This paper is concerned with the unfolding of human marks, beginning with scribbling, and their contribution to developing literacy. The paper argues that children's scribbles reveal a neural substrate destined for marks and influence that substrate significantly, cuing what is distinctly human in linguistic behavior and consciousness, or symbolic thought. The paper proposes and presents evidence that very young children's scribbling serves four critical purposes: (1) to train the brain to pay attention and to sustain attention; (2) to stimulate individual cells and clusters of cells in the visual cortex for line and shape; (3) to practice and to organize the shapes and patterns of thought; and (4) through an increasing affinity for marks, to prepare the human mind for a consciousness organized by literacy. The scribble hypothesis predicts that young children who are encouraged to scribble and draw, to talk and to write, and to compute and to compose about their scribbles and drawings will write more easily and will continue to write for pleasure as well as to disseminate information. Also, they will show an "innate" affinity for geometry, and in general, will think more connectedly and unpredictably, or creatively. By making use of the neuroconstructivist theory and cross-modal teaching and learning strategies such as drawing/writing, the paper asserts that the brain of the child practices thinking as it has evolved to think, using nested and unified systems of marks. The paper concludes with a discussion of the implications for parents and teachers. (Contains 87 references.) (Author/KB)