Thea Singer's article on “The Evolution of Dance” raises so many questions, not only about entrainment—in which motor neurons align with sensory neurons’ detection of auditory signals—and the differences a trained dancer experiences in comparison with one who dances for enjoyment but also about why we dance and why some people and cultures don't dance more.

I dance with my children to feel joyous, connected, energized and relaxed. But I feel uncomfortable studying dance for the way professionals execute movements because I feel that doing so runs the risk of reinforcing inhibitions associated with how dance must look instead of feel in my own culture. It can be highly invalidating when the focus is on who is the leanest and most physically adept for a dance style. Further, taking a look at how different groups and cultures approach dance might open its potential healing aspects, such as for Parkinson's sufferers, to more people.

CATE ANDREWS via e-mail

There appears more to the story of dance than human social evolution. Much of the “sensing” external rhythmic movements and sounds that Singer describes begin in utero. The maternal abdomen and uterus are filled with noises, both maternal and external. After a 45-year career in diagnostic medical sonography and the observations of thousands of fetuses, I am convinced that the ancient human tendency to use ritual rhythm, drumming and dancing is a natural result of these in utero fetal experiences. More research is needed.

TERRY J. DUBOSE Associate professor emeritus, University of Arkansas for Medical Sciences


“Black Holes from the Beginning of Time,” by Juan García-Bellido and Sébastien Clesse, describes the interesting idea that primordial black holes are the constituents of dark matter. But the authors appear to claim credit for a concept that has been in the literature for more than 40 years—see the 1975 paper “Cosmological Effects of Primordial Black Holes” by one of us (Chapline). And in the case of only such black holes of many solar masses making up dark matter, it existed before the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) announced its discovery of gravitational waves in 2016—see a recent preprint paper by one of us (Frampton) at

GEORGE F. CHAPLINE Lawrence Livermore National Laboratory

PAUL H. FRAMPTON University of Salento, Italy

Suppose there was a wide distribution of primordial black hole masses. Over time the lower end of the distribution would gradually evaporate by Hawking radiation. If the total mass involved were significant, then the universe's gravitational mass would be gradually reduced, and the slowing of the universe's expansion from gravitational attraction would decrease. In other words, could the long-term evaporation of a mass distribution of primordial black holes be observed as “dark energy”?

Francis X. Hart University of the South

THE AUTHORS REPLY: In response to Chapline and Frampton: We note in our article that the idea of primordial black holes (PBHs) dates to the 1970s. This is indeed also the case for PBHs as dark matter, which Chapline was one of the first to propose. But his black holes were smaller than 1019 kilograms, which is now ruled out by observations of microlensing events.

We can claim credit for the idea that PBHs (as dark matter) should have both stellar masses and a broad mass distribution, which we proposed in a paper submitted as a preprint in January 2015 ( and published in July of that year. And in a preprint paper we submitted immediately after Advanced LIGO's February 2016 announcement of its first gravitational-wave discovery (—published this past March—we noted that it had probably detected the merging of such PBHs and estimated the rate of events expected in our scenario, which seems to agree with more recent observations.

To answer Hart's question: The evaporation rate of black holes via Hawking radiation is very slow, and our PBHs have a significant abundance only in a range of around one solar mass. It would take trillions of trillions of times the age of the universe for one of them to evaporate. In our model, the fraction of evaporating black holes is completely negligible. But even if evaporating black holes were abundant enough in the early universe, the reduction of the dark matter in PBHs could be compensated by an increase of radiation.

In any case, this would give rise to dark radiation, not dark energy, which is very different. In an expanding universe, radiation density redshifts much faster than matter and soon would become inconsequential. On the other hand, dark energy density stays constant, giving rise to the acceleration of the universe.

Diabetes and Surgery

In “Operation: Diabetes,” Francesco Rubino talks about the role of bile as among the factors in the benefits of using surgery to control type 2 diabetes. But it seems confused on the links among the gallbladder, bile and glucose. For those of us who have had our gallbladders removed, does the lack of the organ tend to increase or decrease the amount of glucose in our blood?


RUBINO REPLIES: Despite a clear role of bile and bile acids—components of bile that act as signaling molecules—in metabolic regulation, there is no clinical evidence that removing the gallbladder induces substantial effects (positive or negative) on diabetes; in fact, blood glucose levels typically remain about the same. This should not be surprising, because the main function of the gallbladder is to store bile, not produce it (as the liver does), and the loss of such function is usually compensated by the main bile duct and the other bile conduits.

Gastrointestinal surgery can instead change the characteristics of bile and bile acids, which can affect the metabolism of glucose. Indeed, by altering the site and timing of bile-nutrient mix within the intestine, this surgery influences the interaction of bile acids with other components of intestinal content, thereby changing their chemical characteristics. For one, this can influence the ability of bile acids to interact with specific cell receptors in the intestinal lining. Further, changes in bile acids inside the gut can influence their reabsorption from downstream segments of the small intestine, thus affecting their levels in the bloodstream. In turn, this can change the way these molecules signal to other tissues involved in metabolic regulation, including the liver. In that organ, circulating bile acids serve as a feedback mechanism that modulates their own synthesis. As I discussed in my article, however, surgery's effect on diabetes is likely the result of a combination of various changes in GI mechanisms, not of bile acids by themselves.


“Black Holes from the Beginning of Time,” by Juan García-Bellido and Sébastien Clesse, incorrectly states that in the 1970s Bernard Carr and Stephen Hawking proposed primordial black holes only with masses smaller than a mountain's that would have evaporated long ago. They had also investigated the possibility of more massive, nonevaporating black holes.