Human thought has many sculptors, and each wields special tools for distinct effects. Is the Internet in the tool kit? That depends on the sculptor.

Natural selection sculpts human thought across generations and at geologic time scales. Fitness is its tool, and human nature, our shared endowment as members of a species, is among its key effects. Although the thought life of each person is unique, one can discern patterns of thought that transcend racial, cultural and occupational differences; similarly, although the face of each person is unique, one can discern patterns of physiognomy â€” two eyes above a nose above a mouth â€” that transcend individual differences.

Is the Internet in the tool kit of natural selection? That is, does the Internet alter our fitness as a species? Does it change how likely we are to survive and reproduce? Debate on this question is in order, but the burden is surely on those who argue no. Our inventions in the past have altered our fitness: arrow heads, agriculture, the control of fire. The Internet has likely done the same.

But has the Internet changed the patterns of thought that transcend individual differences? Not yet. Natural selection acts over generations; the Internet is but one generation old. The Internet is in the tool kit, but has not yet been applied. Over time, as the Internet rewards certain cognitive skills and ignores or discourages others, it could profoundly alter even the basic patterns of thought that we share as a species. The catch, however, is "over time." The Internet will evolve new offspring more quickly thanHomo sapiensÂ and they, rather than the Internet, will alter human nature. These offspring will probably no more resemble the Internet thanÂ Homo sapiensÂ resembles amoebas.

Learning sculpts human thought across the lifetime of an individual. Experience is its tool, and unique patterns of cognition, emotion and physiology are its key effects. Marcel Just and Timothy Keller found that poor readers in elementary school can dramatically improve their skills with six months of intensive training, and that white matter connections in the left hemispheres of their brains are measurably increased in the process.

There are, of course, endogenous limits to what can be learned, and these limits are largely a consequence of mutation and natural selection. A normal infant exposed to English will learn to speak English, but the same infant exposed to C++ or HTML will learn little.

Is the Internet in the tool kit of learning? No doubt. Within the endogenous limits of learning set by one's genetic inheritance, exposure to the Internet can alter how one thinks no less than can exposure to language, literature or mathematics. But the endogenous limits are critical. Multi-tasking, for instance, might be a useful skill for exploiting in parallel the varied resources of the Internet, but genuine multi-tasking, at present, probably exceeds the limitations of the attentional system of Homo sapiens. Over generations, this limitation might ease. What the Internet cannot accomplish as a tool of learning, it might eventually accomplish as a tool of natural selection.

Epigenetics sculpts human thought within a lifetime and across a few generations. Experience and environment are its guides and shifts in gene expression that trigger shifts in cognition, emotion and physiology are its relevant effects. Oberlander and colleagues found that a mother's anxiety can change the expression of the NR3C1 gene in her child, leading to the child's increased reactivity to stress. Childhood abuse can similarly lead to persistent feelings of anxiety and acute stress in a child, fundamentally altering its thought life.

Is the Internet in the toolkit of epigenetics? Possibly, but no one knows. The field of epigenetics is young, and even the basic mechanisms by which transgenerational epigenetic effects are inherited are not well understood. But the finding that parental behavior can alter gene expression and thought life in a child certainly leaves open the possibility that other behavioral environments, including the Internet, can do the same.

Thus, in sum, the relevance of the Internet to human thought depends on whether one evaluates this relevance phylogenetically, ontogenetically or epigenetically. Debate on this issue can be clarified by specifying the framework of evaluation.