so help can be delivered as fast and efficiently as possible.

The impact on the lives of individuals is huge – and obvious to anyone walking around an African city – but it is only beginning to be quantified. Some studies suggest that for every 10 percent increase in the number of mobile phones, per capita GDP increases 0.6 to 0.8 percent – in part by making markets more efficient. Farmers and fishermen, for example, can find out where to get the best prices.

Mobile banking is also catching on. People make purchases and manage their financial affairs on their phones with Wizzit in South Africa, MobiCash in Djibouti, and M-Pesa in Kenya. One study estimates that among rural households that have started using “mobile money,” incomes have increased 5 to 30 percent.

Mobile phones have made life easier, better, more secure, and more interesting for millions of people. Dowden points out that the explosion of cell phone use and its effects came as a surprise to many experts, in part because people’s “desire to communicate” had been greatly underestimated.

That desire is one of the things I love most about Africa – the readiness of people to converse, to argue, to parse, to discuss, to disagree. The channels of communication have always been open, but with mobile phones, people’s ideas travel farther than ever.

And while people in Kenya talk a lot on their phones, this doesn’t seem to have inhibited face-to-fact interactions. When I was there most recently, I had many long and interesting conversations with complete strangers. (Often, they would then ask for my phone number.)

One day, I was walking down the street when an old man asked me where I was from. He wanted to know what I thought of Nairobi, what I was doing there, and so on. I mentioned that it had been a long time since I had been there, and how much I thought the city had changed.

“Everyone has one of these now,” I said, and pulled out my ancient phone.

The man looked at it, and his eyes widened.

“I used to have this exact same phone!” he said. “I loved that phone. It never, ever failed.”