Why haven’t humans evolved eyes in the back of the head? — B. Craft, Wills Point, Tex.
S. Jay Olshansky,a biodemographer at the University of Illinois at Chicago, looks into this query:
As much as we might appreciate the value of detecting predators that approach from behind—or of keeping an eye on the offspring who follow us—it is important to remember that selection is not directed toward the development or formation of anything, let alone “perfect” organs. In other words, just because some feature seems like a good idea, random mutation and selection will not necessarily fashion it.
Body parts that enable us to detect the sights, sounds, smells, tastes, temperature and tactile elements of our environment did not arise from some master plan or blueprint. Rather selection crafted body parts from available components of cells and tissues within existing forms of life, molding ancient and intermediate versions of sensory cells and organs—each elegant in its own right—like lumps of clay over aeons into the shape and form of our modern bodies. There have never been perfectly formed organs for sight or hearing—just versions that get the job done.
The first light-sensitive cell most certainly arose through random mutation among the earliest multicellular creatures. This mechanism of detecting light conferred a selective advantage, however minute, to those individuals possessing these cells. The best evidence for this advantage is the fact that variations on the theme of visual acuity evolved dozens of times, independently, in various invertebrates, with at least nine variations of the eye having emerged—including the camera lens version we know so well.
Although light-sensitive cells are likely to have appeared on different parts of early forms of life, selection seems to favor those that enable creatures to detect light in the direction they are headed rather than the direction from which they came. Forward locomotion probably was a driving force for the current location of light-sensitive cells. Besides, with a simple 90-degree pivot of the head and peripheral vision, we already can see behind us without turning our bodies around. It would appear, however, that rearward vision is already present in parents and teachers—or at least it would seem so to their children and students.
Instead of sequestering carbon dioxide to reduce its effects on global climate, why don’t we split it into harmless carbon and oxygen? —J. Henderson, Devon, Pa.
James E. Miller,a chemical engineer at Sandia National Laboratories, breaks it down:
Splitting carbon dioxide (CO2) into carbon and oxygen can in fact be accomplished, but there is a catch: doing so requires energy. If hydrocarbon fuels, which produce the greenhouse gas in the first place, supply that energy, thermodynamics tells us that the net result will be more CO2 than you started with.
Consider the proposal as a chemical reaction: CO2 plus energy yields carbon and oxygen. This formula essentially reverses coal combustion (carbon plus oxygen yields CO2 and energy). If energy from coal were applied to drive the decomposition reaction, more CO2 would be released than consumed, because no process is perfectly efficient.
Another option would be to harness a carbon-free energy source to drive a reaction that does not merely undo the combustion process but instead uses carbon dioxide as an input to generate useful, energy-rich products. At Sandia National Laboratories, we are working to apply concentrated sunlight to drive high-temperature thermal reactions that yield carbon monoxide, hydrogen and oxygen from CO2 and water. Carbon monoxide and hydrogen are basic chemical building blocks that find use in producing synthetic fuels, so we call this process “sunshine to petrol.”