Throughout the special issue on evolution, a question based on a false premise is asked: What makes human special? As a minor branch on a vast evolutionary bush, modern humans have been roaming the earth for no more than a few hundreds of thousands of years—too little time to demonstrate if the evolution of large brains is a successful strategy for long-term survival of the species. If any life-form were special, it would be bacteria, which will be here long after the human experiment is a distant memory.

Spicewood, Tex.

I find myself likening the attempt to pinpoint where humanity began to preschoolers theorizing about candy they've found scattered about a picnic area. A Hershey bar in the grass suggests chocolate grows there, whereas a Butterfinger by the table surely fell out of a pocket.

Inventing scenarios to try and fit observations are where theories come from. Still, beware that in our eagerness to explain, we risk elevating accidents of the discovery process to primary evidence.

Oxford, Mass.

As John Hawks states in “Still Evolving (After All These Years),” human populations continue to evolve today. But Hawks does not discuss the possible consequences of some of the evolutionary pressures that have been altered in the past century by medicine and public health. Could modern medical intervention inadvertently result in the survival and spread of genetic mutations that would otherwise have been eliminated or in the loss of protective genes?

Stirling, Western Australia

HAWKS REPLIES: A bit of thinking about genetic drift and mutation shows that we need not worry about future generations being “genetically weaker” because of medical technology and other modern advances. If selection is relaxed on a deleterious mutation, its frequency can change only because of random genetic drift. Under drift alone, most rare mutations will become extinct over many generations. A few may increase in frequency, but the speed of genetic drift in a large population is very slow. In our large populations, it would take many thousands of generations for any of today's rare mutations to become common.

I look with wonder and joy on people today living happy lives by managing once fatal genetic disorders. If we can advance medical technology and public health in ways that release people from such lethal disorders for the next few thousand generations, I think we have little to fear from genetic drift.

In “If I Had a Hammer,” Ian Tattersall cites our capacity for symbolic reasoning as one of the traits unique to humans that led to the rise of our species' dominant position on the earth. I believe that our highly developed curiosity is another key, uniquely human trait. Without motivation, our capacity for symbolic reasoning would be of little use, and curiosity could have provided it.

via e-mail

TATTERSALL REPLIES: Human “curiosity,” in the sense in which we understand it today, is clearly enabled by our symbolic capacity to imagine that the world could potentially be different from the one we immediately experience.

I am confused by the apparent contradiction between statements in two articles.

In his article, Tattersall writes that “a population needs to be small if it is to incorporate any substantial innovation, genetic or cultural. Large, dense populations simply have too much genetic inertia to be nudged consistently in any direction.” But in his article, Hawks asserts that “the huge and rapidly increasing population size of our ancestors gave them many more rolls of the dice. As human populations have spread into new parts of the world and grown larger, they have rapidly adapted to their new homes precisely because those populations were so big.”

Berwyn Heights, Md.

THE EDITORS REPLY: Although Tattersall's and Hawks's statements might appear contradictory, they actually refer to different evolutionary scenarios.

Tattersall's article focuses on evolution in small populations of early human ancestors that were isolated from one another and lived in different environments. Under such conditions, random genetic and cultural changes (both beneficial and neutral) could have accumulated rapidly, thereby leading such populations to differentiate and, ultimately, speciate.

In contrast, Hawks's article focuses specifically on adaptive genetic changes within large populations of Homo sapiens. Large populations have more matings and hence more chances for beneficial genetic changes to arise, thus facilitating adaptation to the novel environments our species encountered as it spread out from Africa across the world.

In criticizing the National Institutes of Health's new policy requiring the scientists it funds to use equal numbers of male and female animal subjects in their research [“Vive la Différence,” Forum], R. Douglas Fields claims that doing so would create problems because it would increase data variability. That is balderdash.

First, differences between male and female participants can be captured by many different statistical treatments, which allows researchers to compare groups without increasing error variability.

Second, the idea that reduced variability is the aim of all scientists is scary. I could reduce data variability on chairs by only studying purple ones that were 0.5 meter high. Then I could make only generalizations about those particular chairs.

Laurentian University, Ontario

Fields incorrectly states that when sex is added to an experiment, it cuts the sample size in half and increases variation. In research or experimental design, one reduces variance by eliminating differences within the sample or by building differences into the design. Thus, at the postassessment phase, one might have a “two-by-two design” (experimental group versus control group and male versus female).

Professor Emeritus University of Southern California

FIELDS REPLIES: The essence of experimental research is careful discrimination. Comparing similar groups enables more discerning distinctions. There is no magic statistical method that can overcome the realities that variances add: increasing the number of categories from two to four makes it more difficult to draw conclusions without increasing the sample size.

The scientific method uses deductive reasoning to reject a specific hypothesis. Unlike Whissell's assertion, it does not permit any generalization. Further, only a small fraction of NIH grant applications are funded. The mandate imposes a specific hypothesis to test in every grant and circumvents the normal peer review that considers whether testing a hypothesis in each case is, for instance, prudent or practical.