Why did it take so long to invent the wheelbarrow? Have we hit peak innovation? What our list reveals about imagination, optimism, and the nature of progress. ... The Atlantic recently assembled a panel of 12 scientists, entrepreneurs, engineers, historians of technology, and others to assess the innovations that have done the most to shape the nature of modern life. The main rule for this exercise was that the innovations should have come after widespread use of the wheel began, perhaps 6,000 years ago. That ruled out fire, which our forebears began to employ several hundred thousand years earlier. We asked each panelist to make 25 selections and to rank them, despite the impossibility of fairly comparing, say, the atomic bomb and the plow. (As it happens, both of these made it to our final list: the discovery and application of nuclear fission, which led to both the atomic bomb and nuclear-power plants, was No. 21 of the top 50, ahead of the moldboard plow, which greatly expanded the range of land that farmers could till, at No. 30.) ... Less evident from the final list is what I was fascinated to learn from my talks with many of the panelists. That is the diversity of views about the types of historical breakthroughs that matter, with a striking consensus on whether the long trail of innovation recorded here is now nearing its end.