Mark Zuckerberg

By Craig Newmark

The internet is about people connecting to people, whether for business, politics or socializing. That's something we've all been doing since long before the Internet existed. The real accomplishment is to make those connections so versatile and different that they create a social network that not only reflects your life but maybe expands it.

Mark Zuckerberg, 23, the creator of Facebook, has done just that.

Working from his dorm room at Harvard in 2004, Zuckerberg launched Facebook as a campuswide system and later expanded it to other colleges. The site exploded nationally and globally, and with that came tens of millions of dollars from investors, buyout offers from Yahoo! and Viacom, and a worldwide community of Facebook users.

All that happened because Zuckerberg has remained true to his vision, focusing on building a community rather than on a mere exit strategy—which is why those buyout offers have been declined. Facebook also provides a flexible information ecology, offering a fairly open system that allows users to add small applications to pages. As a result, members are starting to run businesses via the Facebook platform, including loan systems and music distribution. There are challenges, including "Facebook fatigue," which results from just too many invitations. Then, of course, there are issues related to monetization; investors are patient, but not forever. Facebook, however, just keeps growing, with more than 70 million active users so far. That's a lot of people connecting via Zuckerberg's vision—which is just what that vision was always about.

Newmark is the founder of Craigslist, which attracts 30 million people a month

Nicholas Schiff

By Michael Kinsley

If there were a surgical procedure of the year, 2007's winner would have been an operation known as deep brain stimulation (DBS). Most folks are not familiar with it, but 40,000 people (myself included) are intimately so, having undergone it as a radical—and startlingly effective—way to treat the symptoms of Parkinson's disease. Last year a new frontier in DBS was opened when a team led by Dr. Nicholas Schiff of the Weill Cornell Medical College in New York City used it to turn the cognitive lights back on in a man who had spent six years in a near coma as a result of injuries suffered in a mugging and had seemed destined to spend the rest of his life that way.

A DBS procedure is actually two operations. First, surgeons drill a pair of dime-size holes in the skull and thread fine wires down to whatever area of the brain is misfiring or otherwise malfunctioning. A week later, they insert a pacemaker-like device in the chest, thread wires under the skin and connect them to the ones in the brain. When they metaphorically flip the switch (actually, they use magnets to turn the batteries on and off), the symptoms disappear, or at least are greatly reduced.

For the brain-damaged man in last year's procedure, that reduction meant a lot. He is far from cured, but he can recognize his parents and conduct brief conversations. Schiff, 42, led the team that planned and conducted the procedure, though the surgery itself was performed at the Cleveland Clinic by Dr. Ali Rezai, who also performed my procedure and has used DBS on patients with obsessive-compulsive disorder and deep clinical depression. As battlefield medical care gets increasingly sophisticated, more and more soldiers with catastrophic head injuries are surviving combat zones like Iraq, only to come home and find that little can be done to restore their minds. Now, thanks to Schiff and his colleagues, that is changing.

Kinsley, a TIME columnist, underwent DBS to treat Parkinson's in 2006

Paul Allen

By Thomas Insel and Story Landis

If the brain is science's great frontier, you need someone to lead the expedition—someone with smarts and curiosity, not to mention a large bank account to finance the journey. In the terra incognita of the brain, that's Paul Allen.

Co-founder of Microsoft, owner of three sports teams and lead backer of SpaceShipOne, which in 2004 made the first private manned space flight, Allen, 55, is also head of the Allen Institute for Brain Science, which was founded in 2001 with the goal of mapping the brain, cell by cell and gene by gene. His scientific team began with the mouse brain and finished the job in 2006, posting all the data online. Scientists everywhere now have a free neural GPS to learn where and how specific genes are expressed—a vital tool for studying similar functions in human brains.

The institute has now begun its own effort to map human gene expressions, hoping for insight into the molecular basis of consciousness and creativity. Allen founded his institute in the belief that with the right tools you can transform the speed of science itself. Not many people accomplish that even once. Allen is doing it yet again.

Insel heads the National Institute of Mental Health; Landis leads the National Institute of Neurological Disorders and Stroke

Mary Lou Jepsen

By Brewster Kahle

Think it's hard to keep the computers in your kid's school up-to-date? How about the 50% of children worldwide who learn outdoors? If half the planet was not going to be excluded from the Internet age, somebody had to come up with a machine that would work in such unlikely places. Mary Lou Jepsen, 43, did. Along the way, she also survived a terrifying disease.

