It was August 1856, three years before Charles Darwin would publish On the Origin of Species. Workers in a lime quarry in the Neander Valley near Düsseldorf, Germany, unearthed a puzzle in a small cave: a number of ancient-looking bones. Thinking the bones were from a bear, the workers saved some of them for a local schoolteacher and amateur naturalist, Johann Carl Fuhlrott. Fuhlrott later worked with anatomist Hermann Schaaffhausen to study these bones. The two went on to describe and publish the first findings about Neandertals, launching the field of paleoanthropology—and our enduring fascination with this extinct human relative.

Since then, we have learned a lot more about this ancestor from the remains of several hundred additional specimens. Today Neandertals are likely the best known hominid species besides our own—yet they remain mysterious in many ways. Competing theories place them either as an archaic variant of our own species, Homo sapiens, with whom we interbred, or as a separate species altogether.

Either way, Neandertals and H. sapiens had much in common. The cranial capacity of Neandertals was about as large as ours. They stood upright, averaging just a few inches shorter than Cro-­Magnons, with a heavier build and stronger arms and hands. They dabbled in practices—the creation of finely wrought tools and symbolic objects, including jewelry—that humans do routinely, although they did not achieve our cultural heights. They coexisted with us for thousands of years in Europe before they vanished. They had survived for so long, under harsh environmental conditions, which included long frigid periods. Why did such a durable species die out? And in exploring their regrettable demise, can we get a better understanding of what makes modern humans unique?

As science eagerly awaits the treasure trove of data from the first draft of the Neandertal genome, due later this year, researchers have continued to mine clues from the fossil record to answer those questions. In our cover story, “Twilight of the Neandertals,” staff writer Kate Wong relates the subtle and complex factors that may have caused the species’ ultimate collapse. While we also celebrate the 150th anniversary of Darwin’s masterwork, there is, perhaps, a larger practical and evolutionary lesson for modern society to draw from the experience of the Neandertals: climate shifts that made their surroundings increasingly inhospitable may have been the ultimate executioner.

The work of Albert Einstein occurred decades after that of Darwin, but it is no less remarkable in its power to transform our understanding. Some 90 years after the publication of Einstein’s theory of general relativity, which showed that gravity arises from spacetime being curved, physicists are still grappling with its implications. Eduardo Guéron explores one of them—the ability to “swim” through a vacuum without needing to push on anything—in “Adventures in Curved Spacetime." Based on the latest research, the article offers an armchair journey into the mentally stimulating landscape shaped by Einstein’s genius.

Note: This article was originally printed with the title, "Species and Spaces."