Multimillionaire Gordon P. Getty has donated a $5 million challenge grant to the Pasadena-based L.S.B. Leakey Foundation for a study on human origin. The grant, the largest single gift ever for the scientific study of human beginnings, must be matched within two years. "We will have to work like crazy to get that matching money," said F. Clark Howell, a professor of anthropology at the University of California, Berkeley, and co-chairman of the foundation's committee on science and grants. Getty, known for his passions for composing music and the study of anthropology, told foundation members "there is a now-or-never quality to anthropology research. Deforestation threatens the few remaining groups of great apes and hunter-gatherers--human and animal alike."