Jepsen is a veteran of the MIT Media Lab, where she co-created the world's first holographic video system in 1989—back when computer imaging meant straight lines on a cathode screen. In 2005 Jepsen and Media Lab founder Nicholas Negroponte launched the nonprofit, open-source One Laptop Per Child program, which, as the name suggests, was an attempt to get a computer to every child in the world who needs one. The machines would have to work in extreme climate, amid spotty power and Internet connectivity, and be readable in direct sunlight. Oh, and they would have to sell for around $100 each. Negroponte ran the project, but Jepsen was the lead innovator and architect of the hundreds-strong team that would design the machine.

Within two years they succeeded, creating a computer that can run on solar power, with five times the screen resolution of other laptops and a wireless system that creates its own network. The $100 price point has not been met, but $188 has. Jepsen did all that 12 years after receiving a diagnosis of a brain tumor in 1995 that had gone undetected for five years. She beat the disease but must take a dozen pills every day to keep her hormone output stable. Something of a self-taught expert on hormones, she now offers her insights to others who are diagnosed with the same illness. Jepsen is known among her friends as the "light lady" for her work with computer imaging. But the kind of light she's shedding goes far beyond the screen.

Kahle is director and co-founder of the Internet Archive, an open, online library for historical collections

TIME 100: SCIENTISTS AND THINKERS

Daniel C. Dennett///Dr. Robert C. Gallo///Peter J. Gay///Walter Isaacson, Time Magazine///Steven Pinker, MIT///Maxine Singer, President, Carnegie Institute Of Washington///Bruce Sterling

Eric Chivian & Richard Cizik

By Leith Anderson

Scientists and evangelicals slept side by side last summer on the floor of a preschool in the Alaskan village of Shishmaref. We were there to see for ourselves the devastation of climate change on the Inupiaq Eskimos, whose island is eroding into the Chukchi Sea.

The leaders of our small band were the unlikely pair of Eric Chivian, 65 (left, in picture), and Richard Cizik, 56. Chivian shared the Nobel Prize for Peace in 1985 for his efforts to stop nuclear war, and is now a professor of psychiatry at Harvard Medical School and director of its Center for Health and the Global Environment. Cizik, an ordained Evangelical Presbyterian minister and head of the Office of Government Affairs for the National Association of Evangelicals (NAE), came to prominence 25 years ago when he drafted a letter to President Ronald Reagan inviting him to talk to the NAE. That address became Reagan's "evil empire" speech. The event helped give Evangelicals the political clout they still enjoy.

What brings Chivian and Cizik together is a shared passion for the environment, although they act on that passion in very different ways. Chivian is a highly trained scientist who tells stories like a teacher with the bedside manner of the general-practice physician he used to be. Cizik quotes the Bible, carefully referring to "creation care" rather than climate change or global warming, and advocates a brand of pro-life politics that extends well beyond human conception, up through the care of God's creation itself.

They both have their critics. Why would a scientist like Chivian collaborate with Evangelical Christians who talk about the authority of the Bible and their faith in Jesus Christ? Cizik's detractors say there are more important issues for Evangelicals to tackle, and there is no consensus within the community about global warming anyway.

Coming from different directions, the physician and the preacher combine their influence, persuading Americans to take better care of God's creation. And they practice what they preach: Chivian keeps his thermostat at 60°F (16°C) on cold New England nights, and Cizik drives to work in a Prius hybrid.

Anderson is the president of the National Association of Evangelicals, based in Washington

Peter Pronovost

By Kathleen Kingsbury

In science you learn that the simplest answer is often the best. That's a principle sometimes lost in a world of high-tech medicine—but not on Dr. Peter Pronovost. A critical-care researcher at Johns Hopkins University, Pronovost may have saved more lives than any laboratory scientist in the past decade by relying on a wonderfully simple tool: a checklist.

In the U.S., hospital-acquired infections affect 1 in 10 patients, killing 90,000 of them and costing as much as $11 billion each year. Pronovost, 43, began investigating this alarming trend at Johns Hopkins' hospital in 2001 and concluded that arming physicians with a chart reminding them of each step in routine procedures drastically reduces the medical errors that lead to such infections. After he published his results in several prominent journals, the medical community started listening. Michigan hospitals began implementing Pronovost's checklists in ICUs in 2003. Within three months, hospital-acquired infections at typical ICUs in the state dropped from 2.7 per 1,000 patients to zero. More than 1,500 lives were saved in the first 18 months.

Numbers like this get noticed. California and Spain top a long list of places that soon hope to replicate Michigan's results. Pronovost says the checklist protocol could be rolled out nationwide within two years for less than $3 million. With health-care costs mounting, it's nice to know that one solution requires simply returning to basics.