Every human being starts as a single cell – the fusion of an egg and a sperm – and progresses via cell division and cell death through development, birth, growth, and aging. Human health depends on maintaining a proper process of cell division, renewal and death, and humanity's most severe diseases, notably cancer, auto-immune diseases, diabetes, neuro-degenerative and cardiovascular disorders, and the multitude of inherited rare diseases are all the result of specific aberrations in this process.

The history of a person's cells, from conception until any particular moment in time, can be captured by a mathematical entity called a cell lineage tree. The root of the tree represents the fertilized egg, the leaves of the tree represent the person's extant cells, and branches in the tree capture every single cell division in the person's history. Science knows precisely the cell lineage tree of only one organism – a worm called Caenorhabditis elegans that reaches its full size of 1 millimetre and 1,000 cells in 36 hours. By comparison, a newborn mouse, weighing only a few grams, has about 1 billion cells. An average person has about 100 trillion cells.

Understanding the structure and dynamics of the human cell lineage tree in development, growth, renewal, aging, and disease is a central and urgent quest of biology and medicine. The challenge of uncovering the Human Cell Lineage Tree is reminiscent, both in nature and in scope, to the challenge faced by the Human Genome Project at its inception and, in fact, its results will decisively contribute to the functional translation and ultimate understanding of the genome sequence. A technological leap of a magnitude similar to the one that occurred during the Human Genome Project is required for the success of the human cell lineage project, and the biological and biomedical impact of such a success could be of a magnitude similar, if not larger than that of the Human Genome Project.