Which animal was the first to have every cell charted from fertilized egg to adult?

The roundworm Caenorhabditis elegans was the first animal for which every cell lineage has been completely mapped from the fertilized egg to the adult stage. This achievement was significant because C. elegans is a simple organism with a well-defined and invariant cell lineage, making it an ideal model for studying development. Researchers were able to trace the development of each cell, noting how many cells contribute to different tissues and how they differentiate throughout the organism's life.

This groundbreaking work has provided invaluable insights into developmental biology, genetics, and cell differentiation, advancing our understanding of these processes in more complex organisms, including humans. The clear, well-studied anatomy of C. elegans, coupled with its transparent body, facilitates such detailed mapping, allowing scientists to observe cell division and the fate of each cell in real time.

Other options, while also valuable in various biological studies, do not have the comprehensive cell lineage mapping that C. elegans possesses. For example, the fruit fly Drosophila has been extensively studied for genetic research, but mapping every cell from fertilization to adulthood has not been as thoroughly accomplished as in C. elegans. Similarly, while the common house mouse and the zebra fish have been important for various studies,